It is important to note at the outset that atmospheric CO2 enrichment typically has but a small effect on the decomposition rates of senesced plant materials present in soils; yet this fact often leads to significantly greater soil carbon sequestration, as demonstrated by De Angelis et al. (2000), who reported a 4% reduction in the decomposition rate of leaf litter beneath stands of 30-year-old Mediterranean forest species enriched with air of 710 ppm CO2, and who thus concluded that "if this effect is coupled to an increase in primary production [which nearly always occurs in response to elevated CO2] there will be a net rise of C-storage in the soils of forest ecosystems." Similarly, in a study of soybean and sorghum plant residues grown at 705 ppm CO2, where decomposition rates were definitely not impacted by elevated CO2, Henning et al. (1996) still concluded that "the possibility exists for increased soil C storage under field crops in an elevated CO2 world," due, of course, to the greater residue production resulting from CO2-enhanced plant growth.
In a study that revealed how these phenomena once manifested themselves in a field of clover (Trifolium repens L.) at the Swiss Federal Institute of Technology near Zurich, a 71% increase in atmospheric CO2 concentration increased aboveground growth by 146%, while it increased the pumping of newly-fixed carbon into the soil of the CO2-enriched plots by approximately 50% (Nitschelm et al. 1997). In addition, root decomposition in the CO2-enriched plots was found to be 24% less than in the ambient-treatment plots; and, therefore, the researchers concluded that "the occurrence at elevated CO2 of both greater plant material input, through higher yields, and reduced residue decomposition rates would be expected to impact soil carbon storage significantly." And in a similar study of the effects of a doubling of the air's CO2 concentration on three different grass species, Cotrufo and Gorissen (1997) concluded that "elevated CO2 could result in greater soil carbon stores due to increased carbon-input into soils."
One year later, Verburg et al. (1998) grew one-year-old heather plants (Calluna vulgaris L.) for two months in greenhouses maintained at atmospheric CO2 concentrations of 380 and 580 ppm in combination with low and high levels of soil nitrogen before exposing them to 14CO2 for one day, in order to study the fate of recently-fixed carbon in their experimental plant-soil system. This work revealed that the extra CO2 increased net 14C uptake in heather by approximately 43%, irrespective of soil nitrogen content. In addition, soil 14C increased in elevated CO2 plots by 17 and 25% at low and high soil nitrogen levels, respectively. In addition, although total soil respiration was initially higher in the CO2-enriched plots (for two days post 14CO2 labeling), it declined to become significantly lower than the soil respiration rate displayed by plots exposed to ambient air within two weeks; and that trend persisted throughout the remaining four weeks of the study. Thus, it would appear that soil carbon sequestration beneath heather communities will likely increase in the future with further increases in the air's CO2 concentration.
These types of results have also been reported for other shrubs and trees that possess the ability to store more carbon in their associated soils than do grasses, as noted by Gill and Burke (1999)). As an example, Pregitzer et al. (2000) grew aspen seedlings for 2.5 years at 700 ppm CO2 and observed that fine root biomass was 65 and 17% greater than that produced by seedlings growing at ambient CO2 concentration on nitrogen-rich and nitrogen-poor soils, respectively. In commenting on their observations, the researchers stated that such increases in soil carbon inputs "can be substantial," even under low soil nitrogen conditions. Rouhier and Reed (1999) also noted that soil carbon was significantly greater beneath seedlings of birch grown at 700 ppm CO2 than it was beneath seedlings grown at 350 ppm CO2. In fact, Leavitt et al. (1994) found that 10% of the organic carbon present in soils beneath CO2-enriched cotton plants grown for only three years at 550 ppm CO2 came from the extra CO2, which was radiolabelled to trace its path through this woody agricultural species and into the soil.
In a study that included air temperature as a variable, Casella and Soussana (1997) grew perennial ryegrass (Lolium perenne L.) in ambient and elevated (700 ppm) CO2 at two different levels of soil nitrogen and at ambient and elevated (+3°C) temperature for a period of two years, finding that "a relatively large part of the additional photosynthetic carbon is stored below-ground during the two first growing seasons after exposure to elevated CO2, thereby increasing significantly the below-ground carbon pool." At the low and high levels of soil nitrogen supply, for example, the elevated CO2 increased soil carbon storage by 32 and 96%, respectively, "with no significant increased temperature effect," which led the two scientists to conclude that in spite of predicted increases in temperature, "this stimulation of the below-ground carbon sequestration in temperate grassland soils could exert a negative feed-back on the current rise of the atmospheric CO2 concentration." And along these same lines, van Ginkel and Gorissen (1998) and van Ginkel et al. (1999) - who performed similar experiments using Lolium perenne - concluded that the effects of atmospheric CO2 enrichment on increasing plant growth and decreasing decomposition rates of plant litter are "more than sufficient to counteract the positive feedback [on decomposition rates] caused by [an] increase in air temperature."
In further evaluating the effect of temperature on decomposition and soil carbon storage, Fitter et al. (1999) heated upland grass ecosystem soils by nearly 3°C and noted that root production and root death were increased by equivalent amounts. Hence, they concluded that elevated temperatures "will have no direct effect on the soil carbon store [in upland grass communities]." Similarly, Johnson et al. (2000) warmed Arctic tundra ecosystems by nearly 6°C for eight years and reported that warming had no significant effect on ecosystem respiration. In addition, Liski et al. (1999) showed that carbon storage in soils of both high- and low-productivity boreal forests actually increased with temperature along a temperature gradient in Finland. Thus, it is clear that if warming occurs, it will likely have little to no impact on soil carbon sequestration rates; but if and when there is an impact, it may well be positive.
As a consequence of the ongoing rise in the air's CO2 content, one can strongly anticipate that soil carbon storage will increase, which should have wide-ranging positive influences on agriculture. In reviewing the subject of soil carbon storage within the context of global climate change, for example, Rosenzweig and Hillel (2000) concluded that "our management of the soil should be aimed at enhancing soil organic matter for the multiple complementary purposes of improving soil fertility and soil structure, reducing erosion, and helping to mitigate the greenhouse effect." Moreover, in an actual experiment where soybeans were grown at an atmospheric CO2 concentration of 500 ppm, Islam et al. (1999) reported that soil particulate organic carbon content was significantly increased, as were the amounts of dissolved carbon, humic and fulvic acids. These findings thus led them to conclude that "one of the main benefits arising from the greater supply of organic residues to soils under CO2 enrichment is an improvement of soil structure." And in a similar contemporary study, Insam et al. (1999) noted that fumigation of artificial tropical ecosystems with 610 ppm CO2 for about 1.5 years increased humic substances in their soils by nearly 30%.
Studying the subject in New Zealand, Ross et al. (2000) collected soil samples from around a natural CO2 vent in order to determine the effects of elevated CO2 (510-900 ppm vs. 440-460 ppm) on soil carbon and nitrogen contents. The soil at the site was considered to be a gley, while the overlying vegetation was typical of native grasslands in the area, consisting of both C3 and C4 species. This work revealed that several decades of exposure to elevated atmospheric CO2 had significantly increased the soil's organic carbon and total nitrogen contents by 24%, while they increased microbial carbon and nitrogen contents by 116%. These results led Ross et al. to conclude that storage of C and N in gley soils "can increase under prolonged exposure to elevated CO2" and that "increased storage of soil organic matter at such springs can occur, even when soil C concentrations are already high." Thus, it is would appear that as the air's CO2 content continues to rise, vegetated gley soils will sequester increasingly greater amounts of both carbon and nitrogen, and that they will exhibit enhanced biological activity both above- and below-ground.
One year later, Hu et al. (2001) studied carbon and nitrogen relations in the plants and moderately-fertile soil of a sandstone grassland at Stanford University's Jasper Ridge Biological Preserve in central California near the conclusion of a five-year study conducted between 1992 and 1997, where two CO2 treatments (360 and 720 ppm) were maintained within twenty open-top chambers (ten replicates per treatment). This effort revealed that the increase in atmospheric CO2 increased soil microbial biomass at the same time that it increased plant nitrogen uptake, the net effect of which was that less nitrogen was left in the soil for microbes to use, which resulted in decreased microbial respiration per unit biomass and, hence, decreased microbial decomposition and increased ecosystem carbon accumulation.
In a concurrent experiment, King et al. (2001) grew O3-sensitive and O3-tolerant aspen (Populus tremuloides Michx.) clones alone and in mixed stands of paper birch (Betula papyrifera Marsh.) for two years in 30-m-diameter Free-Air CO2 Enrichment (FACE) plots in Rhinelander, Wisconsin, which they maintained at CO2 concentrations of 360 and 560 ppm with and without exposure to elevated O3 (1.5 times ambient concentrations) to study the interactive effects of these parameters on fine-root production and belowground carbon cycling in the soils associated with these stands. This work revealed that the elevated CO2 significantly enhanced the production of fine root biomass by 133 and 83% for aspen and aspen-birch mixed stands, respectively. In contrast, elevated O3 had no effect on fine-root biomass; but simultaneous exposure to elevated O3 and CO2 increased fine-root biomass by approximately 66% for both types of stands. And when averaged across both stands, elevated CO2 also increased dead root biomass by 140%, which is but another example of how elevated CO2 increases carbon inputs to soils.
Also with a paper published in the same year were Andrews and Schlesinger (2001), who in August of 1996, along with a number of other researchers, had established circular FACE plots (30 meters in diameter) which they exposed to air of either 360 or 560 ppm CO2. This modest increase in the air's CO2 content increased the weathering rate of parent rock material, as indicated by a 271% increase in soil mineral cation concentration and a 162% increase in soil alkalinity after the second year of CO2 enrichment. In addition, the elevated CO2 increased the flux of dissolved inorganic carbon compounds to the groundwater by 33%. And extrapolating this phenomenon to the global land area covered by forests, the two researchers remarked that the observed increase in the efflux of dissolved inorganic carbon compounds to groundwater "may act to buffer the rate of CO2 increase in the atmosphere over geologic time periods."
Working in the same plots at the same time, Pritchard et al. (2001) studied the effects of elevated CO2 on belowground root dynamics, as assessed by mini-rhizotrons inserted into the low-nutrient (nitrogen and phosphorus) soils of the experimental plots. And after one year of treatment, they determined that total standing root length and root numbers per minirhizotron were 16 and 34% greater, respectively, in the CO2-enriched plots than in the ambient-air plots. In addition, the elevated CO2 increased the diameter of living and dead roots by 8 and 6%, respectively; and annual root production and root mortality were 26 and 46% greater in the CO2-enriched plots than they were in the control plots, all of which phenomena would be expected to result in enhanced carbon sequestration within the soils in which the trees were rooted.
In another contemporaneous paper, Cardon et al. (2001) wrote that "soil organic carbon (SOC) is the largest reservoir of organic carbon in the terrestrial biosphere." This they affirmed in introducing their experimental study of the potential effects of the ongoing rise in the air's CO2 content on this vast store of material, which was once the "C" in the CO2 of much of earth's atmosphere, where it was freely wafted about prior to being assimilated by plants and sequestered in the soil.
Occupying such a pivotal position as it does in the planetary carbon cycle, SOC is of great interest to scientists who worry about its stability. Among other things, they want to know if allowing more CO2 to be emitted to the atmosphere would lead to even more carbon being sequestered in the soil (a logical hypothesis), or if it would somehow lead to a reduction in what was already there (a not-so-logical hypothesis but one that cannot be ignored, especially in view of what many people consider to be the incredibly high-stakes gamble we are taking with the world's climate as we continue to burn prodigious quantities of fossil fuels).
To further explore this question, Cardon et al. studied soil carbon income and outgo in a number of small microcosms of two annual C3 grassland communities (sandstone and serpentine) of contrived high and low soil-nutrient availability that were maintained out-of-doors in open-top chambers at the Jasper Ridge Biological Preserve in Stanford, California, from October 1994 through August 1996. Key to their study was the utilization of isotopic tracer techniques to determine the sizes of the various SOC pools through time. Helping them in this regard was the fact that they grew the C3 plants in a soil they obtained from a C4 grassland in Colorado, which ensured that the original organic carbon of their experimental soil would have a different isotopic signature from the organic carbon that would be injected into it by the C3 plants that grew upon it. In addition, the carbon of the fossil fuel-derived CO2 supplied to the CO2-enriched chambers had yet a third unique isotopic signature.
So what was learned? First of all, the extra CO2 supplied to half of the mini-ecosystems increased the total root biomass in the serpentine grassland microcosms by a factor of three in both high and low soil-nutrient availability treatments, while it increased total root biomass in the sandstone grassland microcosms by a factor of four in both the high and low soil-nutrient availability treatments. Hence, there was a tremendous CO2-induced increase in the amount of organic material that would eventually become available for incorporation into the soils of both grassland microcosms.
Second, with so much new organic matter being added to the soils of the CO2-erniched microcosms, Cardon et al. felt that previously carbon-limited microbes in these soils would alter their survival strategy and turn from breaking down older more recalcitrant soil organic matter to attack the more abundant and labile rhizodeposits being laid down in the newly-carbon-rich soils of the CO2-enriched microcosms. This rhizodeposition, as they christened it, consists of "all deposition of organic carbon from living root systems to soils, including compounds lost through root exudation, sloughing of dead cells during root growth, and fine root turnover," which, as noted above, was dramatically enhanced by atmospheric CO2 enrichment.
The upshot of this scenario - which seems thoroughly vindicated in light of the observations about to be described - is that the experimentally-imposed increase in atmospheric CO2 concentration actually retarded the decomposition of the older SOC of the imported soil, which had been deposited within that soil over who knows how many prior decades or centuries. Also, this phenomenon effectively increased the turnover time of the original SOC, significantly increasing its stability.
So what do these findings imply about the future? "If this reduction in breakdown of older SOC is sustained," wrote Cardon et al., "an increased retention of carbon in older SOC pools might be expected under elevated relative to ambient CO2." Therefore, not only does atmospheric CO2 enrichment lead to higher rates of carbon input to soils, it likely also leads to slower rates of carbon withdrawal from them. And that's what allows ever more carbon to be locked away in earth's soil bank as the air's CO2 content continues to rise. And that phenomenon appears to keep the air's rate-of-CO2-rise from accelerating too greatly, even in the face of yearly increases in anthropogenic CO2 emissions.
Shifting our perspective just a bit, one might reason, since biological activity generally increases with rising temperatures - especially when the initial temperature is below the freezing point of water - that global warming would enhance rates of soil microbial respiration, leading to increases in the soil-to-air flux of CO2. However, as Neilsen et al. (2001) have noted, "over-winter processes account for a significant portion (20-70%) of annual ecosystem carbon and nitrogen cycling and soil-atmosphere trace gas fluxes." Hence, it was not immediately apparent what the ultimate consequences of warming-induced reductions in the frequency and severity of freezing would really be; and to thus find out for themselves, Neilsen et al. conducted an experiment.
They began by collecting samples of soil from a northern hardwood-dominated forest in New Hampshire, USA. These samples, from nearly pure stands of sugar maple (Acer saccharum Marshall) and yellow birch (Betula alleghaniensis Britton), were placed in small vessels and either maintained at the normal laboratory temperature of 20-25°C or subjected to mild and severe freezes of -3 and -13°C, respectively, for ten days, after which all samples were kept at the normal laboratory temperature for 23 additional days. And to find the answer to their question, Neilsen et al. measured the evolution of CO2 from the soils at the beginning and end of the full 33-day period, as well as at three other times during the course of the experiment.
This work revealed that freezing had a significant effect on CO2 evolution from the soils. Cumulative 33-day totals of respiration (in units of mg carbon per kg of soil) for the soil samples taken from the maple stand were 1497, 2120 and 3882 for the control and -3 and -13°C temperature treatments, respectively, which numbers represent carbon loss enhancements (relative to that of the control) of 42 and 159% for the -3 and -13°C treatments, respectively, or an increased carbon loss of 13 ± 1% for each degree C below freezing. For the soil samples taken from the birch stand, the corresponding respiration numbers were 1734, 2866 and 5063, representing carbon loss enhancements of 65 and 192% for the -3 and -13°C treatments, respectively, or an increased carbon loss of 18 ± 3% for each degree C below freezing.
It can be readily appreciated how these research results relate to the subject of global warming effects on soil carbon sequestration. As temperatures gradually warm over the course of many years and different climate zones move poleward in latitude and upward in elevation, regions that experienced many hard freezes in the past will experience less of them in the future, while other regions will experience a shift from hard freezes to mild freezes. Still other regions that experienced mild freezes in the past will experience fewer - or none - in the future. And in all of these situations, together with every permutation that falls somewhere between them, there will be a tendency for less carbon to be released to the atmosphere, which means that more will remain sequestered in the soil.
Moving ahead another year and focusing on a strikingly different aspect of carbon sequestration in soils, it is readily evident that the great deserts of Africa and Asia have a huge potential for sequestering carbon, because they are currently so barren that their soil carbon contents have essentially nowhere to go but up. The problem with this scenario, however, is that that's where their soils are going: up, up and away, with every wisp of wind that disturbs their surfaces.
The ongoing rise in the air's CO2 content could do much to reverse this trend. At higher atmospheric CO2 concentrations, nearly all plants are more efficient at utilizing water (Morison, 1985). Hence, as the air's CO2 content rises, the vegetation that rings the earth's deserts should be able to encroach upon them and more effectively protect their surfaces from the ravages of the wind, thereby reducing soil and carbon losses due to erosion. Also, rising atmospheric CO2 concentrations should increase the stability of surface soil crusts that are held together by lichens and/or algae (Tuba et al., 1998; Brostoff et al., 2002), which should also help to reduce the deleterious effects of wind erosion (Evans and Johansen, 1999). In addition, many of the algal components of desert soil crusts are nitrogen-fixers (Evans and Belnap, 1999); and their CO2-enhanced presence should lead to more nitrogen being made available to other plants, which should accelerate the development of soil-protecting ecosystems even more.
The end result of all of these phenomena working together is greater carbon storage, both above- and below-ground, in what was previously little more than a source of dust for the rest of the world. And therein lies one of the great unanticipated benefits of the CO2-induced greening of the globe's deserts: less airborne dust to spread havoc throughout the earth.
To better explain this phenomenon, it is helpful to refer to an article in the American Scientist magazine entitled "The Global Transport of Dust." There, Griffin et al. (2002) began their essay with a description of the magnitude of soil materials wafted about by the wind. "By some estimates," they said, "as much as two billion metric tons of dust are lifted into the Earth's atmosphere every year." And riding along on those particles are "pollutants such as herbicides and pesticides and a significant number of microorganisms - bacteria, viruses and fungi." In fact, the four scientists calculated that there are easily enough bacteria thus moved about the planet each year "to form a microbial bridge between Earth and Jupiter."
But is dust from Africa and Asia really capable of stretching that far? Well, it may not traverse interplanetary space; but it does cross both the Atlantic and Pacific Oceans. Griffin et al. reported, for example, that dust storms originating in North Africa "routinely affect the air quality in Europe and the Middle East" and that millions of tons of African sediment "fall on the North Amazon Basin of South America every year." Likewise, Prospero (2001) noted that everyone in the United States living east of the Mississippi River is affected by dust of African origin. And in another example from April of 2001, Griffin et al. reported that a large dust cloud originating over the Gobi Desert of China "moved eastward across the globe, crossing Korea, Japan, the Pacific (in five days), North America (causing sporadic reports of poor air quality in the United States), the Atlantic Ocean and then Europe."
Many of the biological entities associated with the dust particles that are thus dispersed about the planet have serious consequences for plants, animals and humans. Airborne fungi from Africa that frequently make their way to the Americas, for example, cause sugar cane rust, coffee rust and banana leaf spot. Griffin et al. have also described how the scourge of Caribbean sea fans - Aspergillus sydowii - "is also found in the Caribbean atmosphere during African dust events," noting that the region's "sea fans and other coral reef organisms have experienced a steady decline since the late 1970s," when worsening drought in Africa predisposed increasing amounts of soil there to wind erosion (Prospero, 2001). And they also stated that they expect that "future research will show that many other coral diseases are spread by dust from both Africa and Asia."
With respect to human health, Griffin et al. noted that "African dust is reported to be a vector for the meningococcal meningitis pathogen Neisseria meningitis in sub-Saharan Africa," and that outbreaks of the disease often followed localized or regional dust events that resulted in many fatalities. They also reported that there has been a 17-fold increase in the incidence of asthma on the island of Barbados since 1973, "which corresponds to the period when the quantities of African dust in the region started to increase."
Because the dust clouds that reach the Americas from Africa and Asia have traveled such long distances, most of the larger particles they originally contained generally fall out of them along the way. The particles that remain, therefore, are typically very small: so small, in fact, that Griffin et al. report that "once they are inhaled into the lungs they cannot be exhaled." What makes this situation especially serious is that the tiny dust particles typically are heavily coated with iron; and a substantial fraction of that iron is released to the lung tissue when the particles are deposited there. And iron, as Prospero notes, is "particularly efficient in producing an inflammatory response in the lungs."
In light of these incredible-but-true observations, it is clear that the slow but steady acceleration of carbon sequestration in the deserts of Africa and Asia, which is being provided by the ongoing rise in the air's CO2 content, is producing more than just local benefits. Plants and animals far and wide, on land and in the sea, together with people everywhere, will ultimately benefit, if they are not already doing so, from the reduced airborne-dispersal of pathogens responsible for many debilitating diseases, as source-region soils become better protected against the erosive power of the wind. And if natural carbon sequestration tendencies can bring about these ancillary benefits, so too can those of man. Consequently, citizens involved in local carbon sequestration projects can take satisfaction from the fact that their efforts are having a positive impact on the global environment in more ways than one. Even if rising concentrations of atmospheric CO2 have no substantial impact on the world's climate, for example, there are many other reasons to be involved in projects designed to enhance the productivity of the planet's managed and natural ecosystems, not the least of which is the reduction of airborne dust caused by wind-induced soil erosion.
Moving from desert to tropical isle, Dilustro et al. (2002) noted that soils store approximately three times more carbon than plants do, but that almost all of that carbon is transferred to the soil through plants. They also noted that plant root responses to elevated CO2 had been largely overlooked in this regard; and they thus concluded that some of the carbon that is missing from current global carbon cycle models may well be sequestered belowground. And intrigued by the possibility that enhanced carbon transfer to soils via plants responding to the aerial fertilization effect of atmospheric CO2 enrichment may account for much of the carbon that exits the atmosphere each year, the four researchers designed an experiment to provide some potential answers to this important question.
On a small barrier island in the northern part of the Kennedy Space Center, Florida, USA, the group of scientists erected sixteen open-top chambers around clumps of evergreen scrub oaks and associated saw palmetto shrubs that comprised a fire-adapted ecosystem that had historically been maintained by natural fire cycles of 10- to 15-year intervals, which had last been burned in February 1996, just prior to the start of their experiment. Half of the chambers were maintained at the CO2 concentration of the ambient air, while the other half - starting on 15 May 1996 - were continuously maintained at CO2 concentrations approximately 350 ppm above ambient. In addition, in the soils of each of the sixteen chambers, the scientists inserted two mini-rhizotron tubes to a depth of 101 cm, through which they viewed the growth and development of the ecosystem's fine-roots at three-month intervals, from March 1996 to December 1997, via tiny video camera systems.
So what was learned? In the words of Dilustro et al., "our hypothesis that elevated atmospheric CO2 would increase fine-root density, productivity, mortality and turnover was demonstrated." Indeed, by the end of the 21-month study period, the fine-root length density of the re-sprouting trees and shrubs in the ambient-air chambers had attained a mean of 7.53 mm cm-2 in the top 101 cm of soil, while that of the re-sprouting plants in the CO2-enriched chambers had attained a mean of 21.36 mm cm-2, indicative of a CO2-induced increase of 184% in this important root property. Concomitantly, there was also a 55% increase in ecosystem aboveground biomass; and all this happened, as the scientists described it, "despite water and nutrient limited conditions."
What was the significance of these findings? Dilustro et al. stated that "the increased rates of fine root growth coupled with no change in decomposition rate suggest a potential increased rate of carbon input into the soil." Furthermore, their detailed fine-root data for June of 1997 indicated a mean CO2-induced increase in fine-root length density of approximately 75% in the top three-fourths of the soil profile, but an increase on the order of 125% in the bottom quarter. Hence, there were strong indications that the bottom layer of soil was being supplied with a greater proportion of extra carbon than were the upper soil layers.
One year later, in introducing a new study of their own, Ritchie and McCarty (2003) wrote that "recent studies indicate that soil erosion and redeposition may establish an ecosystem disequilibrium that will promote carbon sequestration within the biosphere (Stallard, 1998; Harden et al., 1999)." In this scenario, as they continued, "soil erosion on the uplands moves soil carbon to deposition sites on the landscape and promotes soil carbon replacement at the eroded sites from the production of vegetative biomass." Often, these deposition sites are riparian systems with high net primary productivity, which also leads to increased onsite storage of carbon. And with respect to the validity of these concepts, Ritchie and McCarty noted that "the capacity of riparian and flood plain systems to capture sediments has been documented (Ritchie et al., 1975; Walling et al., 1999) as well as the ability of these systems to store carbon has been documented (Lal et al., 1998)."
The two researchers then went on to describe their newest study, telling how they collected - and analyzed for carbon content - profiles of soils obtained from an upland area and an adjacent riparian system into which the upland area drained. And in doing so, they found that the riparian system acted as a filter, removing eroded soil materials from the overland flow before they reached the stream that drained the area, so that soil carbon content was significantly greater in the riparian soils than in the upland soils. In fact, Ritchie and McCarty determined that carbon storage in the riparian soils was 3.8 times greater than that in the upland soils in the upper 20 cm of the soil profile and 4.7 times greater in the upper 30 cm, while they report that the work of Ritchie and McCarty (2001) suggested that "there may be as much as 10-15 times more carbon in the total profile (0-200 cm) of the riparian soils."
These results put a bright new face on what was long believed to be a phenomenon of absolutely no virtue whatsoever, i.e., precipitation-driven soil erosion. In addition, they provided a whole new reason for protecting earth's wetlands, i.e., preserving an important sink for atmospheric CO2. Last of all, the findings of Ritchie and McCarty provided the groundwork for a system that may one day be actively promoted as a means of sequestering carbon in an overt attempt to slow the rate of rise of the air's CO2 content that has some virtue in and of itself.
Working concurrently, Jongmans et al. (2003) carried out a micro-morphological study of structural development and organic matter distribution in two calcareous marine loam soils on which pear trees had been grown for the prior 45 years. The soil of one of these Dutch orchards exhibited little to no earthworm activity, while the soil of the other orchard exhibited high earthworm activity, which difference was the result of different levels of heavy metal contamination of the soils of the two orchards due to the prior use of different amounts of fungicides.
The absence of earthworms in the first orchard led to topsoil compaction, restricted liter incorporation into the mineral portion of the soil, less fragmentation of particulate organic matter, and restricted mixing of organic matter with the mineral soil's clay fraction. Furthermore, without earthworms there were no earthworm casts; and the five researchers pointed out that "the rate of organic matter decomposition can be decreased in worm casts compared to bulk soil aggregates," citing the findings of Martin (1991) and Haynes and Fraser (1998).
Thus, based on their own findings, as well as those of others, Jongmans et al. concluded that "earthworms play an important role in the intimate mixing of organic residues and fine mineral soil particles and the formation of organic matter-rich micro-aggregates and can, therefore, contribute to physical protection of organic matter, thereby slowing down organic matter turnover and increasing the soil's potential for carbon sequestration." And these points take on added significance when it is realized that elevated levels of atmospheric CO2 tend to increase earthworm populations and activities, as has been demonstrated by Zaller and Arnone (1997, 1999). Thus, the logical conclusion to be drawn from these two sets of facts is that the ongoing rise in the air's CO2 content will help more of the extra organic matter that is produced under CO2-enriched conditions to remain in the soil even longer than it otherwise would remain due to the organic-matter-conserving nature of the increased activities of the increased earthworm populations that have been shown to occur as a consequence of increases in the atmosphere's CO2 concentration.
Moving ahead another year, Prior et al. (2004) speculated that "enhanced aboveground crop growth under elevated CO2, leading to more soil surface residue and greater percent ground cover (Prior et al., 1997) coupled with positive shifts in crop root systems (Prior et al., 2003), may have the potential to alter soil structural characteristics." And, therefore, they decided to see if this inference was indeed true, and if it was true, to see if elevated atmospheric CO2 concentrations tended to enhance or degrade soil physical properties.
With these goals in mind, Prior et al. grew soybean (Glycine max (L.) Merr. cv. Stonewall) and sorghum (Sorghum bicolor (L.) Moench cv. Savanna 5) plants from seed to maturity for five consecutive growing seasons within open-top chambers maintained at atmospheric CO2 concentrations of either 360 or 720 ppm. The soil in which the plants grew had been fallow for more than 25 years prior to the start of the study and was located within a huge outdoor bin, where at the end of each growing season aboveground non-yield residues (stalks, soybean pod hulls and sorghum chaff), including 10% (by weight) of the grain yield, were allowed to remain on the surfaces of the plots to simulate no-tillage farming, after which measurements of soil properties made at the beginning of the experiment were compared with similar measurements made at its conclusion.
These efforts indicated that the elevated CO2 (1) had no effect on soil bulk density in the sorghum plot, but lowered it in the soybean plot by approximately 5%, (2) had no effect on soil saturated hydraulic conductivity in the sorghum plot, but increased it in the soybean plot by about 42%, (3) increased soil aggregate stability in both plots, but by a greater amount in the soybean plot, and (4) increased total soil carbon content by 16% in the sorghum plot and 29% in the soybean plot. Consequently, the soils of both plots experienced some improvements in response to the experimental doubling of the air's CO2 content, although there were more and greater improvements in the soybean plot than in the sorghum plot. And in discussing their several findings, Prior et al. were therefore able to state that they were indicative of a significant "potential for improvements in soil carbon storage, water infiltration and soil water retention, and reduced erosion," which valuable positive consequences they rightly described as "CO2-induced benefits."
Hard on the heels of Prior et al.'s 2004 study came that of Krull et al. (2005), who reported that "colonization of grasslands or savannas by trees over the last 50-100 years, often described as 'thickening', has received attention due to the large potential for carbon sequestration in woody biomass." It was also a hot topic because many studies had attributed thickening to "the increase in atmospheric CO2, causing CO2 fertilization and resulting in increased water-use efficiency in C3 plants," as discussed by Berry and Roderick (2002) and Grunzweig et al. (2003). In addition, they said that "much of the change in atmospheric CO2 occurred over the last 50 years [1953-2003 = 64 ppm] with the most significant changes being in the last 20 years [1983-2003 = 33 ppm]."
Working at a site some 40 km northwest of Longreach, Queensland, Australia, Krull et al. thus measured vertical profiles of δ13C and 14C of bulk and size-separated soil organic matter to infer the time course of changes in these parameters along a transect spanning the dynamic transition zone between C4-dominated grassland and C3-dominated woodland, which ecotone was comprised of different-age specimens of leguminous gidyea trees (Acacia cambagei) that were interspersed with occasional whitewood trees (Atalaya hemiglauca). Then, since the long-time landholder said that thickening by the Acacia trees had occurred "at least since the 1950s," they tested whether the observed changes in soil carbon stocks could be reproduced by the Roth-Carbon turnover model over a 50-year time period.
When all was said and done, it was finally determined, according to the eight researchers, that "much of the vegetation change at this site occurred over the last 50 years." In addition, they measured approximately twice as much total organic carbon in the soil beneath the fully established woodland as what they measured in the soil beneath the pristine grassland. And so it was that they concluded their paper by saying their findings "stress the importance of viewing soils as dynamic systems and indicating the potential for soil organic carbon sequestration in grazed semi-arid woodlands," which land use represents a form of agroforestry, the virtues of which had recently been touted by Mutuo et al. (2005). Also, their findings suggested the operation of an important negative feedback that has the potential to slow the rate-of-rise of the air's CO2 content, wherein the ongoing enrichment of the air with CO2 from the burning of fossil fuels enables woody species to more readily colonize less productive grasslands and thereby extract greater amounts of CO2 from the atmosphere, while at the same time providing many benefits to the soil in which the trees are rooted.
Parenthetically, it is important to note that papers published in journals such as Science and Nature typically attract lots of attention, especially when they deal with high-profile subjects such as global warming, which some have described as a threat worse than nuclear warfare or global terrorism. And when a study describes a phenomenon that could potentially exacerbate that threat, it behooves its authors and the editors of the journal in which it is published to be especially careful in the way they describe what was found and what its implications may be.
A case in point concerns the report of Heath et al. (2005), who studied soil sequestration of root-derived carbon from seedlings of six European tree species and found it to decline in response to atmospheric CO2 enrichment. This finding led them to write that "should similar processes operate in forest ecosystems, the size of the annual terrestrial carbon sink may be substantially reduced, resulting in a positive feedback on the rate of increase in atmospheric carbon dioxide concentration." This outcome was parroted by the journal's editors, who wrote that the new findings "raise the possibility that the future rise in atmospheric CO2 concentrations could be higher than expected," which is exactly the type of scenario climate-alarmists love to promote.
How reasonable are these speculations? To answer this question, it is important to know how closely - or not - the experimental setting of Heath et al.'s study mimicked that of real-world forests or orchards. Was their study a FACE experiment, such as that conducted in the Duke University Forest, where multiple 30-m-diameter plots of initially 13-year-old loblolly pine trees had been exposed continuously to ambient and elevated concentrations of atmospheric CO2 each growing season since August of 1996? Or, was it an out-of-doors open-top chamber study, such as the Phoenix, Arizona sour orange tree experiment, where trees were grown from the sapling stage to mature adults over a period of 17 years of continuous CO2 enrichment? No, it was neither of the above. The Heath et al. experiment was but a 15-month study conducted in small greenhouses, where seedlings were grown in vertical sections of 16-cm-diameter polyethylene tubes supplied with but 10 liters of soil.
To their credit, Heath et al. readily acknowledged the many deficiencies of their study. They stated, for example, that "young trees, grown in mesocosms in a semi-controlled environment and protected from major herbivores, may respond differently from mature trees growing in a natural forest." They also wrote that their experiment "ran for only two growing seasons" and that "the input of leaf litter to the soil was excluded." With respect to these latter two points, they also stated that "the possibility that longer term increased inputs of leaf litter under elevated CO2 could counteract the effect on the sequestration of root-derived carbon cannot be ruled out." And they added that "although soil microbial respiration increased under elevated CO2, the effect of this on the decomposition of native soil carbon is not known."
That these deficiencies likely precluded the discovery of the truth sought by Heath et al. - within the context of their experiment, at least - is revealed by their acknowledgment that "in contrast to our experiment, CO2 enrichment caused an increase in soil carbon sequestration beneath Betula seedlings over the course of one growing season (Ineson et al., 1996)," and that "free-air CO2 enrichment (FACE) also caused an increase in the sequestration of new carbon in C4 soil cores transplanted into former agricultural ground beneath 2- to 3-year-old Populus saplings (Hoosbeek et al., 2004)." Consequently, and after reviewing the results of still other pertinent experiments, they ultimately concluded "there is insufficient evidence to predict with certainty whether plant responses to elevated CO2 will result in increased or decreased sequestration of new carbon in the soils of forest ecosystems."
In light of these astounding observations, which were made in Heath et al.'s own paper, and which suggest there is no compelling reason to believe that their results bear any resemblance to what will actually occur in the real world of nature as the air's CO2 content continues to climb, one wonders why their paper was ever accepted for publication in so prestigious a journal as Science. Could it be that it was not for scientific reasons at all, but for the support it may have been hoped it might provide for the political views of its editor, who strongly supported Kyoto-type regulations of anthropogenic CO2 emissions? We'll most likely never know, but the entire episode certainly reeks of strangeness.
In a contemporary study suffering from some of the same problems as that of Heath et al., soil carbon contents were measured by Bellamy et al. (2005) at 2179 locations scattered across England and Wales between 1994 and 2003, adjacent to points where similar measurements had been made between 1978 and 1983, after which rates of change of soil carbon content were calculated for the quarter-century period 1978-2003. And in so doing, they found that "carbon was lost from soils across England and Wales over the survey period at a mean rate of 0.6% yr-1," which phenomenon they attributed to "climate change," noting that over the period of study "the mean temperature across England and Wales increased by about 0.5°C."
Bellamy et al. thus concluded that "losses of soil carbon in the UK, and by inference in other temperate regions, are likely to have been offsetting absorption by terrestrial sinks," while Associated Press writer Michael McDonough began his review of the study (7 Sep 2005) by stating that "rising temperatures resulting from climate change are likely causing soil in England and Wales to lose large amounts of carbon, possibly further contributing to the greenhouse gas effect." These conclusions, however, were not as well supported as they were portrayed to be.
For starters, Bellamy et al. only resampled 38.5% of the original sites that were sampled between 1978 and 1983, so that the vast majority of England and Wales was not assessed for changes in soil organic carbon (SOC) content, much less "other temperate regions" of the globe. In addition, the five researchers claimed that "the relationship between rate of carbon loss and carbon content is irrespective of land use," which was key to their being able to claim "a link to climate change." However, in an accompanying article that raised several other concerns, Schulze and Freibauer (2005) said that in "re-inspecting the results, we think that the land-use factor has played a role - for example, only alteration in land use and gradual changes in land management can explain why croplands lost more carbon than other areas." Also, in this regard, they stated that studies conducted in China, Finland and Flanders "attribute most of the SOC loss to changes in land use and management." In fact, even Bellamy et al. admitted that "various changes in land use will have contributed to carbon losses from soils across England and Wales over the survey period, both under agricultural uses (drainage schemes, post-war grassland conversion, increased stocking rates) and non-agricultural uses (afforestation on wet soils, increased erosion, increased burning of upland vegetation)." However, they indicated that they did "not have sufficient data at the scale of the National Soil Inventory to explore these effects," so they really did not know the role played by land use, which meant they really did not know the role played by climate change.
Another complaint of Schulze and Friebauer was that the SOC losses observed by Bellamy et al. "occurred independently of soil properties, challenging our knowledge about SOC stability," as this observation was at odds with what had been learned about the subject over the years. They also noted that the carbon losses were proportional to SOC concentration, which implies "a first-order decay of a homogeneous pool" that "contradicts the view that SOC in carbon-rich soils contains a higher fraction of stable carbon than does that in carbon-poor soils."
Last of all, Schulze and Friebauer noted that SOC contents may have changed in deeper soil layers than the top 15-cm layer measured by Bellamy et al., possibly in compensating ways; and they were firm in their opinion that "increased temperature alone seems to be too weak a driver" to have caused the observed changes in SOC. Thus, it would appear that Bellamy et al. merely "scratched the surface" of the controversial topic in a way that has yet to reveal its true nature.
Dropping back in time a couple of years, Callesen et al. (2003) had measured SOC contents of forest floors and mineral soils to a depth of 100 cm in 234 well-drained Danish, Finnish, Norwegian and Swedish forests between latitudes 55 and 68°N and longitudes 6 and 28°E, after which they performed a number of analyses with the data, the first of which revealed that "soil organic carbon in forest floors and mineral soil + forest floors was positively correlated with temperature and precipitation in the study region." They also reported that "a similar increase in SOC with temperature and precipitation was found in nine pine stands on sandy soils within the same latitude range but between 22 and 29°E (Vucetich et al., 2000) representing the same temperature gradient but a lower precipitation range." And they affirmed that the positive correlation with temperature was greatest for coarse-textured soils, less for medium-textured soils, and negligible for fine-textured soils.
In discussing their work, the Nordic scientists stated that "the increase in SOC with temperature and precipitation is interpreted as an indirect effect of higher net primary production," while further noting that (1) in Europe "increasing site productivity has been reported in both nemoral forests and in boreal forests at higher latitudes (Eriksson and Karlsson, 1996; Skovsgaard and Henriksen, 1996; Cannell et al., 1998)," and that (2) this increase "could be attributed to increased atmospheric CO2 concentrations along with the fertilizer effect of nitrogen deposition, and management regimes optimizing forest production." And in light of these broad-based findings, it can be appreciated that just the opposite of what Bellamy et al. had claimed to be occurring in the top 15 cm of soils in England and Wales had actually been happening in the top 100 cm of soils throughout much of Europe, producing a negative feedback to both rising air temperatures and atmospheric CO2 concentrations.
Returning to the original timeline of this summary, Lichter et al. (2005) reviewed what had been learned to that point in time about the effects of an atmospheric CO2 enrichment of 200 ppm on the soil carbon dynamics of the Duke Forest (an aggrading loblolly pine stand near Chapel Hill, North Carolina, USA) during the first six years of the long-term FACE experiment being conducted there. Over this time period, they reported that organic C accumulated in the forest floor of the elevated CO2 plots at a rate that was 52 ± 16 g C m-2 yr-1 greater than what would have been expected during reforestation under ambient CO2 conditions, as represented by the rate of C accumulation in the forest floor of the ambient CO2 plots.
This additional C sink, in their words, "resulted from increased C inputs of 50 ± 30 g C m-2 yr-1 to the forest floor in response to CO2 enhancement of primary production." And since there was "no evidence that the overall rate of decomposition of the forest floor decreased under the elevated CO2 treatment," they concluded that "the additional C sink in the forest floor of the elevated CO2 treatment ... is wholly dependent on the net primary production enhancement and increased C inputs," which after a total of six years had increased the forest floor's organic C content by approximately 27%, as best as could be determined from their plotted data. What is more, the data gave no indication that this trend may be on the verge of declining anytime soon.
With respect to the underlying mineral soil, Lichter et al. said they could detect no statistically significant treatment effects on the C content of the bulk mineral soil or the intra-aggregate particulate organic matter and mineral-associated organic matter fractions after six years of CO2 enrichment. Nevertheless, there was a nearly statistically significant (P = 0.11) increase of 18.5% in the free light fraction of the organic matter in the top 15 cm of the soil profile, as well as a 3.9% increase in the total intra-aggregate particulate organic matter there; and the sum of the organic C in these two categories plus the mineral-associated organic C was 11.5% greater in the CO2-enriched plots than in the ambient treatment plots.
Although Lichter et al. were somewhat pessimistic and continued to believe that "forest soils are unlikely to sequester significant additional quantities of atmospheric C associated with CO2 fertilization because of the low rates of C input to refractory and protected soil organic matter pools," the CO2-enriched trees of their study continued to demonstrate a large and unabated growth advantage over the ambient-CO2 trees; and both the forest floor and the surface soil horizon beneath the CO2-enriched trees continued to accumulate more organic C than the forest floor and surface soil horizon beneath the ambient-CO2 trees. And, therefore, as time proceeds, the un-stimulated refractory and protected soil organic matter pools of which Lichter et al. wrote could yet begin to show increased carbon accumulation.
Moving on, there are a number of naturally occurring phenomena that tend to mute the rate of rise of the anthropogenic-driven increase in the atmosphere's CO2 concentration, while there are other phenomena that perform the same function that owe their existence to human ingenuity. In their report of a study of the latter type, Prior et al. (2005) described a multifaceted field management system that was developed to help farmers conserve resources and increase crop yields, while simultaneously stimulating carbon sequestration in their fields.
This conservation as opposed to conventional management system employed little to no tillage and used special crop rotations. In the southern United States, where Prior et al. had been testing the two approaches for the prior five years, the conventional cropping system consisted of a rotation cycle where grain sorghum and soybean were rotated each year with spring tillage after winter fallow that produced only a light growth of weeds, while in the conservation cropping system, grain sorghum and soybean were also rotated, but in the place of weeds were three cover crops: crimson clover, sunn hemp and wheat, which were similarly rotated but without tillage.
To see how the two management systems compared in terms of crop production and soil carbon sequestration, as well as to see how well they might be expected to compare in the high-CO2 world that is expected to prevail a half century or so from now, the five U.S. Department of Agriculture scientists employed them for a period of four years (two complete cropping cycles) in 7-meter-wide x 76-meter-long x 2-m-deep bins filled with a silt loam soil, upon which they constructed a number of clear-plastic-wall open-top chambers they maintained at atmospheric CO2 concentrations that averaged either 375 ppm (ambient) or 683 ppm (enriched) over the four years of their study.
And what did they learn? In terms of the cumulative residue produced over the two cropping cycles, there was little interaction between management practices and atmospheric CO2 concentration, with conservation practices increasing this parameter by about 90% in both CO2 treatments, with elevated CO2 increasing it by approximately 30% in both of the management treatments, and with conservation practices and elevated CO2 together increasing it by 150%. In terms of the carbon retained and incorporated into the first 5 cm of the soil at the end of the two cropping cycles, however, there were significant interactions. The elevated CO2 increased this important soil property by about 10% in the conventional system, but by 45% in the conservation system, while the application of conservation practices increased 0- to 5-cm soil carbon storage by close to 45% in ambient-CO2 air but by nearly 90% in elevated-CO2 air, while together the two treatments increased surface soil carbon storage by close to 110%.
Clearly, increasing atmospheric CO2 concentrations and best-management conservation practices work hand-in-hand to boost crop yields and residue production while increasing soil carbon storage, with each factor bringing out the best in the other. In addition, as noted by Prior et al., "in an elevated CO2 environment there will be larger amounts of crop residue and consequently more ground cover," so that "accumulation of additional surface litter may improve water infiltration (and storage) and help ameliorate water quality problems by reducing runoff and soil erosion."
Contemporaneously, Jastrow et al. (2005) noted that many field-scale CO2-enrichment studies "have failed to detect significant changes in soil C against the relatively large, spatially heterogeneous pool of existing soil organic matter, leading to the general conclusion that the potential for increased soil C is limited (Hungate et al., 1997; Gill et al., 2002; Hagedorn et al., 2003; Lichter et al., 2005)." And an additional long-held opinion, as they related it, was that "if CO2-stimulated increases in soil organic C do occur, they will be allocated to rapidly cycling, labile pools with little, if any, long-term stabilization," citing Hungate et al. (1997) and Lichter et al. (2005). By the time of Jastrow et al.'s writing, however, after many long and arduous experiments had been conducted and their data properly analyzed, the truth was beginning to be seen to be something quite different.
The long-awaited confirmation of the more optimistic view of the subject was firmly established by Jastrow et al. (2005), who described and further analyzed the findings of (1) the first five years of the deciduous forest FACE study that was being conducted at Oak Ridge, Tennessee (Norby et al., 2001), (2) the entire eight years of the prairie grassland open-top chamber study at Manhattan, Kansas (Owensby et al., 1993), and (3) thirty-five other studies of like nature. So what, exactly, did Jastrow et al.'s review of the pertinent literature reveal?
Atmospheric CO2 enrichment to approximately 200 ppm above ambient, in the words of Jastrow et al., "increased C stocks in the forest soil at an average rate of 44 ± 9 g C m-2 yr-1," while "in the prairie, the incremental increase in C stocks corresponded to an average accrual rate of 59 ± 19 g C m-2 yr-1." And why? Because, as they described it, "both systems responded to CO2 enrichment with large increases in root production," and "even though native C stocks were relatively large, over half of the accrued C at both sites was incorporated into micro-aggregates, which protect C and increase its longevity." Likewise, their meta-analysis of the 35 independent experimental observations indicated that CO2 enrichment ranging from 200 to 350 ppm over periods ranging from two to nine years increased soil C over soil depths ranging from 5 to 20 cm by 5.6% (95% CI = 2.8-8.4%), "supporting the generality of the accrual measured in the forest and prairie experiments."
In commenting on their findings, the seven scientists said they "clearly demonstrate that mineral soil C, including micro-aggregate protected pools, can increase measurably in response to a step-function increase in atmospheric CO2 concentrations," and that "the C storage capacities of mineral soils - even those with large organic matter stocks - are not necessarily saturated at present and may be capable of serving as C sinks if inputs increase as a result of passive CO2 fertilization." In addition, they said that "the meta-analysis, which included some multifactor studies and data collected over a wide range of climatic conditions, suggests that soil C accrual ... is likely to be a general response to CO2 enrichment."
This response, in Jastrow et al.'s words, "is not insignificant." In fact, they noted that "if mineral soil C in the surface 20 cm of the world's temperate forests, temperate grasslands, shrublands, and croplands (234 Pg C ... according to Jobbagy and Jackson, 2000) were to increase by 5.6% or at a rate of 19 g C m-2 yr-1, then 8-13 Pg of C might be accumulated within a 10-year period," which suggests that over a period of 180 years the amount of carbon found in the soils of these biomes could possibly be doubled (234 Pg C divided by 1.3 Pg C per year = 180 years).
Viewed in the light of these several observations, it can be readily appreciated that the soil-carbon-sequestering prowess of earth's vegetation can indeed act as a significant brake on the rate-of-rise of the air's CO2 content and thereby help to mute the magnitude of any CO2-induced impetus for global warming.
Several years earlier, at the Sky Oaks CO2 enrichment site of San Diego State University in California, which is located in chaparral vegetation that is dominated by chemise (Adenostoma fasciulatum) shrubs, twelve 2-m by 2-m by 2-m closed chambers were constructed so as to contain a central individual Adenostoma shrub and its surrounding herbaceous plants. Then, beginning in December 1995, the chambers were continuously maintained at six atmospheric CO2 concentrations ranging from 250 to 750 ppm in 100-ppm increments. And subsequently, at various times throughout 1999, measurements of net ecosystem exchange of CO2 were made, while soil samples were collected for analyses of arbuscular mycorrhizal (AM) fungi and sequestered carbon found in both bulk soil and water-stable aggregates.
In describing the results, Treseder et al. (2003) reported that "plants and soils within the chambers took up more carbon under CO2 enrichment." More specifically, they said that the chambers exposed to 250 to 550 ppm CO2 released an average of 703 g C m-2 year-1, while the chambers in the 650-750-ppm treatments absorbed an average of 160 g C m-2 year-1. Likely driven by these dramatic CO2-induced differences in net ecosystem exchange of CO2, it was not surprising, as they described it, that "pools of total carbon in bulk soil and in water-stable aggregates increased 1.5- and three-fold, respectively, between the 250- and 650-ppm treatments." In addition, they found that "the abundance of live AM hyphae and spores rose markedly over the same range of CO2." And thus it was that Treseder et al. concluded that the augmentation of the carbon pools found in their study, "if common in other ecosystems, appears substantial enough to influence sequestration of CO2 originating from fossil fuel burning and deforestation." And, of course, the nature of that "influence" would be to greatly increase soil carbon sequestration.
In another study from the same location that utilized the identical experimental chambers, Allen et al. (2005) assessed the various ways by which carbon entered the soil and was sequestered there. This work revealed that the "total allocation of carbon to soil increased significantly through the study period with elevated CO2," as did "new carbon inputs into macro-aggregates." This latter observation was very important, as these aggregates, in their words, "have increasing concentrations of glomalin, a glycoprotein produced by arbuscular mycorrhizal fungi (Rillig et al., 1999)," which substance acts to create and stabilize soil aggregates and protect the carbon they contain. In addition, they said that CO2 effects on soil bacteria "were not detectable." In fact, they reported that microbial mass was actually "negatively affected by increasing CO2," noting that "under extended nitrogen limitation the plants ultimately garner the nitrogen," and that the plants "ultimately outcompete microbes for these scarce soil resources," citing Hu et al. (2001).
In concluding their discussion of their findings, Allen et al. remarked that "undisturbed arid shrublands may not fix comparatively large amounts of carbon, but they may sequester a large fraction of that carbon." Noting that "carbon allocated to arbuscular mycorrhizal fungi forms a large part of the macro-aggregate structure in the form of glomalin (Rillig et al., 2002)," and that those aggregates "may be protected from decomposition," they thus concluded that the enhanced formation of such aggregates in CO2-enriched air forms "an important [carbon] sequestration pathway" in chaparral ecosystems.
One year later, while working in the Mojave Desert at the Free-Air CO2 Enrichment facility located near Mercury, Nevada, USA, where various shrubs and perennial grasses grow, Billings and Schaeffer (2004) examined the effects of atmospheric CO2 enrichment (to 550 ppm throughout each growing season since April 1997) on soil nitrogen (N) dynamics via measurements of foliage C and N contents and isotope composition, and by measuring resin-available N and rates of soil respiration in the field in conjunction with assessments of potential C evolution and net N mineralization derived from long-term soil incubations. This work revealed that "effects of elevated CO2 on soil C and N dynamics are variable and complex, with many competing processes," and they noted that "changes in soil microbial activity with elevated CO2 ... could affect both mineralizing and immobilizing microbial processes." Nevertheless, the bulk of their observations suggested that "elevated CO2 may increase root and/or soil microbial activity," which more often than not "can result in periodic increases in resin-available N, particularly when soil moisture is available," and they said that these several interrelated phenomena "may translate into more plant available N at these times." In the simplest of terms, the two researchers thus concluded that "if increases in plant-available N are maintained, particularly when soil moisture is available, arid ecosystems may be able to sustain any increases in productivity induced by elevated CO2."
In studying the potential for the long-term storage of carbon in earth's soils, Lagomarsino et al. (2006) noted that an increase of labile carbon below ground, such as is typically provided by atmospheric CO2 enrichment, "could induce two mechanisms acting in opposite ways: (1) an enhanced soil organic matter decomposition due to the stimulation of microbial activity through the so-called priming effect (Kuzyakov et al., 2000); and (2) a retarded mineralization of native soil organic carbon due to the preference of microbes for easily decomposable substrates (Cardon et al., 2001)." So which mechanism is likely the stronger of the two?
In a study designed to broach this question at the POPFACE experimental plantation in central Italy, where clones of Populus alba, Populus nigra and Populus x euramericana had been grown since 1999 with nitrogen fertilization throughout the 2002-2004 growing seasons, Lagomarsino et al. conducted a number of physical and chemical analyses of soils that they sampled in June and October of 2004. This work revealed that Hoosbeek et al. (2004) did indeed observe a priming effect of the newly incorporated litter in the first rotation cycle of trees exposed to air containing an approximate 50% increase in atmospheric CO2 concentration, but that in the second rotation cycle Hoosbeek et al. (2006) observed an accumulation of carbon in the soil of that treatment. In harmony with this latter observation, Lagomarsino et al.'s 2004 data revealed no increase in carbon mineralization activity under elevated CO2, but rather a decrease of microbial basal respiration in the non-rhizospheric soil of the CO2-enriched treatment. And noting that "microbial carbon immobilization was the dominant process under elevated CO2, limiting the carbon losses from soil," Lagomarsino et al. concluded that their results suggested "a possible positive trend for carbon storage on the long term, independent of soil nitrogen availability."
Moving ahead another year, Cheng et al. (2007) conducted a two-year free-air CO2 enrichment (FACE) study of sorghum (Sorghum bicolor (L.) Moench) near Phoenix, Arizona (USA), where they studied the dynamics of soil organic carbon (SOC) pools comprised of labile and recalcitrant SOC of short and long mean residence time (MRT), respectively, under Control conditions (360 ppm CO2) and FACE conditions (560 ppm CO2), together with water-adequate (wet) and water-deficient (dry) treatments. In doing so, they noted that because soils typically contain large amounts of carbon compared to what they sequester annually, it is difficult to measure changes in total SOC content over periods of only a few years; and their study proved no exception to this general rule, as no significant differences in total SOC could be detected between the Control and FACE treatments over the two years of the sorghum experiment.
Nevertheless, much was learned by other means, such as stable-carbon isotopic (δ13C) tracing, which revealed that 53% of the final SOC in the FACE plot was in the recalcitrant or long MRT carbon pool and 47% in the labile or short MRT pool, whereas in the Control plot 46% and 54% of the final SOC was in the recalcitrant and labile pools, respectively, indicating, in the words of the ten researchers, that "elevated CO2 transferred more SOC into the slow-decay carbon pool." In addition, they reported that "isotopic mixing models revealed that increased new sorghum residue input to the recalcitrant pool mainly accounts for this change, especially for the upper soil horizon (0-30 cm) where new carbon in recalcitrant soil pools of FACE wet and dry treatments was 1.7 and 2.8 times as large as that in respective Control recalcitrant pools." In addition, Cheng et al. stated that "old C in the recalcitrant pool under elevated CO2 was higher than that under ambient CO2, indicating that elevated CO2 reduces the decay of the old C in [the] recalcitrant pool."
Therefore, because "higher recalcitrant C content and lower labile C content in the soils were detected under elevated CO2 relative to ambient CO2 treatments, suggesting that SOC under elevated CO2 becomes more stable against chemical and biological degradation," the ten scientists said their results implied that terrestrial agro-ecosystems may play a critical role in sequestering CO2 under future atmospheric conditions.
In a significant contemporary study, Bockheim (2007) explained that "cryoturbation is a dominant process in permafrost regions and refers collectively to all soil movements due to frost action," while additionally reporting that several prior studies had suggested that cryoturbation "was particularly active during mid-Holocene warming periods in the arctic." And from that observation came the key question addressed in this paper. "What effect will sustained warming have on redistribution of soil organic carbon, and will this redistribution exacerbate or mitigate release of CO2 to the atmosphere?"
Bockheim's study focused on the amount of soil organic carbon (SOC) that had been incorporated via cryoturbation into the active layer and near-surface permafrost of 21 sites in northern Alaska, 10 of which were located in the Arctic Coastal Plain and 11 of which were in the Arctic Foothills. There, the University of Wisconsin researcher determined that, based on data acquired from the 21 sites, "55% of the SOC density of the active layer and near-surface permafrost could be attributed to redistribution from cryoturbation," while listing "five lines of evidence suggesting that increased cryoturbation from arctic warming will result in increased storage of SOC."
First, Bockheim stated that "once cryoturbation has moved SOC to the cold, deeper soil layers, little or no biological decomposition will take place." Second, "major organic horizons that are cryoturbated ... are 10 to 50% more dense than the equivalent uncryoturbated horizons," and "low-density SOC may be more susceptible to decomposition than high-density SOC." Third, "low-molecular-weight neutrally charged organic compounds are more biodegradable than high-molecular fractions." Fourth, "Kaiser et al. (2007) reported lower decomposition rates of redistributed SOC in Siberian subsoils than in equivalent material collected from the surface." And fifth, "mechanistic models (Waelbroeck et al., 1997) predict that sustained arctic warming will result in permafrost thawing and a delayed long-lasting increase in SOC storage."
In the concluding words of Bockheim, "these results suggest that continued warming of the arctic may accelerate cryoturbation," and that "this, in turn, will increase the incorporation of dense, high-molecular-weight SOC at depth, thereby enabling the soil to store more SOC than at present and reducing the loss of CO2 to the atmosphere from soil respiration," which is essentially just the opposite of what climate alarmists usually contend.
In another study from the same year, Pendall and King (2007) conducted a series of long-term (170-330 days) laboratory incubation experiments to examine changes in soil organic matter pool sizes and turnover rates in soil collected from an open-top chamber (OTC) atmospheric CO2 enrichment study in the shortgrass steppe of northeastern Colorado, USA, where the air in the ambient CO2 chambers (ACs) and elevated CO2 chambers (ECs) had atmospheric CO2 concentrations of 360 and 720 ppm, respectively, and where this degree of CO2 enrichment enhanced both above- and below-ground plant growth by 15-35%. In so doing, they also discovered that "active pool carbon increased in EC relative to AC treatments systematically over the first 3 years of exposure to elevated CO2 in topsoils and to a lesser degree in subsoils," and they remarked that "these results are consistent with independent results from the same OTC study showing that rhizo-deposition rates doubled and root production increased under elevated CO2." In addition, they determined that "new carbon turnover was not enhanced by elevated CO2," confirming that "new carbon inputs under elevated CO2 are not simply lost to mineralization" and that "pool sizes may continue to increase under elevated CO2." Thus, the two researchers concluded that "these results suggest that soil carbon storage may increase in semi-arid grasslands under elevated CO2," and they opined that this phenomenon, in turn, would tend to mitigate the degree of global warming thought by many to accompany increases in the air's CO2 content.
Moving ahead a year, and working with undisturbed soil cores with and without visible wheat residues that were extracted at the conclusion of the third year of a mini-FACE (free-air CO2 enrichment) experiment conducted in a field planted annually to spring wheat (Triticum aestivum L.) near Hohenheim, Germany, Marhan et al. (2008) studied the effect of elevated atmospheric CO2 concentration (an extra 160 ppm) on the decomposition of the wheat residues present in the soil by measuring CO2 evolution from the cores, as well as the leaching of inorganic and organic carbon from them, during 191 days of core incubations in the laboratory. This work revealed that cumulative residue decomposition was not affected by elevated CO2 when no wheat residues were visible in the cores. When such residues were visible, however, decomposition was found to be "significantly lower" (by 19%) in the elevated compared to the ambient CO2 treatment, which for the more common 300-ppm degree of atmospheric CO2 enrichment roughly translates to a decomposition reduction of 36%. In addition, they found that more dissolved inorganic carbon (DIC) was leached from the elevated CO2 treatment cores, both with and without visible plant residues, than from similar cores from the ambient CO2 treatment (47.2% and 29.5%, respectively, for their degree of CO2 enrichment, which equates to about 88% and 55%, respectively, for a 300-ppm increase in atmospheric CO2 concentration); and they said that these extra amounts of DIC represented "an additional possible mechanism for carbon sequestration in soils of arable cropping systems under future elevated CO2 concentrations." Finally, on top of everything else, they reported that stubble and root biomass tended to be higher by 12.0 and 9.44%, respectively, in soil cores taken from the elevated CO2 plots at the end of the study, which equates to approximate stubble and root biomass enhancements of 22% and 18%, respectively, for a 300-ppm increase in atmospheric CO2 concentration. With respect to the potential for enhanced carbon sequestration in wheat (and other cereal-crop) fields in a CO2-enriched world of the future, therefore, the six scientists concluded that "increased input of plant residues and reduced decomposition of plant-derived carbon" are, indeed, "possible mechanisms for enhanced carbon sequestration under elevated atmospheric CO2 concentration."
One year later, Hopkins et al. (2009) wrote that there were "two sets of long-term experimental plots which have been under constant and known management for over a century and for which historical data exist that allow comparison over recent decades to determine what, if any, changes in SOC have occurred." These unique plots were the Palace Leas Meadow Hay Plots in northeast England, which were established in 1897, and the plots of the Park Grass Continuous Hay Experiment established in 1856 at Rothamsted in southeast England. And in studying them, Hopkins et al. determined "there were no significant differences between 1982 and 2006 for the Palace Leas plots or between 1959 and 2002 for the Park Grass plots," which led them to conclude that "there has been no consistent decrease in SOC stocks in surface soils under old, permanent grassland in England in recent decades, even though meteorological records for both sites indicate significant warming of the soil and air between 1980 and 2000." And as for why they found this to be the case, they speculated that "the lack of a consistent decline in SOC content linked to increased soil temperature since 1980 may be due to a compensatory increase in primary production," citing the work of Jenkinson et al. (1991), which is something that would logically be expected to occur in a CO2-accreting atmosphere.
Contemporaneously, Martens et al. (2009) wrote that "the generally higher above and belowground productivity of C3 plants under elevated CO2 leads to the conclusion that more rhizo-depositions (roots and exudates) are transferred into soils, potentially increasing soil carbon content," but they indicated that most free-air CO2-enrichment (FACE) and outdoor chamber studies have failed to detect significant changes in soil organic carbon (SOC) due to the typically large amount and spatially heterogeneous nature of pre-existing SOC. Thus, in an attempt to overcome these difficulties, Martens et al. cultivated well watered and fertilized spring wheat (cv. Minaret) within stainless steel cylinders forced into the soil of control and free-air CO2-enriched (to 180 ppm above ambient) FACE plots at the experimental farm of the Federal Research Institute in Braunschweig, Germany, where between stem elongation and beginning of ripening the plants were repeatedly pulse-labeled with 14CO2 and thereafter monitored daily for soil-borne total CO2 and 14CO2 until harvest, after which the distribution of 14C was analyzed in all plant parts, soil, soil mineral fractions and soil microbial biomass.
This work revealed, in the words of the four researchers, that "in comparison to ambient conditions, 28% more 14CO2 and 12% more total CO2 was evolved from soil under elevated CO2," and that "in the root-free soil 27% more residual 14C was found in the free-air CO2-enriched soil than in the soil from the ambient treatment." In addition, they said that in soil samples from both treatments about 80% of residual 14C was "integrated into the stable, clay bound soil organic matter pool," which suggests, in their words, that "under FACE conditions a considerable contribution was made to the long-term storage of soil carbon in this soil." And so it was that Martens et al. were able to "show for the first time," as they described it, "that a crop plant grown under FACE conditions deposited significantly more carbon to soil than those grown under ambient CO2 in the field," and that "the additional carbon input under elevated CO2 did not induce an accelerated degradation of pre-existing soil organic matter (no positive priming effect)," thereby demonstrating that "wheat plants grown under elevated CO2 can contribute to an additional net carbon gain in soils," which is especially good news for the biosphere.
Drawing one year closer to the present, Springsteen et al. (2010) wrote that "woody plant expansion within grassland ecosystems is a worldwide phenomenon, and dramatic vegetation shifts from grassland to savanna/woodlands have occurred over the past 50-100 years in North America," while noting that one of the chief factors that has contributed to this phenomenon is believed by many to have been the concomitant historical increase in the air's carbon dioxide concentration, as suggested in the studies of Archer et al. (1995), Polley (1997), Bond and Midgley (2000) and Bond et al. (2003). They also indicated that once shrublands are established, they tend to persist for a number of possible reasons, one of which is a type of feedback phenomenon referred to as islands of fertility, which "occurs when resources accumulate in soils beneath woody plants due to litterfall, interception of wet and dry deposition, nitrogen fixation, and animal droppings," as described by Schlesinger et al. (1990), Archer et al. (1995), Reynolds et al. (1999) and Lopez-Pintor et al. (2006). And they report, in this regard, that "changes in soil attributes under woody vegetation have been documented in the arid grasslands of the southern Great Plains, including increases in soil carbon and nitrogen," citing the work of Reynolds et al. (1999), Hibbard et al. (2001, 2003), McCulley et al. (2004), Schade and Hobbie (2005) and Liao et al. (2006).
But getting around to their work at the USDA-ARS Northern Great Plains Research Laboratory near Mandan, North Dakota (USA), Springsteen et al. examined near-surface (upper 15 cm) soil biogeochemistry along a 42-year (1963-2005) chrono-sequence, which encompassed grassland, woodland, and grassland-woodland transition zones in a northern Great Plains grassland, in order to determine the influence of woody plant expansion on soil carbon and nitrogen contents. And as a result of what they learned, the four researchers were able to report that total soil carbon content rose by 26% across the chrono-sequence from grassland to woodland within the 0-15 cm soil depth, while total soil nitrogen content rose by 31%. And they added that the rate of woody shrub expansion from 1963 to 1988 (25 years) was ~1,800 m2 per year at their study site, while from 1988 to 2005 (17 years) it was ~3,800 m2 per year, or just a little more than doubled.
And so it is that as ever more experiments of this type are conducted at ever more sites around the world, it is becoming increasingly evident that soil carbon sequestration driven by woody-plant invasions of grasslands (which are driven to a significant degree by the ongoing rise in the air's CO2 content), as well as the increases in soil nitrogen content required to sustain them, are growing ever greater with the passage of time, as the greening of the earth continues.
Concomitantly, Yang et al. (2010) wrote that "soil stores more than twice as much carbon than does vegetation or the atmosphere," citing Schlesinger (1997), while noting that many people believe that "climate warming is likely to accelerate the decomposition of soil organic carbon which could lead to increased carbon release from soils, providing a positive feedback to climate change (Davidson and Janssens, 2006)." But is this view correct?
To find out, Yang et al., "conducted five consecutive regional soil surveys in China's grasslands during 2001-2005 and sampled 981 soil profiles from 327 sites across the northern part of the country," after which they compared their results "with data of 275 soil profiles derived from China's National Soil Inventory during the 1980s." And as a result of this effort, the seven scientists determined that the organic carbon stock in the upper 30 cm of soil in northern China's grasslands "did not show significant association with mean annual temperature," and that "grassland soil organic carbon stock did not change significantly over the past two decades."
In concluding their paper, Yang et al. thus wrote that "it has been often asserted that soil will act as a carbon source because of its sensitivity to global environmental change (e.g., Melillo et al., 2002; Bellamy et al., 2005; Schipper et al., 2007)," but that "in contrast to these previous reports, our results indicate that soil organic carbon stock in northern China's grasslands has not experienced significant changes during the past two decades, despite measureable climate change," i.e., global warming. Hence, there is good reason (i.e., real-world data) to not believe that "climate warming is likely to accelerate the decomposition of soil organic carbon which could lead to increased carbon release from soils, providing a positive feedback to climate change," as some have suggested based on primarily theoretical considerations.
Last of all, Zhou et al. (2013) took advantage of a long-term field experiment with increased temperature and precipitation - which was established in late April 2005 in a semiarid temperate steppe in Duolun County, Inner Mongolia, China - "to investigate the effects of warming, increased precipitation and their interactions on SOC [Soil Organic Carbon] fraction" by quantifying "labile SOC, recalcitrant SOC and stable SOC at 0-10 and 10-20 cm depths." The results of this effort revealed that "neither warming nor increased precipitation affected total SOC and stable SOC at either depth," but that "increased precipitation significantly increased labile SOC at the 0-10 cm depth" and that "warming decreased labile SOC and marginally but significantly increased recalcitrant SOC at the 10-20 cm depth." And they found that there were also "significant interactive effects of warming and increased precipitation on labile SOC and recalcitrant SOC at the 0-10 cm depths."
When all was said and done - and the several pluses and minuses accounted for - Zhou et al. ultimately concluded that "given that the absolute increase of SOC in the recalcitrant SOC pool was much greater than the decrease in labile SOC, and that the mean residence time of recalcitrant SOC is much greater, our results suggest that soil C storage at 10-20 cm depth may increase with increasing temperature in this semiarid grassland," which represents a net negative feedback on predicted global warming, as well as a tremendous benefit for the terrestrial biosphere.
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