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Soil Water Status (Field Studies) - Summary
CO2-induced global warming has long been predicted to increase evapotranspiration, causing decreases in soil moisture that may offset concomitant increases in continental precipitation and lead to greater aridity in water-limited ecosystems around the world (Manabe and Wetherald, 1986; Rind, 1988; Gleick, 1989; Vlades et al., 1994; Gregory et al., 1997; Komescu et al., 1998).  In this summary we thus review what has been learned about the topic from open-top chamber and FACE studies that have been conducted out-of-doors in real-world field situations.

In an experiment conducted over a single eight-month growing season, Hungate et al. (2002) measured evapotranspiration and soil moisture in mature scrub-oak communities growing within open-top chambers maintained at atmospheric CO2 concentrations of 380 and 700 ppm.  They found that the extra CO2 reduced the mean daily evapotranspiration rate of this ecosystem by 19%, in spite of the fact that 20-40% increases in leaf area index were observed in the CO2-enriched chambers relative to the ambient-air chambers.  In addition, they found that the elevated CO2 significantly increased the soil moisture content of the 3- to 10-cm soil-depth interval.

In another single-season study, Higgens et al. (2002) constructed open-top chambers about portions of an annual grassland located in a Mediterranean-type climate in California, USA.  Continuously fumigating the chambers with air of either 360 or 720 ppm CO2, they observed an 18% increase in soil moisture content in the CO2-enriched chambers at the end of the season.

In a study spanning two growing seasons, Morgan et al. (2001) constructed open-top chambers in a native shortgrass steppe ecosystem in Colorado, USA, and exposed the enclosed plants to atmospheric CO2 concentrations of either 360 or 720 ppm.  They too observed that the plant communities enriched with CO2 tended to have more moisture in their soils than the communities exposed to ambient air; and this phenomenon likely contributed to the less negative and, hence, less stressful plant water potentials that were measured in the plants exposed to the CO2-enriched air.

In another two-year grassland study, Reich et al. (2001) grew 16 perennial species as monocultures within FACE plots maintained at atmospheric CO2 concentrations of 360 and 560 ppm and low and high levels of soil nitrogen, finding that the extra CO2 enhanced soil water contents by 9, 6 and 4% in plots containing C3 grasses, C4 grasses and forbs, respectively.  Likewise, in a two-year study of a horticultural crop, Bunce (2001) grew strawberry plants in the field for two years in open-top chambers maintained at atmospheric CO2 concentrations of 350, 650 and 950 ppm, finding that when soil water potentials were measured during several dry summer days, an increasingly greater amount of soil moisture was indicated for each step increase in the air's CO2 concentration.

Adding artificial warming to the mix of parameters studied, Zavaleta et al. (2003) conducted a two-year study of an annual-dominated California grassland at the Jasper Ridge Biological Preserve near Stanford, California, USA, where they heated a number of FACE plots (enriched with an extra 300 ppm of CO2) with infrared heat lamps that warmed their soil surfaces by 0.8-1.0C.  The researchers found that the individual effects of atmospheric CO2 enrichment and soil warming were of similar magnitude; and acting together they enhanced mean spring soil moisture content by about 15%.  The effect of CO2 was produced primarily as a consequence of its ability to cause partial stomatal closure and thereby reduce season-long plant water loss via transpiration, while in the case of warming there was an acceleration of canopy senescence that further increased soil moisture by reducing the period of time (length of the growing season) over which transpiration losses occur, all without any decrease in total plant production.  Commenting on their findings, Zavaleta et al. say they "illustrate the potential for organism-environment interactions to modify the direction as well as the magnitude of global change effects on ecosystem functioning," specifically stating that "in at least some ecosystems, declines in plant transpiration mediated by changes in phenology can offset direct increases in evaporative water losses under future warming."

Moving up the experiment longevity ladder, in a three-year study stretching from May 1999 to October 2001, Ferretti et al. (2003) investigated plant and soil water relations in a mixed C3/C4 grassland in the western Great Plains region of the USA within a set of open-top chambers maintained at ambient and twice-ambient atmospheric CO2 concentrations.  In addition to documenting a mean plant biomass increase of 50% in the elevated CO2 treatment, they observed significantly wetter soils in the high-CO2 chambers, which they say was "most likely a result of improved soil-water conservation as a result of reduced stomatal conductance under elevated CO2."  Noting that "elevated CO2 had the effect of increasing soil-water conservation as has been previously found (e.g., Morgan et al., 2001; Volk et al., 2000)," and that "reduced evaporation was mainly responsible for greater soil water content under elevated CO2," Ferretti et al. conclude that "the most significant effect of elevated CO2 on the hydrologic budget in water limited ecosystems is likely to be an increase in soil water storage (Jackson et al., 1998)."

At the conclusion of a five-year FACE study of a pasture on the North Island of New Zealand that imposed an atmospheric CO2 enrichment of approximately 100 ppm upon half of the experimental plots, Newton et al. (2003) measured the water repellency of the pasture soil, finding, in their words, that "at field moisture content the repellence of the ambient soil was severe and significantly greater than that of the elevated CO2 soil."

Commenting on the significance of this result, Newton et al. note that water repellency "is a soil property that prevents free water from entering the pores of dry soil (Tillman et al., 1989)," and they report that it "has become recognized as a widespread problem, occurring under a range of vegetation and soil types (agricultural, forestry and amenity; sand, loam, clay, peat and volcanic) (Bachmann et al., 2001) and over a large geographical range (Europe, USA, Asia, Oceania) (Bauters et al., 1998)."  Specifically, they note that water-repellency-induced problems for land managers include "increased losses of pesticides and fertilizers, reduced effectiveness of irrigation, increased rates of erosion, and increased runoff," and they note there are water-repellency-induced problems "in the establishment and growth of crops (Bond, 1972; Crabtree and Gilkes, 1999) and implications for the dynamics of natural ecosystems, particularly those subject to fire (DeBano, 2000)."  Hence, it is clear that the CO2-induced reduction of soil water repellency discovered in this study portends a wide range of very important benefits for both agro- and natural ecosystems as the air's CO2 content continues to rise in the years and decades ahead.

In another five-year study that concentrated on soil properties, Prior et al. (2004) grew plots of soybean and sorghum plants from seed to maturity within open-top chambers maintained at atmospheric CO2 concentrations of either 360 or 720 ppm in a huge outdoor bin filled with soil that had been fallow for more than 25 years prior to the start of their study.  Also, at the end of each growing season, they allowed aboveground non-yield residues (stalks, soybean pod hulls and sorghum chaff), including 10% of the grain yield, to remain on the surfaces of the plots to simulate the effects of no-tillage farming.  Measurements of soil properties made at the beginning of the experiment were then compared with similar measurements made at its conclusion.

So what did Prior et al. find?  They report 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 beneath both crops experienced 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.  Prior et al. say these results "indicate potential for improvements in soil carbon storage, water infiltration and soil water retention, and reduced erosion," which valuable positive consequences they rightly describe as "CO2-induced benefits."

In yet another five-year study, Nelson et al. (2004) used open-top chambers to examine the effects of elevated CO2 (720 vs. 360 ppm) on plant water relations, ecosystem water use efficiency, soil moisture dynamics and root distributions of the semi-arid shortgrass steppe (SGS) of Colorado, USA.  Their work revealed that seasonal soil moisture throughout the soil profile (0-105 cm) was increased under elevated CO2 compared to ambient CO2 for much of the study period, with the greatest relative increase (16.4%) occurring in the 75-105 cm depth increment.  Noting that "increased soil moisture under elevated CO2 at the deepest soil depth suggests that water percolated deeper into the soil profile and that less moisture was lost to evapotranspiration under elevated CO2," they say that "this phenomenon enhances water storage in the deep fine sandy loam soils underlying large portions of the SGS," and they go on to report that "this increase in soil moisture has been shown to be the major controlling factor in improved carbon assimilation rates and increased total aboveground biomass in this system (LeCain et al., 2003) and will likely decrease the susceptibility of the SGS to drought."

Another important finding of Nelson et al.'s study was that leaf water potential was enhanced by 24-30% under elevated CO2 in the major warm- and cool-season grass species of the SGS.  These results are similar to those of studies involving other C3 and C4 grass species (Owensby et al., 1993; Jackson et al., 1994); and Nelson et al. note that the enhanced leaf water potential, "which reflects improved plant water status and increased drought tolerance (Tyree and Alexander, 1993)," may lead to increased leaf turgor and allow the grasses "to continue growth further into periods of drought."  Hence, it is not surprising that when averaged over the five years of their study, plant water-use efficiency was 43% higher in the elevated-CO2 plots than in the ambient-air plots.

In discussing the broader implications of these findings, Nelson et al. say their results "suggest that a future, elevated CO2 environment may result not only in increased plant productivity due to improved water use efficiency, but also lead to increased water drainage and deep soil moisture storage in this semi-arid grassland ecosystem."  And they say that "this, along with the ability of the major grass species to maintain a favorable water status under elevated CO2, should result in the SGS being less susceptible to prolonged periods of drought."

That Nelson et al.'s findings are the norm, and not the exception, is confirmed by their noting that "previous studies have reported increased soil moisture under elevated CO2 in semi-arid C3 annual grasslands in California (Fredeen et al., 1997), mesic C3/C4 perennial tallgrass prairie in Kansas (Owensby et al., 1993, 1999; Ham et al., 1995; Bremer et al., 1996), and mesic C3 perennial grasslands in Switzerland (Niklaus et al., 1998) and Sweden (Sindhoj et al., 2000)."  Hence, we can validly expect the beneficent effects of atmospheric CO2 enrichment revealed in this impressive study to be found in grasslands throughout the world as the air's CO2 content continues to rise to double-and-beyond its current concentration.

But what if air temperature rises concurrently?  Actually, things could get even better under that scenario.  Nelson et al. note, for example, that "air temperature was on average 2.6C higher inside the chambers than outside," and they say that this warming "was implicated in the 36% enhanced biomass production observed in chambered-ambient compared to non-chambered plots."  Consequently, since this already-enhanced biomass production was the starting point from which the 41% increase in biomass elicited by the doubling of the air's CO2 content was calculated, the increase in biomass caused by the concurrent actions of both factors (increasing air temperature and CO2 concentration) could well be something on the order of 90%.

Topping the longevity scale of the pertinent CO2-enrichment experiments we have reviewed is the study of Owensby et al. (1999), who constructed open-top chambers on a pristine tallgrass prairie in Kansas, USA, and fumigated them with ambient and twice-ambient concentrations of atmospheric CO2 for eight consecutive growing seasons.  In every year of this landmark study, the CO2-enriched plots were found to contain more soil moisture than the plots exposed to ambient air, suggesting that CO2-enriched prairie ecosystems should be better able to cope with the adverse consequences of water stress in especially dry years; and in testing this hypothesis, Owensby et al. did indeed find that atmospheric CO2 enrichment significantly increased both above- and belowground biomass in all years of below average rainfall.

Rounding out this Subject Index Summary is the literature review of Morgan et al. (2004), who describe plant gas exchange, biomass and species responses of five native or semi-native temperate and Mediterranean grasslands and three semi-arid ecosystems to atmospheric CO2 enrichment, with an emphasis on water relations.  In the words of this group of 15 scientists, "increasing CO2 led to decreased leaf conductance for water vapor, improved plant water status, altered seasonal evapotranspiration dynamics, and in most cases, periodic increases in soil water content," such that "across the grasslands of the Kansas tallgrass prairie, Colorado shortgrass steppe and Swiss calcareous grassland, increases in aboveground biomass from CO2 enrichment were relatively greater in dry years."  Although they remark that "vegetative and reproductive responses to CO2 were highly varied among species and ecosystems, and did not generally follow any predictable pattern in regard to functional groups," they say that, considered in their entirety, the literature results they reviewed "suggest that the indirect effects of CO2 on plant and soil water relations may contribute substantially to experimentally induced CO2-effects."

In conclusion, it would appear from these many real-world studies that the ongoing rise in the atmosphere's CO2 concentration should be gradually increasing the soil moisture contents of the world's soils, in contrast to the many model-based projections cited in the introductory paragraph of this Summary.  That such is indeed the case is suggested by the findings of Robock et al. (2000), who developed a massive collection of soil moisture data from over 600 stations spread across a variety of climatic regimes, including the former Soviet Union, China, Mongolia, India and the United States.  In analyzing these observations, they determined that "in contrast to predictions of summer desiccation with increasing temperatures, for the stations with the longest records, summer soil moisture in the top 1 m has increased while temperatures have risen," which is truly good news for the biosphere.

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Bond, R.D.  1972.  Germination and yield of barley when grown in a water-repellent sand.  Agronomy Journal 64: 402-403.

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Last updated 9 February 2005