As the CO2 content of the air increases, nearly all of earth's plants respond favorably by increasing their photosynthetic rates and producing greater amounts of biomass. Invariably, this phenomenon leads to greater inputs of carbon to the soil in the form of roots, root exudates and senesced plant material. Much research has been conducted to determine if these biological inputs will increase the carbon sequestering abilities of soils, particularly if air temperatures rise, which has been predicted by some to enhance plant litter decomposition rates to the point that global warming may be exacerbated. Thus, we here summarize recent research results that have addressed these important questions.
To begin, it is important to note that atmospheric CO2 enrichment typically reduces, or does not affect, decomposition rates of senesced plant materials present in soils (see our Subject Index Summary on Decomposition). This simple phenomenon, or non-phenomenon, often leads to 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 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 of how these principles work 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 24% less than in the ambient-treatment plots. Consequently, the authors 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."
These types of results have also been reported for woody plants, which possess the ability to store more carbon in their associated soils than do grasses (Gill and Burke, 1999). Pregitzer et al. (2000), for example, 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 authors say 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." The authors thus concluded 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." 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. Moreover, 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 led the authors 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." Similarly, 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%.
In conclusion, it is clear that atmospheric CO2 enrichment increases plant growth and plant-mediated carbon inputs to soils; and the resulting increased soil carbon contents will likely not be reduced if air temperatures rise (in fact, it is possible they may even be enhanced), allowing soil carbon sequestration to increase with increasing atmospheric CO2 concentration. Finally, with more carbon in soils, soil structure and fertility should be improved, providing a positive feedback that further enhances plant growth and soil carbon sequestration.
References
Casella, E. and Soussana, J.-F. 1997. Long-term effects of CO2 enrichment and temperature increase on the carbon balance of a temperate grass sward. Journal of Experimental Botany 48: 1309-1321.
Cotrufo, M.F. and Gorissen, A. 1997. Elevated CO2 enhances below-ground C allocation in three perennial grass species at different levels of N availability. New Phytologist 137: 421-431.
De Angelis, P., Chigwerewe, K.S. and Mugnozza, G.E.S. 2000. Litter quality and decomposition in a CO2-enriched Mediterranean forest ecosystem. Plant and Soil 224: 31-41.
Fitter, A.H., Self, G.K., Brown, T.K., Bogie, D.S., Graves, J.D., Benham, D. and Ineson, P. 1999. Root production and turnover in an upland grassland subjected to artificial soil warming respond to radiation flux and nutrients, not temperature. Oecologia 120: 575-581.
Gill, R.A. and Burke, I.C. 1999. Ecosystem consequences of plant life form changes at three sites in the semiarid United States. Oecologia 121: 551-563.
Henning, F.P., Wood, C.W., Rogers, H.H., Runion, G.B. and Prior, S.A. 1996. Composition and decomposition of soybean and sorghum tissues grown under elevated atmospheric carbon dioxide. Journal of Environmental Quality 25: 822-827.
Insam, H., Baath, E., Berreck, M., Frostegard, A., Gerzabek, M.H., Kraft, A., Schinner, F., Schweiger, P. and Tschuggnall, G. 1999. Responses of the soil microbiota to elevated CO2 in an artificial tropical ecosystem. Journal of Microbiological Methods 36: 45-54.
Islam, K.R., Mulchi, C.L. and Ali, A.A. 1999. Tropospheric carbon dioxide or ozone enrichments and moisture effects on soil organic carbon quality. Journal of Environmental Quality 28: 1629-1636.
Johnson, L.C., Shaver, G.R., Cades, D.H., Rastetter, E., Nadelhoffer, K., Giblin, A., Laundre, J. and Stanley, A. 2000. Plant carbon-nutrient interactions control CO2 exchange in Alaskan wet sedge tundra ecosystems. Ecology 81: 453-469.
Leavitt, S.W., Paul, E.A., Kimball, B.A., Hendrey, G.R., Mauney, J.R., Rauschkolb, R., Rogers, H., Lewin, K.F., Nagy, J., Pinter Jr., P.J. and Johnson, H.B. 1994. Carbon isotope dynamics of free-air CO2-enriched cotton and soils. Agricultural and Forest Meteorology 70: 87-101.
Liski, J., Ilvesniemi, H., Makela, A. and Westman, C.J. 1999. CO2 emissions from soil in response to climatic warming are overestimated - The decomposition of old soil organic matter is tolerant of temperature. Ambio 28: 171-174.
Nitschelm, J.J., Luscher, A., Hartwig, U.A. and van Kessel, C. 1997. Using stable isotopes to determine soil carbon input differences under ambient and elevated atmospheric CO2 conditions. Global Change Biology 3: 411-416.
Pregitzer, K.S., Zak, D.R., Maziaasz, J., DeForest, J., Curtis, P.S. and Lussenhop, J. 2000. Interactive effects of atmospheric CO2 and soil-N availability on fine roots of Populus tremuloides. Ecological Applications 10: 18-33.
Rosenzweig, C. and Hillel, D. 2000. Soils and global climate change: Challenges and opportunities. Soil Science 165: 47-56.
Rouhier, H. and Read, D. 1999. Plant and fungal responses to elevated atmospheric CO2 in mycorrhizal seedlings of Betula pendula. Environmental and Experimental Botany 42: 231-241.
van Ginkel, J.H. and Gorissen, A. 1998. In situ decomposition of grass roots as affected by elevated atmospheric carbon dioxide. Soil Science Society of America Journal 62 : 951-958.
van Ginkel, J.H., Whitmore, A.P. and Gorissen, A. 1999. Lolium perenne grasslands may function as a sink for atmospheric carbon dioxide. Journal of Environmental Quality 28: 1580-1584.