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Elevated CO2 May Slow Plant Decomposition Rates, Increasing Soil Carbon Storage
In a previous essay, we indicated that contrary to the long-held assumption that global warming would increase soil respiration rates and reduce soil carbon storage, thereby adding to the growing burden of atmospheric CO2, elevated temperatures may actually enhance soil carbon storage, thereby slowing the rate-of-rise of the air's CO2 content.  We here describe a second natural phenomenon that does much the same thing.

As plants grow and develop, they shed various organs (lose their leaves or drop their fruit, for example) at different stages of their life cycles, ultimately leaving behind all of their remaining biomass upon their death.  This litter, which was constructed from CO2 acquired during photosynthesis, is then subjected to the process of decomposition, which returns some of its carbon back to the atmosphere, once again in the form of CO2.

At first glance, it might appear that the process of decomposition would have little net impact on terrestrial carbon sequestration.  However, in reviewing the published scientific literature on the topic, we find that litter from plants grown at elevated CO2 concentrations often decomposes at a slower rate, or to a lesser degree, than litter from plants grown at the air's current CO2 concentration.  This phenomenon results in greater carbon retention times within decaying litter; and it provides greater time for more of the litter's carbon to become incorporated into more stable compounds that can be sequestered for longer periods of time within soils.  And, of course, it leaves a greater amount of carbon to be thus sequestered.

Nitschelm et al. (1997), for example, studied root decomposition rates in large plots of white clover, observing that a 250 ppm increase in the air's CO2 content reduced decomposition rates by 24%.  Similarly, atmospheric CO2 enrichment significantly reduced litter decomposition rates in an alpine grassland species (Hirschel et al., 1997), in seedlings of yellow poplar (Scherzel et al., 1998), and in sorghum and soybeans (Tobert et al., 1998).  In addition, in studying litter decomposition rates in Lolium perenne grasslands, Van Ginkel et al. (1996) determined that root decomposition rates were 19 and 14% slower at atmospheric CO2 concentrations of 700 ppm than they were at ambient CO2 concentrations after one and two years of treatment exposure, respectively.  Likewise, Van Ginkel and Gorissen (1998) grew this same perennial ryegrass at 700 ppm CO2 and noted a 42% increase in both root and soil microbial biomass, while root decomposition rates dropped by 13% relative to those measured at 350 ppm CO2.

More recently, Van Ginkel et al. (1999) used their earlier experimental results to test whether or not global warming and atmospheric CO2 enrichment, acting in unison, would amplify plant residue decomposition rates in Lolium perenne grasslands and lead to a net loss of carbon from them.  Their results indicate that the addition of global warming will not increase plant residue decomposition rates enough to turn such ecosystems into carbon sources, as opposed to the sinks they are currently; for CO2-induced increases in plant growth and CO2-induced decreases in plant decomposition rates "are more than sufficient to counteract the positive feedback caused by an increase in temperature."

Not all studies have indicated that atmospheric CO2 enrichment will reduce litter decomposition rates, however, as demonstrated by the experiments of Dukes and Field (2000) on native California grassland species, Hirschel et al. (1997) on plants from lowland calcareous grasslands and moist tropical forests, Scherzel et al. (1998) on eastern white pine, and Henning et al. (1996) on soybean and sorghum.  In fact, in an analysis of several dozen such studies, Norby et al. (2001) concluded that elevated CO2 had no consistent effect on leaf litter decomposition rate.  Even in the face of no net change in litter decomposition, however, more carbon will still be sequestered in soils at higher atmospheric CO2 concentrations, since the aerial fertilization effect of elevated CO2 will lead to the production of more plant biomass; and an unchanged rate of decomposition will thus still result in more carbon eventually being retained in the soil under these conditions.

In summation, it is clear from experimental results described in the scientific literature that as the air's CO2 content continues to rise, earth's vegetation will likely respond with increasing photosynthetic rates and biomass production.  As a consequence of these phenomena, more plant litter will be returned to the soil where it should be retained for longer periods of time, as elevated atmospheric CO2 concentrations tend to decrease or, at the very minimum, maintain current rates of litter decomposition.  Thus, the carbon sequestering abilities of earth's natural ecosystems should increase in tandem with the CO2 content of the atmosphere; and they should do so even in the face of any global warming that might occur concurrently.

Dr. Craig D. Idso Dr. Keith E. Idso

References
Dukes, J.S. and Field, C.B.  2000.  Diverse mechanisms for CO2 effects on grassland litter decomposition.  Global Change Biology 6: 145-154.

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.

Hirschel, G., Korner, C. and Arnone III, J.A.  1997.  Will rising atmospheric CO2 affect leaf litter quality and in situ decomposition rates in native plant communities?  Oecologia 110: 387-392.

Nitschelm, J.J., Luscher, A., Hatrwig, 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.

Norby, R.J., Cotrufo, M.F., Ineson, P., O'Neill, E.G. and Canadell, J.G.  2001.  Elevated CO2, litter chemistry, and decomposition: a synthesis.  Oecologia 127: 153-165.

Scherzel, A.J., Rebbeck, J. and Boerner, R.E.J.  1998.  Foliar nitrogen dynamics and decomposition of yellow-poplar and eastern white pine during four seasons of exposure to elevated ozone and carbon dioxide.  Forest Ecology and Management 109: 355-366.

Torbert, H.A., Prior, S.A., Rogers, H.H. and Runion, G.B.  1998.  Crop residue decomposition as affected by growth under elevated atmospheric CO2Soil Science 163: 412-419.

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., Gorissen, A. and van Veen, J.A.  1996.  Long-term decomposition of grass roots as affected by elevated atmospheric carbon dioxide.  Journal of Environmental Quality 25: 1122-1128.

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.