Saturday, 15 July 2006

Responses of Plant Growth and Nutrient Cycling under Elevated CO2: A Meta-Analysis.

Kees Jan van Groenigen1, Marie Anne de Graaff1, Johan Six1, Bruce Hungate2, Nico Van Breemen3, and Chris van Kessel1. (1) Dept of Plant Sciences, Univ of California-Davis, One Shields Avenue, Davis, CA 95616, (2) Dept of Biological Sciences and Merriam-Powell Center for Env. Research, Northern Arizona Univ, PO Box 5640, Building 21, Flagstaff, AZ 86011, (3) Laboratory for Soil Science and Geology, Wageningen Univ, PO Box 37, Wageningen, Netherlands

The current rise in atmospheric CO2, a consequence of human activities such as fossil fuel burning and deforestation, is thought to stimulate plant growth in many ecosystems. If increased C assimilation by plants is translated into increased soil organic carbon (SOC), terrestrial ecosystems might help mitigate rising CO2 emissions. However, higher plant growth rates in a CO2-rich world can only be sustained if the soil supplies plants with additional nutrients. Therefore, the effect of elevated CO2 on nutrient cycling is of key importance when predicting the potential for C storage in plant biomass and soils. The impact of higher CO2 levels on the flow of C and nutrient through ecosystems depends on a set of complex interactions between soil and plants. We therefore need field experiments under realistic conditions to make accurate predictions on the effects of rising atmospheric CO2. Over the last two decades, many Open Top Chamber (OTC) and Free Air Carbon dioxide Enrichment (FACE) experiments have been conducted, covering a wide range of terrestrial ecosystems. Yet, because of spatial variability, the sensitivity of these experiments to detect CO2 induced changes is low. A quantitative integration of results across multiple studies might overcome this problem. Meta-analytic methods enable placing confidence limits around effect sizes; therefore they provide a robust statistical test for overall CO2 effects across multiple studies. Using meta-analysis, we summarized the results of 116 studies on plant biomass production, SOM dynamics and biological N2 fixation in FACE and OTC experiments. Averaged over all studies, elevated CO2 stimulated above- and belowground plant biomass by 20% and 30%, respectively. Despite the stimulation of microbial respiration by 7.5%, elevated CO2 still caused soil C contents to increase by 0.9% per year. These results suggest that the effect of elevated CO2 on soil C input outweighs its effect on microbial decomposition. However, elevated CO2 had a relative strong effect on potential mineralizable and soluble C, suggesting that C sequestration mostly occurred in labile pools with limited storage potential. Stimulation of gross N immobilization at elevated CO2 was significant (+22%), but gross and net N mineralization rates remained unaffected. Combined with a significant increase in microbial N contents (+6%), this suggests that higher CO2 levels enhanced microbial N demand. An increase in competition for available N between soil microbes and plants could potentially limit plant growth and thus soil C input under elevated CO2. Indeed, elevated CO2 only increased soil C contents when N was added at rates exceeding typical N deposition. These results are in line with the Progressive Nitrogen Limitation (PNL) theory, which states that when elevated CO2 stimulates biomass production in unfertilized ecosystems, the resulting increase in soil C inputs will gradually reduce N availability. Some believe that an increase in atmospheric CO2 will stimulate N2 fixation, thereby providing N needed for C sequestration. However, averaged over all experiments, N2 fixation was unresponsive to elevated CO2 unless other essential nutrients were added. Thus, our meta-analysis corroborates the untested hypotheses that increased soil C input and soil C sequestration under elevated CO2 are limited directly by N availability, and indirectly by nutrients needed to support N2 fixation. Together, these findings suggest that the potential for rapid soil C storage under elevated CO2 will be small.

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