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Rising concentrations of CO2 in the atmosphere are likely to stimulate plant growth, leading to increased biomass and soil carbon stocks, thereby lessening the rate of the rise in atmospheric CO2 levels. Van Groenigen et al. (p. 508, published online 24 April) report the results of a meta-analysis and modeling that show that increasing the concentration of atmospheric CO2 also stimulates microbial decomposition of organic carbon in soils, by roughly the same amount that it increases soil organic carbon, leading to lower equilibrium soil carbon inventories and limiting the accumulation of carbon. Thus, soils may not provide as much carbon storage as hoped.
Soils contain the largest pool of terrestrial organic carbon (C) and are a major source of atmospheric carbon dioxide (CO2). Thus, they may play a key role in modulating climate change. Rising atmospheric CO2 is expected to stimulate plant growth and soil C input but may also alter microbial decomposition. The combined effect of these responses on long-term C storage is unclear. Combining meta-analysis with data assimilation, we show that atmospheric CO2 enrichment stimulates both the input (+19.8%) and the turnover of C in soil (+16.5%). The increase in soil C turnover with rising CO2 leads to lower equilibrium soil C stocks than expected from the rise in soil C input alone, indicating that it is a general mechanism limiting C accumulation in soil.