A call to investigate drivers of soil organic matter retention vs. mineralization in a high CO2 world

Understanding how elevated atmospheric CO2 alters the formation and decomposition of soil organic carbon (SOC) is important but challenging. If elevated CO2 induces even small changes in rates of formation or decay of SOC, there could be substantial feedbacks on the atmosphereʹs concentration of CO2. However, the long turnover times of many SOC pools – decades to centuries – make the detection of changes in the soilʹs pool size difficult. Long-term CO2 enrichment experiments have offered unprecedented opportunities to explore these issues in intact ecosystems for more than a decade. Increased NPP with elevated CO2 has prompted the hypothesis that SOC may increase at the same time that increased vegetation nitrogen (N) uptake and accumulation indicates probable declines in SON. Varying investigators thus have hypothesized that SOC will increase and SON will decline to explain increased NPP with elevated CO2; researchers also invoke biogeochemical theory and stoichiometric constraints to argue for strong limitations on the co-occurrence of these phenomena. We call for researchers to investigate two broad research questions to elucidate the drivers of these processes. First, we ask how elevated CO2 influences compound structure and stoichiometry of that proportion of NPP retained by soil profiles for relatively long time periods. We also call for investigations of the mechanisms underlying the decomposition of mineralizable organic matter with elevated CO2. Specifically, we need to understand how elevated CO2 influences microbial priming (driven by enhanced microbial energy needs associated with increases in biomass or activity) and microbial mining of N (driven by enhanced microbial N demand associated with greater vegetative N uptake), two processes that necessarily will be constrained by the stoichiometry of both substrates and microbial demands. Applying technologies such as nuclear magnetic resonance and the detection of biomarkers that reveal organic matter structure and origins, and studying microbial stoichiometric constraints, will dramatically improve our ability to predict future patterns of ecosystem C and N cycling.