Dominant role of nitrogen stoichiometric flexibility in ecosystem carbon storage under elevated CO.

Interactions between the carbon (C) and nitrogen (N) cycles can impact on the sensitivity of terrestrial C storage to elevated atmospheric carbon dioxide (CO) concentrations (eCO). However, the underlying mechanisms associated with CN interactions that influence terrestrial ecosystem C sequestration (C) remains unclear. Here, we quantitatively analyzed published C and N responses to experimentally eCO using a meta-analysis approach. We determined the relative importance of three principal mechanisms (changes in the total ecosystem N amount, redistribution of N between plant and soil pools, and flexibility of the C:N ratio) that contribute to increases in ecosystem C storage in response to eCO. Our results showed that eCO increased C and N accumulation, resulted in higher C:N ratios in plant, litter, and soil pools and induced a net shift of N from soils to vegetation. These three mechanisms largely explained the increment of ecosystem C under eCO, although the relative contributions differed across ecosystem types, with changes in the C:N ratio contributing 50% of the increment in forests C, while the total N change contributed 60% of the increment in grassland C. In terms of temporal variation in the relative importance of each of these three mechanisms to ecosystem C: changes in the C:N ratio was the most important mechanism during the early years (~5 years) of eCO treatment, whilst the contribution to ecosystem C by N redistribution remained rather small, and the contribution by total N change did not show a clear temporal pattern. This study highlights the differential contributions of the three mechanisms to C, which may offer important implications for future predictions of the C cycle in terrestrial ecosystems subjected to global change.