Dominant role of nitrogen stoichiometric flexibility in ecosystem carbon storage under elevated CO₂

Interactions between the carbon (C) and nitrogen (N) cycles can impact on the sensitivity of terrestrial C storage to elevated atmospheric carbon dioxide (CO₂) concentrations (eCO₂). However, the underlying mechanisms associated with CN interactions that influence terrestrial ecosystem C sequestration (C_seq) remains unclear. Here, we quantitatively analyzed published C and N responses to experimentally eCO₂ 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 eCO₂. Our results showed that eCO₂ 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_seq under eCO₂, although the relative contributions differed across ecosystem types, with changes in the C:N ratio contributing 50% of the increment in forests C_seq, while the total N change contributed 60% of the increment in grassland C_seq. In terms of temporal variation in the relative importance of each of these three mechanisms to ecosystem C_seq: changes in the C:N ratio was the most important mechanism during the early years (~5 years) of eCO₂ treatment, whilst the contribution to ecosystem C_seq 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_seq, which may offer important implications for future predictions of the C cycle in terrestrial ecosystems subjected to global change.