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 (eCO(2)). However, the underlying mechanisms associated with C-N interactions that influence terrestrial ecosystemC sequestration (C-seq) remains unclear. Here, we quantitatively analyzed published C and N responses to experimentally eCO(2) 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(2). Our results showed that eCO(2) 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 Cseq under eCO(2), 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 mechanismduring the early years (similar to 5 years) of eCO(2) treatment, whilst the contribution to ecosystem C-seq byN 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. (C) 2020 Elsevier B.V. All rights reserved.