Nitrogen cycle responses to elevated CO2 depend on ecosystem nutrient status

Nitrogen (N) limitation of terrestrial ecosystems is a crucial factor for predicting how these ecosystems respond and feedback to climate change. Nitrogen availability for plants in terrestrial ecosystems depends on the internal soil N cycle and inputs to the ecosystem via biological N-2 fixation. We reviewed the effect of elevated atmospheric CO2 concentrations (eCO(2)) on gross soil N transformations to advance our understanding of ecosystem responses to eCO(2). Overall, neither gross mineralization nor gross nitrification was altered by eCO(2). However, emerging from ecosystem specific analysis, we propose a new conceptual model for eCO(2) effects on gross mineralization based on ecosystem nutrient status: gross mineralization is only stimulated in N limited ecosystems, but unaffected in phosphorus limited ecosystems. Moreover, the ratio of ammonium oxidation to immobilization is decreased under eCO(2), indicating a tighter N cycle with reduced ecosystem N losses. This new conceptual model on N cycle responses to eCO(2) should be tested in the future in independent experiments and it provides a new concept for refining mechanistic models of ecosystem responses to climate change.