Nitrogen cycle responses to elevated CO2 depend on ecosystem nutrient status
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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 N2 fixation. We reviewed the effect of elevated atmospheric CO2 concentrations (eCO2) on gross soil N transformations to advance our understanding of ecosystem responses to eCO2. Overall, neither gross mineralization nor gross nitrification was altered by eCO2. However, emerging from ecosystem specific analysis, we propose a new conceptual model for eCO2 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 eCO2, indicating a tighter N cycle with reduced ecosystem N losses. This new conceptual model on N cycle responses to eCO2 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.
KeywordsElevated CO2 Climate change Nitrogen cycle Gross rates Nutrient limitation Progressive nitrogen limitation Phosphorus limitation
Prof. L. Klemedtsson for valuable discussion; financially support by the strategic research area BECC (Biodiversity and Ecosystem services in a Changing Climate, www.becc.lu.se) and the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS).
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