The effects of nitrogen enrichment on soil CO 2 fluxes depending on temperature and soil properties

Aim The continuous increase in anthropogenic nitrogen ( N ) may have a substantial impact on soil carbon ( C ) fluxes; thus, understanding the dynamics of soil C fluxes under N enrichment is important. Our main goal was to resolve the conflicting results presented to date and to expand our knowledge about the response of soil respiration ( R s) to N enrichment, which may be affected by the physico‐chemical properties of soil and environmental factors. Methods We assembled a large dataset for meta‐analysis, including 563 datasets on annual and seasonal R s with N enrichment from 154 published papers at 163 sites, covering seven types of biomes world‐wide. Results (1) N enrichment was not significantly related to global R s but we found a negative relationship in forests and a positive one in other biomes. (2) R s showed a negative correlation with the N levels in forests and croplands and a positive correlation with the N levels in deserts; heterotrophic respiration exhibited negative correlations with N levels globally, and its response was correlated with the incubation environment. (3) The response of R s to N enrichment was also correlated with mean annual temperature and soil properties, with 15 ° C being the threshold for switching between increasing and decreasing R s. (4) The estimated total C flux for global terrestrial lands was 97.01 P g C year −1 , and 1 kg of N enrichment at ha −1 year −1 induced an average efflux of 1.33 kg C ha −1 year −1 . Main conclusions The response of R s to future N enrichment should be predicted separately for each biome. The association between changes in R s and temperature and soil properties under N enrichment makes soil C flux a more complex challenge in the context of future increases in temperature and N deposition.