The effects of nitrogen enrichment on soil CO2 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 (Rs) to N enrichment, which may be affected by the physicochemical properties of soil and environmental factors. Methods We assembled a large dataset for meta-analysis, including 563 datasets on annual and seasonal Rs 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 Rs but we found a negative relationship in forests and a positive one in other biomes. (2) Rs 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 Rs 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 Rs. (4) The estimated total C flux for global terrestrial lands was 97.01 Pg 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 Rs to future N enrichment should be predicted separately for each biome. The association between changes in Rs 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. [ABSTRACT FROM AUTHOR]