Research articleSoil nitrate accumulation explains the nonlinear responses of soil CO2 and CH4 fluxes to nitrogen addition in a temperate needle-broadleaved mixed forest

•The responses of soil CO2 and CH4 fluxes to multi-level nitrogen addition were nonlinear.•The critical levels of N addition for the alteration of soil C fluxes were determined.•Reduced NH4+ and oxidized NO3− had contrasting effects on soil CO2 emission and CH4 uptake.•Soil nitrate accumulation dominates the nonlinear responses of soil C fluxes to N addition.

The responses of soil-atmosphere carbon (C) exchange fluxes to growing atmospheric nitrogen (N) deposition are controversial, leading to large uncertainty in the estimated C sink of global forest ecosystems experiencing substantial N inputs. However, it is challenging to quantify critical load of N input for the alteration of the soil C fluxes, and what factors controlled the changes in soil CO2 and CH4 fluxes under N enrichment. Nine levels of urea addition experiment (0, 10, 20, 40, 60, 80, 100, 120, 140 kg N ha−1 yr−1) were conducted in the needle-broadleaved mixed forest in Changbai Mountain, Northeast China. Soil CO2 and CH4 fluxes were monitored weekly using the static chamber and gas chromatograph technique. Environmental variables (soil temperature and moisture in the 0-10 cm depth) and dissolved N (NH4+-N, NO3−-N, total dissolved N (TDN), and dissolved organic N (DON)) in the organic layer and the 0-10 cm mineral soil layer were simultaneously measured. High rates of N addition (≥60 kg N ha−1 yr−1) significantly increased soil NO3−-N contents in the organic layer and the mineral layer by 120%-180% and 56.4%-84.6%, respectively. However, N application did not lead to a significant accumulation of soil NH4+-N contents in the two soil layers except for a few treatments. N addition at a low rate of 10 kg N ha−1 yr−1 significantly stimulated, whereas high rate of N addition (140 kg N ha−1 yr−1) significantly inhibited soil CO2 emission and CH4 uptake. Significant negative relationships were observed between changes in soil CO2 emission and CH4 uptake and changes in soil NO3−-N and moisture contents under N enrichment. These results suggest that soil nitrification and NO3−-N accumulation could be important regulators of soil CO2 emission and CH4 uptake in the temperate needle-broadleaved mixed forest. The nonlinear responses to exogenous N inputs and the critical level of N in terms of soil C fluxes should be considered in the ecological process models and ecosystem management.