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

Abstract 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 CO 2 and CH 4 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 CO 2 and CH 4 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 (NH 4 + -N, NO 3 − -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 NO 3 − -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 NH 4 + -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 CO 2 emission and CH 4 uptake. Significant negative relationships were observed between changes in soil CO 2 emission and CH 4 uptake and changes in soil NO 3 − -N and moisture contents under N enrichment. These results suggest that soil nitrification and NO 3 − -N accumulation could be important regulators of soil CO 2 emission and CH 4 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.