Nonlinear responses of soil nitrous oxide emission to multi-level nitrogen enrichment in a temperate needle-broadleaved mixed forest in Northeast China

Abstract The responses of nitrous oxide (N 2 O) emission from forest soils to increasing atmospheric nitrogen (N) deposition are controversial. In this study, our objectives were to explore the response curves of soil N 2 O flux to multi-level N inputs, as well as to examine the key factors dominating the changes in soil N 2 O emission caused by N enrichment in the temperate needle-broadleaved mixed forest, Northeast China. The study consists of nine levels of urea addition (0, 10, 20, 40, 60, 80, 100, 120, 140 kg N ha − 1  yr − 1 ) with 4 replicates for each treatment. Soil N 2 O fluxes were monitored weekly using the static chamber and gas chromatograph technique. NH 4 + -N, NO 3 − -N, total dissolved N (TDN), dissolved organic N (DON), and auxiliary variables (soil temperate and moisture in 0–10 cm depth) were measured at the same frequency to examine the regulation of soil N 2 O flux. The results showed that high rates of urea inputs (> 60 kg N ha − 1  yr − 1 ) significantly increased soil NO 3 − -N concentrations in litter layer and mineral layer (0–10 cm depth) by 120–180% and 56.4–84.6%, respectively. Soil N 2 O flux increased exponentially with increase in the rates of urea addition by 194% to 334% for the 60 to 140 kg N ha − 1  yr − 1 treatments relative to the control. The critical level of N input for the significant alternation of soil N accumulation and N 2 O emission was approximately 70 kg N ha − 1  yr − 1 . The changes in soil N 2 O flux elicited by N addition were positively related to those of soil NO 3 − -N and soil moisture contents. These results suggest that exogenous N input at the rate below the critical load will not significantly promote soil N 2 O emission over the short term, which is favorable to carbon sequestration of the temperate needle-broadleaved mixed forest, Northeast China.