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

- Urea addition only significantly led to the accumulation of soil NO3−-N.
- Soil N2O flux increased exponentially with increase in the rates of urea addition.
- The critical load of N deposition for soil N2O emission was about 70 kg N ha− 1 yr− 1.
- Soil NO3− had a higher regulation to soil N2O emission relative to soil NH4+.

The responses of nitrous oxide (N2O) 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 N2O flux to multi-level N inputs, as well as to examine the key factors dominating the changes in soil N2O 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 N2O fluxes were monitored weekly using the static chamber and gas chromatograph technique. NH4+-N, NO3−-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 N2O flux. The results showed that high rates of urea inputs (> 60 kg N ha− 1 yr− 1) significantly increased soil NO3−-N concentrations in litter layer and mineral layer (0-10 cm depth) by 120-180% and 56.4-84.6%, respectively. Soil N2O 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 N2O emission was approximately 70 kg N ha− 1 yr− 1. The changes in soil N2O flux elicited by N addition were positively related to those of soil NO3−-N and soil moisture contents. These results suggest that exogenous N input at the rate below the critical load will not significantly promote soil N2O emission over the short term, which is favorable to carbon sequestration of the temperate needle-broadleaved mixed forest, Northeast China.