Responses of soil nitrous oxide flux to soil environmental factors in a subtropical coniferous plantation: A boundary line analysis

Soil N2O responds differently to various soil variables; moreover these driving variables usually have low explanatory capacity of soil N2O flux variability. Anthropogenic nitrogen (N) deposition inputs to the subtropical plantation forests markedly increase N2O emission from soils, but the relative contribution of deposited NH4+ and NO3− is not well understood. Based on a two-type (NH4Cl and NaNO3) and three-level (0, 40, 120 kg N ha−1 yr−1) N addition experiment, we simultaneously determined soil N2O flux and related soil variables once a week in 2015. Boundary line analysis was used to describe the maximum responses of soil N2O fluxes to different soil variables. Four years of N addition led to significant accumulations of soil NH4+-N and NO3−-N, thereby accelerating soil acidification. Nitrogen addition significantly increased soil N2O fluxes by 4.7-19.3 folds relative to control; moreover, the promotions to soil N2O emission and soil acidification were greater with ammonium-N addition than with nitrate-N addition. The relationships between the maximum soil N2O fluxes and individual soil variables were well fitted to the Gaussian equations, and the optimum values of soil temperature, moisture, pH, and NO3−-N content indicate that nitrification dominated soil N2O production. Overall, our results suggest that exogenous NH4+ input to the subtropical plantation appears to have greater negative effects on soil acidification and N2O emission compared with NO3− input. Boundary line analysis well depicts the shape and magnitude of response functions of soil N2O fluxes against soil variables, which should be incorporated into terrestrial ecosystem process models.