Soil 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.