Contrasting response of soil N2O release to ammonium, nitrate, and urea addition rates is determined by substrate availability and microbial community abundance and composition

Although nonlinear responses of soil nitrous oxide (N2O) release to exogenous nitrogen (N) input have been reported, the contrasting effects of oxidized and reduced N forms and their driving mechanisms have rarely been investigated. Here, a 91-day microcosm incubation experiment with three forms (ammonium, nitrate, and urea) and eleven rates (0, 1, 2, 4, 6, 8, 10, 12, 14, 28, and 56 g N m−2 yr−1) of N addition was conducted to determine the response dose, saturation threshold, and turning point of N2O release from the temperate forest soil. Simultaneously, soil biochemical properties, N-cycling functional gene abundances, and bacterial and fungal community composition were determined to assess the biotic and abiotic contributions to changes in soil N2O release. The responses of soil N2O release rate to ammonium, nitrate, and urea addition rates exhibited saturation, sigmoidal, and exponential curves, respectively. The response dose of soil N2O release was higher in urea addition than in ammonium and nitrate additions (28 g N m−2 yr−1 vs. 8 g N m−2 yr−1). The tipping point and saturation thresholds for ammonium and nitrate additions were estimated to be 14 g N m−2 yr−1 and 56 g N m−2 yr−1, respectively, whereas neither occurred in urea addition. Except high rate of ammonium addition (≥28 g N m−2 yr−1), N inputs significantly increased AOB abundance and promoted soil NO3−-N accumulation. Also, N addition, especially high-rate ammonium, led to a shift in bacterial community from oligotrophic to copiotrophic groups. Oxidized nitrate input increased soil NO3−-N content and changed bacterial and fungal community structure, whereas reduced ammonium and urea additions increased soil NH4+-N content and AOB abundance, as well as altered bacterial community. These findings highlight that N forms contrastingly affect soil N2O release. Substrate availability, functional gene abundance, and microbial community composition equally contribute to soil N2O release, which should be considered in terrestrial ecosystem process models.