Inhibitory effect of ammonium conversion by deep nitrogen application effectively reduces gaseous nitrogen losses

Abstract

Increasing the depth of nitrogen (N) fertilizer application is an effective agronomic management measure for mitigating NH3 volatilization and N2O emissions. However, the information about reasons for the reduction of gaseous N loss and the responses of soil N transformation processes to deep N fertilization is limited. This field experiment monitored NH3 volatilization, N2O emission, and soil NH4+/NO3- dynamics under three N application depths (8, 16, and 24 cm) in the typical soil of Loess Plateau (Cumulic Anthrosol) during 2021–2022 and 2022–2023. The N transformation rate under different soil depths was determined by laboratory 15N tracing experiments, and driving contributions of soil physicochemical properties and biological characteristics to gaseous N emissions were considered comprehensively. Our field results verified that increasing N fertilization depth effectively reduces NH3 volatilization and N2O emission by 20.7–55.6% and 17.9–45.8%; Simultaneously, deep N fertilization delayed the peak time of NH3 or N2O and retained a higher concentration of NH4+ in deep soil (37.3–46.4 mg kg−1). The 15N tracing experiments revealed that the lower soil enzymatic activity and microbial abundance in deep soil significantly decreased N mineralization, NH4+ consumption, and nitrification rates by 30.6–44.1%, 37.1–44.9%, and 22.5–31.1% (P < 0.05), compared with the shallow soil. Random forest analysis explored that the predictive factors of NH3 volatilization were soil urease activity, NH4+ consumption rate, soil NH4+, and TOC (P < 0.01). Interestingly, N2O emission was also mainly affected by soil urease activity, hydroxylamine oxidoreductase (HAO) activity, soil NH4+, and NO3- (P < 0.01); Through partial least squares path models (PLS-PMs), deep N fertilization mainly reduced risk of gaseous N loss by changing the environmental conditions required for N transformation and affecting the N mineralization and nitrification processes to limit NH4+ generation and consumption. This study provides a theoretical foundation for the demonstration and promotion of deep fertilization in the Loess Plateau.