Potential risk of soil reactive gaseous nitrogen emissions under reclaimed water irrigation in a wheat-maize rotation system

Reactive nitrogen gas (Nr) originating from the soil is highly susceptible to the influence of agricultural water and fertilizer management practices, making it a critical gas that can substantially contribute to atmospheric pollution. Understanding the emission patterns of Nr in the surface soil under reclaimed water (RW) irrigation is essential for guiding RW irrigation practices and informing policies aimed at reducing Nr emissions. In this study conducted in Beijing, North China, field sites were selected with different irrigation management strategies and corresponding fertilizer application methods. The two types of irrigation water quality comprised RW irrigation and groundwater (UW) irrigation, whereas the two fertilizer types utilized were calcium ammonium nitrate with a high nitrate nitrogen content and ammonium sulfate with ammonium nitrogen. Nr emissions and concentrations in the soil profile (0-30 cm depth) were monitored throughout key agricultural events, including cultivation, irrigation, fertilization, and harvest, spanning the period from 2020 to 2021. It is observed that RW irrigation effectively reduces cumulative ammonia (NH3) emissions compared to groundwater (UW) treatment. However, it results in increased concentrations of nitrous oxide (N2O) (12.33%-73.82%) and nitric oxide (NO) (13.74%-36.59%) in the soil within the depth range of 0-30 cm. Of particular importance is the notable increase in the abundance of soil denitrifying and ammonia-oxidizing archaea (AOA-amoA) genes due to RW irrigation, indicating the potential for elevated N2O and NO emissions. Furthermore, it is noteworthy that the soil Nr emissions and concentrations are more strongly affected by nitrogen type rather than water quality. Conversely, the inclusion of nitrate has proven to be effective in mitigating N2O and NO emissions in RW-irrigated farmland.