Soil pH differently affects N2O emissions from soils amended with chemical fertilizer and manure by modifying nitrification and denitrification in wheat-maize rotation system

Emissions of nitrous oxide (N₂O), a potent greenhouse gas, from farmland have been recognized to be affected by soil pH and nitrogen (N) fertilizer application. However, the interactive effects of soil pH and N fertilizer type on N₂O emissions and their influencing mechanism are poorly understood. A field experiment was conducted to elucidate the impacts of synthetic fertilizer and manure on soil properties and N₂O fluxes along a soil acidity gradient (soil pH = 6.8, 6.1, 5.2, and 4.2) in the Huai River Basin, and a lab incubation experiment was performed to understand the underlying mechanisms of changed N₂O flux. Low soil pH inhibited the ammonia-oxidizing bacteria abundance and thereby reduced the N₂O production by nitrification under both synthetic fertilizer and manure application. The N₂O production by denitrification was also reduced with declining soil pH, likely due to the decreased nirS and nirK abundances, and lower NO₃⁻. However, low soil pH reduced the nosZ abundance and increased (nirS + nirK)/nosZ ratio, resulting in the increased N₂O/(N₂O + N₂) ratio. Finally, with the decreased nitrification and denitrification, soil N₂O emission was significantly reduced with declining soil pH regardless of fertilizer types. Compared with synthetic fertilizer, manure application increased soil nutrients (total N, dissolved organic C, and NO₃⁻), nirK abundance, and (nirS + nirK)/nosZ ratio in the soils with pH of 5.2 and 4.2, thereby promoting N₂O production by denitrification and N₂O/(N₂O + N₂) product ratio in acidic soils. Consequently, soil N₂O emission was increased with manure application in acidic soils. This study provides novel insight and improves our understanding of how soil pH regulates nitrification, denitrification, and N₂O emissions from soils amended with chemical fertilizer and manure, which gives guidance on developing N management strategies for sustainable production and N₂O mitigation in acid soils.