Soil acidification and ammonia-oxidizing archaeal abundance dominate the contrasting responses of soil N₂O emissions to NH₄⁺ and NO₃^- enrichment in a subtropical plantation

Atmospheric nitrogen (N) deposition markedly increases nitrous oxide (N2O) emissions from subtropical plantation soils. However, the contrasting effects of deposited NH4+ and NO3- on soil N2O emissions and the underlying mechanisms are not well understood. Based on the long-term N fertilization manipulative experiment with two types (NH4Cl and NaNO3) and three levels (0, 40, and 120 kg N ha-1 yr- 1), we investigated the response of soil N2O emissions to N addition, as well as the environmental and microbial control on soil N2O emissions using flux observations and molecular biology assays. Nitrogen addition significantly decreased soil pH by 0.30-0.67 units and increased soil N2O emissions by 6- to 21-fold relative to the control; furthermore, the effect of NH4Cl addition was significantly stronger than that of NaNO3 addition. Nitrogen addition significantly increased the abundance of ammonia-oxidizing archaea (AOA), whereas high levels of NH4Cl addition (120 kg N ha-1 yr- 1) significantly decreased the abundances of nirS, nirK, and nosZ. In addition, NH4Cl addition decreased the Shannon diversity indices of AOA, nirK, and nosZ communities. Changes in soil N2O flux were positively correlated with changes in soil NO3- -N concentration and AOA abundance, whereas negatively correlated with changes in soil pH. Soil acidification and enhanced AOA-dominated nitrification led to higher soil N2O emissions under NH4Cl addition relative to NaNO3 addition. Overall, the contrasting effects of reduced NH4+ and oxidized NO3- should be incorporated into ecosystem process models to accurately assess the response of N2O emissions from subtropical plantation soils to exogenous N input.