Nitrogen reduction processes in paddy soils across climatic gradients: Key controlling factors and environmental implications

Nitrogen reduction processes play crucial roles in removal and retention of nitrogen (N) in soil environments. However, the key biogeochemical controls on dissimilatory nitrate/nitrite reduction processes in paddy soils across climatic gradients remain poorly understood. Here, we used a ¹⁵N-isotopic tracing approach to quantify the potential rates of denitrification, anaerobic ammonium oxidation (anammox) and dissimilatory nitrate reduction to ammonium (DNRA) and elucidated the key factors controlling the three processes in paddy soils from temperate to tropical climates. Potential rates of denitrification, anammox and DNRA varied greatly across the studied paddy soils, ranging from 8.62 to 19.1, 0.02 to 4.27 and 0.20 to 5.01 nmol N g⁻¹ h⁻¹, respectively. Potential rates of denitrification were significantly higher in north subtropical than in south subtropical climates, and the rates of anammox were significantly higher in central subtropical than in both central temperate and north subtropical climates. Potential rates of DNRA exhibited no significant difference between the climatic gradients. Potential rates of the three processes were not significantly related to climatic temperature. Generalized linear models revealed that soil NO₃ ^–, labile organic carbon and nosZ abundance were the key regulators of denitrification, soil NO₂ ^– and hszB abundance were the key variables controlling anammox, while total organic carbon, Fe(II), and nrfA abundance were the key controls on DNRA across these paddy soils. Soil variables explained 69%, 89% and 96% of total variances of denitrification, anammox and DNRA, respectively, along the climatic gradients. N-retention index (DNRA/(denitrification + anammox)) was significantly correlated with water content and dissolved organic carbon. These results suggest that soil variables rather than climatic temperature control the geographical variances of N reduction processes in paddy soils along climatic gradients.