Patterns and determinants of nitrification and denitrification potentials across 24 rice paddy soils in subtropical China

Nitrification and denitrification processes are major pathways for nitrogen (N) losses in agricultural soils, and highly depend on soil properties. Improved understanding of soil nitrification and denitrification processes is essential for better N management in agroecosystems. However, the effects of geochemical properties on nitrification and denitrification potentials remain largely uncertain, especially in paddy soils. Here, we investigated the patterns and controlling factors of nitrification and denitrification potentials across 24 paddy soil sampling sites (varied in soil texture, pH, and contents of soil organic carbon (SOC), nutrients, and exchangeable metals) in major rice-producing areas of Hubei Province, China. The results showed substantial variations in the potential rates of nitrification (0.46-17.65 mg N kg- 1 soil d-1) and denitrification (6.31-18.12 mg N kg- 1 soil d-1) across the paddy soil sampling sites. Soil pH and SOC were the predominant factors regulating nitrification and denitrification potentials, respectively. Nitrification potential strongly increased with soil pH at value above 6.2, while showed an insignificant correlation with pH at value below 6.2. Increased soil pH stimulated nitrification potential via enhanced NH4+ availability, likely due to increased soil active iron (Fe) and manganese (Mn) oxides facilitating chemisorption. Denitrification potential strongly positively correlated with SOC at value below 15 g kg- 1, dissolved organic carbon and nitrate contents, but had little relationship with active Fe and Mn properties and clay contents. The potentials of nitrification uncoupled with denitrification, possibly resulting from their distinct controlling factors across the paddy soils. Overall, these findings could help improve the understanding of the key factors controlling potential nitrification and denitrification across paddy soils, and provide a theoretical basis for optimizing agricultural management to mitigate N losses.