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.

Biochar, an environmentally friendly material, was found to passivate lead (Pb) in contaminated soil effectively. This study utilized spectroscopic investigations and partial least squares path modeling (PLS-PM) analysis to examine the impact of coconut-fiber biochar (CFB) on the translocation, accumulation, and detoxification mechanisms of Pb in soil-rice systems. The results demonstrated a significant decrease (p < 0.05) in bioavailable Pb concentration in paddy soils with CFB amendment, as well as reduced Pb concentrations in rice roots, shoots, and brown rice. Synchrotron-based micro X-ray fluorescence analyses revealed that CFB application inhibited the migration of Pb to the rhizospheric soil region, leading to reduced Pb uptake by rice roots. Additionally, the CFB treatment decreased Pb concentrations in the cellular protoplasm of both roots and shoots, and enhanced the activity of antioxidant enzymes in rice plants, improving their Pb stress tolerance. PLS-PM analyses quantified the effects of CFB on the accumulation and detoxification pathways of Pb in the soil-rice system. Understanding how biochar influences the immobilization and detoxification of Pb in soil-rice systems could provide valuable insights for strategically using biochar to address hazardous elements in complex agricultural settings.