Soil texture affects the nitrogen fate in the methane fermentation digestate applied to paddy soil

Waste fluid (digestate) from methane fermentation (MFW) is increasingly used as a fertilizer in paddy fields to reduce greenhouse gas emissions and mitigate climate change. However, the influence of soil texture on the fate of nitrogen (N) following MFW application remains unclear. This study aimed to elucidate the effects of MFW application rates on rice growth and N pathways in soils with contrasting textures to minimize environmental N loss during rice cultivation. Two-year pot experiments were conducted using sandy loam (SL) and clayey loam (CL) soils. Five treatments were applied, each with four replicates: a chemical fertilizer (NH₄Cl, 200 mg N pot⁻¹), three MFW rates of total N (100, 200, and 400 mg N pot⁻¹), and a no-fertilizer control. To trace N derived from the inorganic fertilizer (Ndfif), ¹⁵NH₄Cl was added to account for 20% of the inorganic N in each treatment, while maintaining total N input constant. Increasing MFW application rates significantly enhanced the rice growth and yield, with a more pronounced effect in CL soil compared to SL soil. Higher MFW application rates further increased Ndfif uptake by plant, soil N retention, and NH₃ volatilization in CL soil, whereas SL soil showed more significant Ndfif leaching. Chemical fertilizers showed significantly higher Ndfif recovery but were more prone to leaching in SL soil compared to MFW-derived N. Based on normalized comparisons of rice yield and N loss, optimal MFW application rates were estimated at approximately 180 mg N-MFW pot⁻¹ for both soils, beyond which further yield increases were accompanied by substantial N leaching and emissions. Soil texture critically affects the fate of N following MFW application, especially regarding NH₃ volatilization in CL soils and N leaching in SL soils. Optimizing MFW application rates is crucial for achieving enhanced rice yields with minimal environmental N loss, thereby facilitating sustainable rice production. These findings emphasize the importance of soil texture in determining optimal nutrient management for sustainable rice production.