Effects of Mulching Practices on Soil Soluble Nitrogen in Orchards in Red Soil Hilly Areas

Background and Aims Mulching practices significantly contribute to soil and water conservation and soil ecological regulation in red soil hilly regions. However, it remains unclear how grass cover and plastic mulching affect the composition of soil soluble nitrogen by modifying microenvironments and functional microbial communities. Methods We examined the effects of grass cover and plastic mulching on nitrogen-related microbial communities and soluble nitrogen components in the 0–60 cm soil layer of citrus orchards in a red soil hilly area. Results Both grass cover and plastic mulching significantly increased soluble nitrogen content in the 0–20 cm soil layer. Compared to the control, grass cover increased ammonium nitrogen, nitrate nitrogen, and soluble organic nitrogen by 20.2%, 18.3%, and 56.7%, respectively, while plastic mulching raised them by 3.3%, 11.5%, and 38.9%. Grass cover not only induced greater increases in soil nitrogen fractions than plastic mulching but also promoted a higher relative abundance of key nitrogen-cycling microorganisms such as Proteobacteria, Actinobacteriota, Verrucomicrobiota, and Acidobacteriota. Structural equation modeling revealed that soil microorganisms (path coefficient = 0.68) had the strongest influence on total soluble nitrogen, exceeding the effects of physical (0.33) and chemical (0.49) properties. Among chemical factors, total nitrogen had the strongest direct effect on ammonium and dissolved organic nitrogen, while the relative abundance of Proteobacteria most strongly promoted nitrate nitrogen. Conclusion Grass cover is more effective than plastic mulching or no treatment in enhancing soil soluble nitrogen components and enriching nitrogen-related microbial communities in red soil hilly regions.