Nitrogen substitution practice improves soil quality of red soil (Ultisols) in South China by affecting soil properties and microbial community composition

Substituting synthetic nitrogen (N) fertilizer with organic amendments is considered an effective practice to improve soil quality. However, information on the effects of N substitution practices on soil quality in the red soil area in South China is still limited. In this study, therefore, four kinds of organic materials, including maize straw, cow manure, biochar and biogas residue, were applied to sweet maize farmland for six consecutive cropping seasons to substitute 20% synthetic N fertilizer in the Pearl River Delta Plain, which is a typical red soil area in South China. We compared the effects of different N substitution practices with conventional fertilization (CK) on the soil physical, chemical and biological properties and the microbial community structure. A minimum dataset method was utilized to calculate the soil quality index (SQI) under different treatments. The results demonstrated that the N substitution practice improved the SQI by 55%−70% compared to the CK in the red soil of South China by improving the soil physicochemical properties and biological characteristics. Meanwhile, the N substitution practice showed positive effects on changing the microbial community structure. Cow manure substitution showed the best effect due to the increased relative abundance of beneficial phyla in the soil microbiota, such as Proteobacteria, Chloroflexi and Ascomycota, and the decreased relative abundance of Basidiomycota. Finally, the structural equation model illustrated that the N substitution practice greatly improved the SQI in red soil by improving the soil bulk density, pH, β-glucosidase activity, urease activity and microbial community composition. Collectively, all four N substitution practices significantly improved soil quality, and cow manure substitution was the most effective way to improve soil microbial community composition and soil quality. This study provides a research basis for improving soil quality in the red soil region in South China and valuable information for the development of clean agricultural production in typical subtropical areas in East Asia.