Partial organic fertilizer substitution promotes soil multifunctionality by increasing microbial community diversity and complexity

Partial substitution of synthetic nitrogen (N) with organic fertilizers (PSOF) is of great significance in improving soil ecosystem functions in systems that have deteriorated due to the excessive application of chemical N fertilizer. However, existing studies typically focus on individual soil functions, neglecting the fact that multiple functions occur simultaneously. It remains unclear how PSOF influences multiple soil functions and whether these impacts are related to soil microbial communities. Here, we examined the impacts of partial substitutions (25%-50%) of chemical N fertilizer with organic form (pig manure or municipal sludge) in a vegetable field on soil multifunctionality, by measuring a range of soil functions involving primary production (vegetable yield and quality), nutrient cycling (soil enzyme activities, ammonia volatilization, N leaching, and N runoff), and climate regulation (soil organic carbon sequestration and nitrous oxide emission). We observed that PSOF improved soil multifunctionality, with a 50% substitution of chemical N fertilizer with pig manure being the best management practice; the result was strongly related to the diversities and network complexities of bacteria and fungi. Random forest analysis further revealed that soil multifunctionality was best explained by the bacterial-fungal network complexity, followed by available phosphorus level and bacterial diversity. The PSOF also shifted the composition of bacterial and fungal communities, with increased relative abundances of dominant bacteria phyla, such as Bacteroidetes, Gemmatimonadetes, and Myxococcota, and fungal phyla, such as Basidiomycota and Olpidiomycota. The observed increases in soil multifunctionality were consistent with significant increases in the relative abundances of keystone taxa such as Blastocladiomycota, Chaetomiaceae, and Nocardiopsaceae. Together, these findings indicate that PSOF can enhance interactions within and among microbial communities and that such practices have the potential to improve soil ecosystem multifunctionality and contribute to the development of sustainable agriculture.