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Low soil nutrient availability severely constrains plant growth and productivity in acidic soil ecosystems. Diazotrophs and arbuscular mycorrhizal fungi (AMF) are the important microbial groups that improve the bioavailability of soil nitrogen (N) and phosphorus (P), respectively. Therefore, the objective was to investigate the biogeographic patterns and co-occurrence networks of the diazotrophic and AMF communities in the acidic soil ecosystem. We collected 52 soil samples from four long-term field observation stations with spatial distances of 961.3 km in the acidic soil region of southern China, and analyzed the community composition and biogeographic distributions of the diazotrophs and AMF using high-throughput sequencing of the functional gene amplicons. The variation partition analysis indicated that soil physicochemical factors provided more contributions than geographic distance, sampling stations and land-use types in the assembly of the diazotrophic and AMF community communities. Distance-decay relationships and Mantel tests showed that soil physicochemical factors had remarkable effects on both microbial community structures, whereas geographic distance only significantly affected the diazotrophic community variations. Co-occurrence network analysis revealed that AMF network was more complex and stable than the diazotrophic network. Furthermore, the dominant genera Bradyrhizbium and Glomus might be the more important keystone taxa in the diazotrophic and AMF networks, respectively. These findings suggest that the diazotrophic and AMF communities exhibit different assembly mechanisms and co-occurrence ecological networks in the acidic soil region of southern China, and both community assemblies were mainly driven by soil physicochemical factors.

The nitrogen (N) cycle process in agricultural soil is affected by fertilization. However, there is limited information on the abundance of N-cycling microbial community and its response to the different fertilization strategies in acidic soil. Using quantitative PCR, we investigated the microbial abundance involving 11 functional genes of the N cycle (nifH, chiA, aprA, archaeal and bacterial amoA, napA, narG, nirS, nirK, norB and nosZ) in an acidic agricultural soil. The soils were subjected to no fertilizer (Control), chemical NPK fertilizer (NPK), NPK combined with quicklime (NPKCa), NPK combined with crop residues (NPKS), NPK combined with quicklime and crop residues (NPKSCa), NPK combined with manure (NPKM) and only manure (M) fertilization for 27 years. Results showed that fertilization improved crop yield and soil nutrient availability, changed soil pH levels and affected the abundance of N-cycling functional genes. Manure application markedly increased the abundance of all genes and was associated with the largest shift in community structure. The variation of microbial community in the fertilization treatments (NPK, NPKCa, NPKS, NPKSCa, M and NPKM) resulted from the distinct contribution of functional genes, in which the nifH, napA and archaeal amoA genes highly contributed to the community variations between manure fertilization and Control. Soil variables, particularly pH, NO3−-N and AP, significantly affected the N-cycling microbial community structure. Overall, the different fertilization strategies demonstrated varying influential patterns on the N-cycling microbial abundance and community by altering soil properties and affecting specific functional gene abundance; in particular, manure treatments had a greater influence compared to chemical fertilizers.