Objectives Studies have shown that continuous straw returning to cropland can improve soil fertility and crop yield and alleviate soil degradation caused by chemical fertilizer application in fluvo-aquic soil of Huang-Huai-Hai region, north China. However, the contribution of the bacterial community (especially the co-occurrence patterns) to soil nutrient availability and enzymatic activity is still unclear. The overall objective of this study was to investigate the bacterial community diversity, composition and intraspecific interactions as influenced by chemical fertilizer application and straw returning.

Methods The tested soils were collected from three fertilization regimes (CK, no fertilization control; NPK, fertilization with chemical N, P, and K fertilizers; NPKS, straw returning combined with chemical N, P, and K fertilizers) in a 10-year field experiment located in Dezhou city, Shandong Province. Bacterial communities were characterized using high-throughput sequencing of the 16S rRNA gene V4-V5 region. Cross-treatment spearman correlation network was constructed to explore bacterial co-occurrence patterns.

Results Chemical fertilizer application and straw incorporation (P<0.05) improved soil fertility and enzymatic activity. Compared with CK, NPK and NKPS increased SOM by 23.19% and 34.82%, decreased pH by 0.06, bacterial diversity by 0.90% and 0.91%, and bacterial evenness by 1.11% (P<0.05). NPK (P<0.05) increased the activity of β-glucosidase (β-GC) and alkaline phosphatase (ALP) by 47.91% and 50.35%; NPKS (P<0.05) increased the activity of β-GC, ALP and dehydrogenase (DHA) by 78.31%, 46.53%, and 50.91%, respectively. The dominant species in CK were oligotrophic (i.e., Acidobacteria and Planctomycetes) and nitrogen-fixing bacteria (i.e., Hyphomicrobiaceae and Burkholderiaceae). The indicator species in NPK belong to Proteobacteria (i.e., Oxalobacteraceae, Rhodanobacteraceae, and Xanthomonadaceae), Actinobacteria, and Bacteroidetes, which play important roles in recalcitrant organic decomposition. Continuous straw returning in NPK stimulated the growth of Firmicutes (i.e., Bacillus), which was famous for cellulose degradation. We explored the distribution patterns of indicator species by co-occurrence network analysis. We found that the indicator species grouped in distinct modules reflecting different fertilization regimes were closely related to soil nutrient content and enzyme activity. The indicator species in NPKS (i.e., Bacillus) formed specific species clusters through a strong symbiotic relationship. This was positively related to β-GC, ALP and DHA, indicating that members in this module could improve soil microbial activity, and promote soil carbon and phosphorus transformation.

Conclusions These findings suggest that long-term straw returning combined with chemical fertilizer could improve soil fertility, optimize bacterial community composition, regulate the interaction between bacterial species, and improve soil enzyme activity in the Huang-Huai-Hai plain.