Straw return drives soil microbial community assemblage to change metabolic processes for soil quality amendment in a rice-wheat rotation system

Straw return has been widely implemented to sequester soil organic carbon (SOC) and enhance soil quality in rice-wheat cropping systems, however, the mechanism through which it influences microbial assemblages into mediating biochemical metabolic pathways in soil remains ambiguous. This study aimed at investigating the composition and assembly of soil microbial communities and the soil metabolome prevailing across four straw return practices (control: no straw return; RR: rice-straw return; WR: wheat-straw return; DR: both rice- and wheat-straw return) during wheat cultivation on a rice-wheat rotation field. Straw return primarily altered the abundance of lipids (LL), organic acids (OA), organic nitrogen compounds, and benzenoids, with DR having a greater impact on soil metabolites than WR or RR. Besides being predominantly present in the lipid (LM) and amino acid (AAM) metabolic pathways, these differentially expressed metabolites (DEMs) were also dispersed throughout the nucleotide, xenobiotic, and secondary metabolite pathways. Straw return substantially elevated homogeneous selection in the order DR > WR > RR > control. The links of taxa to LM and AAM occupied by 64.9% in the DEMs-microbial taxa correlation network, with majority of taxa being derived from the keystone modules of the microbial network. Furthermore, these taxa potentially modulated variations in microbiota assembly by 27-45.1%, soil metabolites by 13.3-70.7%, and soil nutrient properties by 24.8-49.6%; and their removal evidently mitigated the natural connectivity of the microbial network. Structural equation modeling depicted that straw return exerted positive effects on LL and OA by regulating microbial assembly. Therefore, our results demonstrated that straw return drive microbial community assembly to dominate the taxa mediating soil LM and AAM for enhancing SOC sequestration and soil quality.