Metagenomic reconstruction of microbial structure and carbon cycling for annual crop productivity: influence of long-term straw mulching and nitrogen

Soil organic carbon (SOC) sequestration is crucial in sustaining agroecosystem productivity. However, the mechanism through which microbial functional gene assemblages and rhizosphere metabolites drive SOC sequestration remains elusive. Ten-year field experiments examined how straw mulching (0 and 7500 kg ha⁻¹) with nitrogen (0, 120, and 180 kg N ha⁻¹) reshape microbial functional potential, drive soil carbon sequestration, and enhance annual crop productivity in the wheat–maize rotation system. Compared with no mulch control, long-term straw mulching with N fertilization increased annual wheat (12.9–25.5 %) and maize (39.6–57.7 %) yields and SOC content (15.8–22.7 %). It also promoted the conversion of labile SOC to slow and passive SOC, owing to the microbial carbon pump effect. Proteobacteria and Actinobacteria emerged as primary microbial phyla that stimulated functional potential involved in organic C oxidation, carbohydrate and lipid metabolism, particularly enhancing functional genes for cellulose oxidation (cellobiosidase) and lignin degradation (benzoyl-CoA reductase). Differentially expressed rhizosphere metabolites, including organic acids, lipids, and phenylpropanoids, were mostly associated with converting labile C to passive SOC. These results indicated that straw mulching with chemical N addition drove the assemblage of microbes to regulate functional genes that participated in organic carbon oxidation and altered the metabolic profile of phenylpropanoids, lipids, and organic acids to increase soil carbon sequestration and annual crop productivity. Our findings provide a framework for optimizing residue-nutrient management to reconcile soil carbon sequestration with agricultural productivity in dryland farming systems.