The Microbial mechanisms of soil organic carbon pool transformation under long-term chemical fertilizer and straw application: Insights from an 18-year field experiment

Crop straw return is a widely adopted agricultural practice, yet the effects of 18 years of continuous straw incorporation combined with chemical fertilization on soil organic carbon (SOC) transformation remain poorly understood. Based on an 18-year field experiment in a rice-wheat rotation system in the Jianghan Plain, this study investigated the effects of four treatments(CK, no fertilizer or straw; NPK, chemical fertilizer alone; S, straw return alone; NPKS, chemical fertilizer combined with straw return) on SOC fractions, amino sugars, lignin phenols, and associated microbial mechanisms. Results demonstrated that long-term NPKS treatment significantly increased SOC content by 49.29% compared with CK, with a greater increase in labile fractions: dissolved organic carbon (DOC, +104.23%), microbial biomass carbon (MBC, +90.76%), readily oxidizable carbon (ROC, +77.65%), and particulate organic carbon (POC, +111.29%). Straw return (S and NPKS) boosted microbial and plant carbon pools, significantly increased fungal necromass carbon, and raised the GluN/GalN ratio by 8.43–10.84%, suggesting a greater contribution of fungi to SOC pools. Concurrently, lignin phenol contents (vanillin, syringic acid) significantly increased under NPKS, highlighting the retention of plant-derived recalcitrant carbon. Importantly, compared with CK, NPKS application significantly increased wheat and rice grain yields by 130.39% and 86.78%, respectively. Metagenomic analysis revealed that straw return significantly altered soil microbial community composition, increasing the relative abundances of Gemmatimonadota, while decreasing those of Planctomycetota. Straw return was associated with increased relative abundances of carbon degradation genes ( amyA , abfA , bglX , etc. ) and CAZyme genes, which are closely linked with organic matter decomposition. Meanwhile, genes involved in carbon fixation ( rbcL , accA , aclB ) were also upregulated under straw return. These findings support the hypothesis that straw return is associated with enhanced microbial catabolic and carbon fixation gene potentials, as well as SOC pool accumulation. This study provides correlative evidence supporting straw return as a potential strategy for enhancing carbon sequestration and key soil quality indicators (organic carbon fractions, microbial carbon metabolism, and nutrient availability) in intensive agricultural systems.