Increasing soil organic carbon content: The key roles of straw return duration and maize growth stages in driving microbial community shifts

Long-term straw return is a key measure for enhancing soil organic carbon (SOC). However, the mechanisms by which prolonged straw return influences SOC accumulation through soil microbial community regulation according to maize growth stage remain unclear. This study was based on a long-term field plot experiment in which soil biochemical properties were measured at three critical maize growth stages—jointing (V12), tasseling and silking (R1), and milk (R3)—under straw return practices of 5, 7, and 9 years, as well as under a no-straw-return control. Effects on SOC content, microbial communities, and their functional attributes were investigated. Long-term straw return generally increased the contents of SOC, microbial biomass carbon (MBC), total nitrogen (TN), available phosphorus, and total phosphorus; however, the response varied according to duration of straw return. Straw return duration and maize growth stage jointly shaped microbial community structure; fungal diversity showed greater sensitivity to growth stage variation. During growth progression, the relative abundance of Actinobacteria increased, enhancing soil capacity for straw decomposition. Predictive functional profiling suggested that long-term straw return can enhance bacterial chemoautotrophic pathway activity and aerobic nutrient-related processes, while promoting straw assimilation and utilization. Fungal communities, primarily characterized by saprotrophic nutrition, were predicted to play major roles in the potential decomposition of cellulose, hemicellulose, and lignin, thus regulating soil carbon cycling. Structural equation modeling revealed that a 7-year straw return strategy achieves optimal SOC sequestration by positively regulating fungal communities; MBC and TN served as key mediating factors in SOC enhancement. This study establishes an optimized field management strategy for straw return in black soil regions and provides theoretical support for advancing sustainable agriculture through coordination of the straw–microorganism–SOC interaction. However, considering soil type specificity and environmental variability, further research across diverse agroecosystems is required to verify the broader applicability of these mechanisms.