Deep tillage with straw retention increased organic carbon sequestration and enhanced homogenization of microbial communities and functions across soil depths

Intensive agricultural practice has resulted in a rapid decline in soil organic carbon (SOC) stocks due to insufficient organic material replenishment in the black soils of Northeast China. Straw retention has emerged as a promising management strategy for improving soil productivity and optimizing straw resource utilization. This study aimed to analyse the changes in SOC and its dynamics by examining the correlations between SOC fractions, carbon-related microbial functions and microbial community composition under different tillage methods combined with straw retention. Compared with conventional tillage without straw retention (CT), deep tillage with straw retention (DS) significantly increased the SOC content in the 0–40 cm soil layer (30.5 %–36.5 %), and no-tillage with straw mulching (MS) significantly increased the SOC content by 45.5 % in the 0–20 cm soil layer. In addition, the DS and MS treatments significantly contributed to the proportion of recalcitrant organic carbon (ROC) to SOC. The increase in lignin phenol (LP) content was more pronounced than that of microbial necromass carbon (MNC) in the soil layers with straw retention. N-acetyl-β-glucosaminidase (NAG) and ROC were the most important factors affecting SOC. Microbial community analysis revealed that the bacterial composition underwent more significant changes under the DS treatment than under the MS treatment, particularly in the 20–40 cm soil layer. Semi-labile organic carbon (LOC II) and MNC were key determinants of the microbial community structure. These findings suggest that the DS treatment enhances subsoil carbon sequestration and facilitates homogenization of microbial functions and communities across soil depths, potentially enhancing soil carbon capture and storage.