Distribution characteristics of straw return and cultivation pattern regulated soil organic carbon fraction in black soil

The formation and transformation of soil organic carbon (SOC) are complex processes, and their responses to agricultural management practices often exhibit a lag effect. To better understand the distribution characteristics and driving mechanisms of microbial residue carbon within different SOC fractions, this study investigated the effects of four improved tillage and cropping systems (deep plowing-continuous cropping, deep plowing-rotation, shallow rotation-continuous cropping, shallow rotation-rotation, using conventional tillage fields as controls). These results showed that mineral-associated organic carbon (MAOC) was a major component of SOC in the study region. Although total SOC and particulate organic carbon (POC) did not significantly differ among treatments (P>0.05), MAOC content was significantly affected (P<0.05), with the highest value observed in the control [(11.67±0.11) g/kg] and the lowest in the deep plowing-rotation treatment [(10.20±0.36) g/kg]. The contribution of microbial residue carbon to MAOC ranged from 21.07% to 28.76%, with fungal residues [(14.69±0.19) g/kg] being the dominant source. Notably, the shallow rotary tillage with continuous cropping treatment significantly enhanced microbial residue carbon accumulation in MAOC, thereby improving SOC stability, while the shallow rotation-rotation system showed a trend toward increasing POC content, potentially enlarging the active carbon pool. This contrast may be attributed to differences in straw incorporation patterns, as compared with shallow rotary tillage, deep plowing promotes the uniform incorporation of straw throughout the tillage layer (0–25 cm), thereby facilitating a more even distribution of carbon inputs and alleviating the accumulation of plant residues in the surface layer (0–10 cm). Overall, soil nutrient status (total phosphorus, C∶N and C∶P ratios), together with mineralogical and biological properties (free iron and microbial residue carbon), are key determinants of the relative proportions of particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) in soils. Agricultural management practices may indirectly affect the content and reactivity of Fed through regulating carbon input intensity, redox conditions, and soil nutrient status, which in turn modulates the formation and accumulation of microbial residue carbon and ultimately determines the partitioning between POC and MAOC. These findings provide a theoretical basis for optimizing farmland carbon management and supporting policy development for carbon sequestration in black soils of northeast China.