Effects of tillage practices on aggregate-associated soil organic carbon fractions and maize yield

Tillage practices can disrupt soil surface, creating loose, erodible soil layers. This significantly impacts soil organic carbon (SOC) fractions, including microbial biomass carbon (MBC), dissolved organic carbon (DOC), particulate organic carbon (POC), readily oxidizable organic carbon (ROC), and heavy fraction organic carbon (HFOC). SOC fractions may depend on soil aggregate size. However, the variation patterns of SOC fractions under different soil aggregate sizes and tillage practices remain unclear. The objectives of this study were to assess the impact of tillage practices on SOC fractions (MBC, DOC, POC, ROC, and HFOC) within soil aggregates and to quantify the relationship between aggregate size and these SOC fractions. Seven treatment combinations were selected: two conventional tillage practices of rotary tillage (RTS) and plow-tillage (PTS) with 100% harvest maize stover mulching, and five conservation tillage practices of subsoiling+100% mulching (STS), no-tillage+30% (NTS1) and 100% (NTS2) mulching, and no-tillage & ridging+30% (NTSR1) and 60% (NTSR2) mulching. Results showed that tillage practices significantly impacted SOC fractions and maize yield (P < 0.05). Specifically, conservation tillage outperformed conventional tillage in enhancing SOC and its labile labile organic carbon (LOC, including MBC, DOC, POC, ROC) by retaining crop residues, increasing carbon inputs, and reducing mechanical disturbance. Mineral-associated organic carbon (MAOC) and heavy fraction organic carbon (HFOC) are classified as stable SOC fractions, and conservation tillage also promoted the accumulation of stable SOC. NTS2 was optimal for SOC accumulation in the 0-20 cm soil layer. STS promoted root carbon input in the 20-40 cm soil layer. Maize yield was the highest under STS and the lowest under RTS. Particularly, ROC (<0.053 mm) in the 0-20 cm soil layer and MBC in the 20-40 cm soil layer (>2 mm, 2-0.25 mm) remarkably affected maize yield. Overall, this study demonstrates that the STS treatment can provide an optimal pathway for achieving both high SOC sequestration and maize yield. Future research should further elucidate the coupling mechanism between microbial functional redundancy and carbon turnover dynamics under long-term tillage practices to provide theoretical support for precision agricultural management.