Soil Aggregate-Associated Organic Carbon Cascading Process and Priming Mechanism Affected by Tillage and Organic Amendments

Clarifying SOC sequestration via physical and microbial processes is key for improving farmland fertility, yet the relative contributions of agronomic practices to carbon fractions and aggregate sizes remain unclear. This study (2010–2019, rice–wheat rotation, Funiu Mountain eastern plain, central China) examined tillage and organic amendment effects on SOC dynamics and underlying mechanisms across aggregate sizes under six treatments (conventional/reduced tillage with no fertilizer, chemical fertilizer, or organic manure). SOC, particulate organic carbon (POC), and mineral-incorporated organic carbon (MOC) were measured in bulk soil and water-stable aggregates (>2000, 250–2000, 53–250, <53 μm), and physical fractionation and phospholipid fatty acid (PLFA) analysis were conducted to assess interactions among aggregates, carbon quality, and microbial communities. Results showed that, compared with conventional tillage without fertilization, both conventional tillage and reduced tillage with organic manure significantly increased bulk SOC by 92–122% and macroaggregate (>250 μm) mass by 15–110%. The combined application of organic manure and reduced tillage redirected SOC from micro- to macroaggregates. Moreover, POC and MOC were the primary contributors to bulk SOC, with POC showing a strong direct effect on SOC accumulation. Furthermore, a positive priming effect was detected exclusively in macroaggregates, identifying them as key sites for SOC turnover and confirming that optimized tillage with manure shifts aggregates to larger sizes and boosts SOC through physical protection. The micro-to-macro cascade offers a robust framework for SOC dynamics, and its persistence under diverse climates warrants future research for sustainable management.