Driving mechanisms of the soil aggregate breakdown-formation on soil organic carbon mineralization under splash erosion

Splash erosion initiates water erosion and significantly affects soil organic carbon (SOC) dynamics by fragmenting soil particles and influencing SOC mineralization. However, the mechanisms linking soil aggregate turnover to SOC mineralization and CO₂ emissions remain unclear. To investigate the fate of soil aggregates and SOC under erosion conditions, raindrop splash erosion experiments were conducted at rainfall intensities of 60, 90, and 120 mm/h. Four types of rare earth oxides were used to label soil aggregates of various sizes: large (2–5 mm), medium (1–2 mm), small (0.25–1 mm), and micro (<0.25 mm), followed by 56-day soil incubation. The results indicated that the breakdown and formation of soil aggregates were significantly influenced by rainfall intensity. The average cumulative breakdown rate of soil aggregates increased with higher rainfall intensities: 120 mm/h (12.02 %) > 90 mm/h (9.49 %) > 60 mm/h (8.19 %). In contrast, the average cumulative formation rate soil aggregates exhibited the opposite trend: 60 mm/h (12.25 %) > 90 mm/h (10.90 %) > 120 mm/h (8.32 %). The primary mode of soil aggregate breakdown was from medium to small, with the highest breakdown rate on day 0, which increased with rainfall intensity: 120 mm/h (58.49 %) > 90 mm/h (48.07 %) > 60 mm/h (43.61 %). Simultaneously, as rainfall intensity increased, the SOC mineralization rate and associated CO₂ emissions consistently rose, and the total CO₂ emissions were: 120 mm/h (23.853 mg·kg⁻¹) > 90 mm/h (21.827 mg·kg⁻¹) > 60 mm/h (19.522 mg·kg⁻¹). Furthermore, a structural equation model was developed to elucidate the relationship between soil aggregate turnover and SOC dynamics, highlighting that direct soil aggregate breakdown effects on SOC mineralization were significantly greater than its indirect effects via other pathways. This direct effect intensified with increasing rainfall intensity, and its path coefficients followed the order: 120 mm/h (0.625) > 90 mm/h (0.545) > 60 mm/h (0.533). These results underscore the significant role of soil aggregate turnover in driving SOC dynamics, thereby providing a theoretical foundation for soil carbon sequestration strategies and enhancing carbon capture capacity.