Drying-wetting cycles affect soil structure by impacting soil aggregate transformations and soil organic carbon fractions

Drying-wetting (DW) cycles influence the formation and breakdown of soil aggregates and consequently impact the physical protection of soil organic carbon (SOC). However, the interaction between soil structural dynamics and SOC fraction changes driven by DW cycles is still unclear. Herein, aggregation pathways were tracked by using rare earth oxides (REOs) as tracers. The recombined soil columns were subjected to different DW frequencies (0, 1, 2, 4 and 8 cycles) and two DW intensities (low intensity, Q1; high intensity, Q2). During a 56-day incubation, aggregate distributions, mean water diameter (MWD), REO concentrations, the contents of SOC and its fractions (particulate organic carbon, POC; mineral-associated organic carbon, MOC) were measured. The results showed that MWD markedly increased with DW frequencies regardless of DW intensities and was significantly related to the relative changes of 2-0.25 mm aggregates (P < 0.05). Based on transformation paths, DW cycles stimulated the breakdown of > 0.25 mm aggregates and the aggregation of < 0.053 mm aggregates, thus leading to the increasing formation of 0.25-0.053 mm aggregates. Soil aggregate turnover time was affected by DW intensities and extended with DW frequencies (P < 0.05). Soil aggregate turnover time was displayed in the order of < 0.25 mm aggregates < 2-0.25 mm aggregates < 5-2 mm aggregates. MOC concentration exhibited an increasing trend with DW frequencies. A higher loss of SOC under high DW intensity resulted from more exposure of POC through soil aggregate breakdown. POC concentration was sensitive most to the 2-0.25 mm aggregates turnover time. The structural equation model (SEM) revealed that DW cycles exerted remarkable effects on MWD by impacting SOC fractions and soil aggregate transformations. Overall, our findings suggested that soil aggregate breakdown and formation processes, rather than aggregate size distributions, were more of a concern when evaluating the effects of DW cycles on soil structural stability.