Soil type-dependent effects of drying-wetting sequences on aggregates and their associated OC and N

Evaluating the impacts of drying-wetting (DW) cycles on soil aggregates and their associated organic carbon (OC) and nitrogen (N) is crucial to understand the OC and N cycles. Soils are likely subjected to DW cycles with different sequences depending on seasons or in agroecosystems. However, studies on how DW sequences influence OC and N dynamics within aggregates, and whether this effect is dependent on soil type, are relatively limited. Herein, two DW sequences, i.e., drying-wetting-drying-wetting (2DW) and wetting-drying-wetting-drying (2WD) treatments were designed, and a consistent wetting (CW) was set as a control to assess the effects of DW sequences. Four soils (Entisol, Ultisol, Anthrosol and Mollisol) varying in texture and OC content were used. The aggregate size distribution, the OC, total N (TN), readily oxidizable OC (ROOC) and mineral N (Min-N) content in aggregates were determined. Results showed that 2DW treatment increased but 2WD treatment decreased the large aggregates of Entisol and Ultisol, while 2DW and 2WD treatments synchronously increased the large aggregates of Anthrosol but decreased them of Mollisol. Two DW treatments increased the OC in each aggregate of Entisol, Anthrosol and Mollisol but decreased them of Ultisol. The 2DW didn't affect but 2WD treatment decreased ROOC in 1–2 mm aggregates of Entisol and <0.25 mm aggregates of Ultisol and Mollisol. The 2DW and 2WD treatments minimally affected TN but potentially influenced Min-N in aggregates. The 2DW and 2WD treatments both decreased the Min-N in each aggregate size class of Entisol and Mollisol, while 2DW increased but 2WD treatment decreased Min-N in each aggregate of Ultisol and Anthrosol. These results indicated that the varied effects of DW sequences and the interactive effects of soil type with DW sequences on aggregate turnover and OC and N cycling should be reconsidered to provide more precisive evidences for global C and N cycles under the scenario of future climate changes.