Background Soil aggregate stability is an important indicator of soil quality and susceptibility to runoff and erosion, and acid rain pollution is a serious environmental problem in Chongqing city. The change of soil physicochemical properties caused by acid rain will affect aggregate stability. In this study, the yellow soil on slope farmland in Chongqing city was used as the research object, and the effects of acid rain wetting on aggregates stability was explored.

Methods Four initial aggregate size classes (>5-7, >3-5, >2-3 and ≥1-2 mm) were slowly capillary-wetted with three levels of simulated acid rain (treatment T_pH1 with acid water pH=1; treatment T_pH3 with acid water pH=3; treatment T_pH5 with acid water pH=5). Soil aggregate stability was determined according to Le Bissonnais to distinguish three breakdown mechanisms: slaking, mechanical breakdown by raindrop impact and disaggregation by differential swelling, and expressed as the mean weight diameter (MWD). During the experiment, the stability of aggregates was measured with three treatment methods involving fast wetting (FW), shaking after pre-wetting (WS), and slow wetting (SW) every 5 days. The MWD of aggregates after drying-wetting cycle with deionized water was taken as background value.

Results 1) Slaking was the main mechanism of aggregate breakdown. 2) Compared with background value, for T_pH1, T_pH3, and T_pH5 treatment, the average change rates of MWD of fast wetting (MWD_FW) were 1.99, 1.30, and 1.43, respectively; the average change rates of MWD of shaking after pre-wetting (MWD_WS) were 1.03, 1.22, and 1.19, respectively; the average change rates of MWD of slow wetting (MWD_SW) were 1.48, 0.78, and 0.88, respectively. 3) The average change rates of MWD for treatment T_pH1, T_pH3, and T_pH5 were 1.50, 1.10, and 1.16, respectively. There was no significant difference in the change of aggregate stability between treatment T_pH3 and T_pH5. But for treatment T_pH1, aggregate stability changed much more than that of T_pH3 and T_pH5. 4) Acid rain wetting promoted < 0.1 mm fractions to flocculate. For treatment T_pH3 and T_pH5, the average change rates of >2 mm proportion in >5-7, >3-5, and >2-3 mm aggregates were 0.87, 0.98, and 0.23, respectively, the >2 mm proportion in >2-3 mm were significantly reduced. The difference of initial aggregate size classes led to different responses to acid rain.

Conclusions For three breaking mechanisms, the influence of acid rain wetting on the aggregate stability is different. Acid rain wetting significantly enhances resistance to slaking of aggregates. In general, aggregate stability increases after being wetted by acid rain, but at the cost of soil acidification. The results have certain reference value for soil and water conservation in acid rain area.