Hydrological stress regimes regulate effects of binding agents on soil aggregate stability in the riparian zones

Aggregate stability, playing a key role in resistance to soil erosion, has been greatly impacted by hydrological stresses. Yet, the mechanism of soil aggregate stabilization has been rarely clarified under hydrological stress regimes. In this study, soils across the water-level-fluctuating riparian zone of the Three Gorges Reservoir (TGR) were investigated to identify how the hydrological stresses regulate the binding agents of soil aggregate stability. Water-stable aggregates (large macro-aggregates: > 2 mm, small macro-aggregates: 0.25-2 mm, micro-aggregates: 0.053-0.25 mm, and silt and clay: 0-0.053 mm) were fractionated by the method of wet sieving. Organic and inorganic binding agents were represented by organic carbon fractions (light free organic carbon, coarse particulate organic carbon, fine particulate organic carbon, and mineral-associated organic carbon) and oxides (free state of iron, aluminum, manganese, silicon oxides, amorphous iron, aluminum, manganese, and silicon oxides), respectively. Our results revealed that soil properties in the riparian zone of the TGR were modified under the hydrological stresses induced by around 10 years' impoundment of the TGR. Soil organic carbon, cation exchange capacity, iron, manganese and silicon oxides were all engaged in mediating the impact of hydrological stresses on aggregate stability. At the strong hydrological stress level, around 48% of variations of the stability index were explained by inorganic factors, among which, the oxides and cation exchange capacity, explaining both 13% of variations, played a more important role in soil aggregation than organic carbon fraction that explained only 2% of variations. Interestingly, the strongest aggregate stability was found at the intermediate hydrological stress level for the Anthrosols, Luvisols, and Regosols with mean weight diameter (MWD) at 1.10, 0.90, and 0.85 mm, respectively. Organic carbon, explaining more than half of variations of the MWD, was the main binding agent for soil aggregation. At the weak and without hydrological stress levels, totally, less than one third of the variations (20% and 15% for the weak stress level and non-stress, respectively) in the linear models, and free silicon oxides played a positive role in stabilizing aggregates. By implication, the eroded soil restoration and reinforcement should consider different soil types and hydrological stress levels in the riparian zone.