Effects of ridge tillage on physical crust pore characteristics and soil infiltration in the black soil region

To investigate the effects of ridge tillage on physical crust pore development characteristics and soil infiltration in the black soil region of Northeast China, this study took crusted soil formed under non-ridging and the structural crust and sedimentary crust soil under two ridge directions (contour ridge tillage and downslope ridge tillage) as research objects. Sampling and in-situ experiments were conducted in May and September 2023, respectively. The changes in soil micro-pores caused by crust development were systematically analysed through CT scanning technology and in-situ infiltration experiments, and the relationship between crust characteristics and infiltration was quantified. The results showed that ridge tillage leads to the differentiation of crust characteristics between ridges and furrows. Contour ridge tillage promoted the development of sedimentary crust, and the proportion of water-stable aggregates in the furrows increased significantly. Longitudinal ridge tillage intensified the sorting effect of runoff on soil particles. The porosity of the crust layer under contour ridge tillage increased with time and the pore size gradually approached. The porosity showed an increasing trend from the crust surface to the subsoil layer. The soil pores of the crust under longitudinal ridge tillage showed a double closure phenomenon, the number of throats in the sedimentary crust decreased significantly while more isolated pores were formed. Under this condition, the stronger concentrated runoff in the furrows drastically reduces sediment particles, resulting in the sedimentary crust being thinner than that formed under contour ridge tillage. The large pore of the crust layer was better preserved under non-ridging conditions. The average infiltration rate of crust soil ranked as non-ridging (0.58 mm/min) > contour ridge tillage (0.15 mm/min) > longitudinal ridge tillage (0.11 mm/min). Ridge tillage mainly regulates soil infiltration by forming different types of crusts. After the formation of crusts, the soil water-holding capacity increases, allowing the soil more rapidly to reach saturation. The high proportion of capillary pores in crusted soil prolongs water retention time, leading to a significant decline in the infiltration of crusted soil. The initial infiltration rate is mainly controlled by the bulk density of the surface crust and subsoil, while the steady infiltration rate is primarily dominated by the pore structure influenced by crust thickness and water-stable aggregates. The formation of different types of crusts induced by ridge tillage is the main pathway affecting the infiltration rate. Under contour ridge tillage conditions, the pore networks are more developed. By improving soil porosity, the soil infiltration capacity is correspondingly enhanced. The bulk density and saturated water holding capacity of the subsoil, as well as the WSA_{> 0.25} and thickness of the crust layer, can effectively predict the initial infiltration rate and steady infiltration rate of crusted soil, respectively. The results can provide important theoretical support for reasonable ridge tillage practices to reduce soil erosion.