Temporal Changes of Soil Water Retention Behavior as Affected by Wetting and Drying Following Tillage

Soil structure‐dependent properties are subject to changes with time and depth under the influences of agronomic practices and environmental factors. Wetting and drying (W/D) cycles following tillage alter the structure and pore‐related functions of field soils. This study investigates the effects of W/D cycles on temporal changes of soil porosity and water retention curves (SWRCs) of a tilled layer (0‐ to 15‐cm) during a post‐tillage period. Soil volumetric water content (θ) and matric potential (ψ m ) dynamics at three depths were monitored continuously, and the total porosity ( P t ) was determined gravimetrically. Nine W/D cycles were identified in the experimental period. Soil P t decreased gradually with time and depth and became relatively stable after four W/D cycles, and the SWRCs shifted toward higher θ values at a specific ψ m . The magnitude of SWRC change was relatively small during the first to third W/D cycles when the degrees of saturation were relatively low, reached the maximum after the fourth W/D cycle during which the soil was nearly saturated, and became less significant thereafter. Soil water holding capacity was improved during the W/D processes mainly due to the reduction of effective porosity and development of residual porosity. During the earlier four W/D cycles, a higher initial θ also contributed to the increase of the residual porosity, and thus enhanced the soil water holding capacity. The difference, however, tended to disappear in later W/D cycles. The data show clearly that the θ–ψ m relationships following tillage are dynamic.