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 (theta) and matric potential (psi(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 theta values at a specific psi(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 theta 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 theta-psi(m) relationships following tillage are dynamic.