Soil hydraulic properties of cropland compared with reestablished and native grassland

Abstract

Conversion of cropland to perennial grasses will, over time, produce changes in soil hydraulic properties. The objective of this study was to characterize and compare hydraulic properties of fine-textured soils on adjacent native grassland, recently tilled cropland, and reestablished grassland in the Conservation Reserve Program (CRP) at three locations in the Southern Great Plains. A tension infiltrometer was used to measure unconfined, unsaturated infiltration over a range of supply pressure heads (nominally, h=−150, −100, −50, and −5 mm H 2O) at the soil surface. Intact soil cores were sampled within the Ap and Bt horizons to determine bulk density and water desorption curves, θ( h), at potentials ranging from −0.15 to −100 kPa. Unsaturated hydraulic conductivity K( h) over the range in supply pressure heads was estimated using Wooding's equation for steady-state flow from a disc source. The van Genuchten water retention model was fitted to θ( h) data to estimate parameter values. Soils in CRP had greater surface bulk densities than their grassland and cropland counterparts. The shape of the soil water retention curve for grassland and CRP land were similar, suggesting that converted croplands had fully reconsolidated. Mean near-saturated hydraulic conductivities of cropland at h=−5 mm were not significantly different from grassland. However, at −150 mm supply pressure head, cropped soils had a mean unsaturated conductivity 2.3 and 4.1 times greater than CRP land and grassland, respectively. Sites in CRP had the lowest ( P<0.05) near-saturated hydraulic conductivities ( h=−5 mm), which suggest that after 10 years, grasses had not fully ameliorated changes in pore structure caused by tillage. Comparison of unsaturated conductivities for grassland and CRP land suggest that long-term structural development on native grasslands was principally confined to effective pore radii greater than 300 μm. Land use practices had a greater effect on water movement than did soil series, indicating that the modifying effects of tillage, reconsolidation, and pore structure evolution on hydraulic properties are important processes governing water movement in these fine-textured soils.