Hydraulic Properties Affected by Topsoil Thickness in Switchgrass and Corn-Soybean Cropping Systems

Loss of productive topsoil by erosion with time can reduce the productive capacity of soil and can significantly affect soil physical and hydraulic properties. This study evaluated the effects of reduced topsoil thickness and perennial switchgrass (Panicum virgatum L.) vs. a corn (Zea mays L.)-soy-bean [Glycine max (L.) Merr.] rotation on soil bulk density (rho(b)), saturated hydraulic conductivity (K-sat), soil water retention, and pore size distributions. The experiment was conducted at the University of Missouri South Farm on a Mexico silt loam (a Vertic Epiaqualf). Plots with the corn-soybean rotation and switchgrass were established in 2009 with four replicates. Twenty-seven years after establishment, plots with selected topsoil thickness were grouped into four treatments (TopA [4 cm], TopB [11 cm], TopC [22 cm], and TopD [36 cm]). Undisturbed soil cores, 76 by 76 mm, were collected by 10- cm depth increments from the surface to the 40-cm depth. Results showed that soil under switchgrass had 53 and 27% higher macroporosity (>1000- mu m diameter) and coarse mesoporosity (60-1000-mu m diameter), respectively, than row crop management. This led to 73% greater K-sat and 11% higher water content at saturation than row crop management. Thicker topsoil treatments (TopC and TopD) had consistently lower rho(b) and greater K-sat than the shallow topsoil treatments (TopA and TopB) for all sampling depths. These results imply that switchgrass production systems significantly improve soil hydraulic properties, particularly in eroded claypan landscapes.