Near-Surface Soil Property Responses to Perennial Forages in a Semiarid Region.

Enhancement of near-surface soil properties is essential to minimize erosion and conserve nutrients and water.  Perennial forages can serve to enhance near-surface soil properties, yet there is limited information regarding soil responses to different perennial crop types and phase lengths in semiarid regions.  This study was conducted to evaluate effects of five perennial forages [alfalfa (Medicago spp.), intermediate wheatgrass (Thinopyrum intermedium (Host) Barkw. & D.R. Dewey subsp. intermedium), switchgrass (Panicum virgatum L.), intermediate wheatgrass-alfalfa mixture, switchgrass-alfalfa mixture] and annual cropping [continuous wheat (Triticum aestivum L.)] on near-surface soil properties.  The study was located approximately 1 km southwest of Mandan, ND USA on a Parshall fine sandy loam (coarse-loamy, mixed, superactive, frigid Pachic Haplustoll).  Soil bulk density, pH, water-stable aggregates (WSA), C:N ratio, and particulate organic matter (POM) were affected by perennial forages within the surface 15 cm.  Intermediate wheatgrass and the intermediate wheatgrass-alfalfa mixture possessed lower soil bulk density at 0-5 cm compared alfalfa and annual cropping (Mean = 1.39 vs. 1.48 Mg m^-3, respectively).  Soil pH at 0-5 cm was significantly lower under annual cropping (5.7) compared to perennial forages (Range = 6.2 to 6.7) (P=0.0116).  Phase length affected water-stable aggregates (WSA) at 0-15 cm (P=0.0139), with significantly greater WSA in perennial forages maintained for three to five years compared to forages maintained for two years.  Soil C:N ratio at 0-5 cm was lowest in alfalfa (9.4), intermediate in the switchgrass-alfalfa mixture (9.6), and greatest in annual cropping, switchgrass, intermediate wheatgrass, and the intermediate wheatgrass-alfalfa mixture (Mean = 10.2) (P=0.0300).  Particulate organic matter C at 5-10 cm was greater in the intermediate wheatgrass-alfalfa mixture compared to annual cropping and other perennial forages (P=0.0050).  Results from this study suggest perennial forages can improve near-surface soil properties, though accrual of such improvements may require phase lengths of three or more years under semiarid conditions.