Switchgrass Affects On Soil Property Changes in the Great Plains.

The capacity of perennial grasses to affect change in soil properties is well documented but soil property information on switchgrass managed for bioenergy is limited.  Potential improvements in near-surface soil function are important should switchgrass be included as a perennial phase within existing cropping systems.  An on-farm study (10 fields) located in North Dakota, South Dakota, and Nebraska was sampled using transects across fields prior to switchgrass establishment and after five years to determine changes in soil bulk density, pH, soil P, and equivalent mass soil organic carbon (SOC).  Changes in soil bulk density were largely constrained to near-surface depths (0 to 0.05 m) where plant biomass inputs and management influences are greatest.  Soil bulk density increased (0 to 0.05 m) at the Nebraska locations (Mean = 0.16 Mg m^-3), while most South Dakota and North Dakota locations showed declines in soil bulk density (Mean = -0.18 Mg m^-3; Range = -0.42 to 0.07 Mg m^-3).  Soil pH change was significant at five of the 10 locations at near surface depths (0 to 0.05 m), but absolute changes were modest (Range = -0.67 to 0.44).   Available phosphorous (P) declined at all sites where it was measured (North Dakota and South Dakota locations), with decreases most prevalent at the 0 to 0.05 m depth.  When summed across the surface 0.3 m depth, annual decreases in available P averaged 1.5 kg P ha^-1 yr^-1 (Range = 0.5 to 2.8 kg P ha^-1 yr^-1).  Averaged across locations, equivalent mass SOC increased by 0.5 Mg C ha^-1 yr^-1 and 2.4 Mg C ha^-1 yr^-1 for the 2500 Mg ha^-1 and 10000 Mg ha^-1 soil masses, respectively.  Results from this study underscore the contribution of switchgrass to affect changes in soil properties over time, though considerable variation in soil properties exists within and across locations.  Further long-term evaluations of soil property changes under perennial bioenergy systems are needed at multiple ecoregions and spatial scales.