Field‐scale soil property changes under switchgrass managed for bioenergy

The capacity of perennial grasses to affect change in soil properties is well documented but information on switchgrass (Panicum virgatum L.) managed for bioenergy is limited. An on‐farm study (10 fields) in North Dakota, South Dakota, and Nebraska was sampled before switchgrass establishment and after 5 years to determine changes in soil bulk density (SBD), pH, soil phosphorus (P), and equivalent mass soil organic carbon (SOC). Changes in SBD were largely constrained to near‐surface depths (0–0.05 m). SBD increased (0–0.05 m) at the Nebraska locations (mean=0.16 Mg m−3), while most South Dakota and North Dakota locations showed declines in SBD (mean=−0.18 Mg m−3; range=−0.42–0.07 Mg m−3). Soil pH change was significant at five of the 10 locations at near surface depths (0–0.05 m), but absolute changes were modest (range=−0.67–0.44 pH units). Available P declined at all sites where it was measured (North Dakota and South Dakota locations). When summed across the surface 0.3 m depth, annual decreases in available P averaged 1.5 kg P ha−1 yr−1 (range=0.5–2.8 kg P ha−1 yr−1). Averaged across locations, equivalent mass SOC increased by 0.5 and 2.4 Mg C ha−1 yr−1 for the 2500 and 10 000 Mg ha−1 soil masses, respectively. Results from this study underscore the contribution of switchgrass to affect soil property changes, though considerable variation in soil properties exists within and across locations.