Soil Carbon Storage by Switchgrass Grown for Bioenergy

Abstract

Life-cycle assessments (LCAs) of switchgrass (Panicum virgatum L.) grown for bioenergy production require data on soil organic carbon (SOC) change and harvested C yields to accurately estimate net greenhouse gas (GHG) emissions. To date, nearly all information on SOC change under switchgrass has been based on modeled assumptions or small plot research, both of which do not take into account spatial variability within or across sites for an agro-ecoregion. To address this need, we measured change in SOC and harvested C yield for switchgrass fields on ten farms in the central and northern Great Plains, USA (930 km latitudinal range). Change in SOC was determined by collecting multiple soil samples in transects across the fields prior to planting switchgrass and again 5 years later after switchgrass had been grown and managed as a bioenergy crop. Harvested aboveground C averaged 2.5 0.7 Mg C ha1 over the 5 year study. Across sites, SOC increased significantly at 030 cm (P=0.03) and 0120 cm (P=0.07), with accrual rates of 1.1 and 2.9 Mg C ha1 year1 (4.0 and 10.6 Mg CO2 ha1 year1), respectively. Change in SOC across sites varied considerably, however, ranging from 0.6 to 4.3 Mg C ha1 year1 for the 030 cm depth. Such variation in SOC change must be taken into consideration in LCAs. Net GHG emissions from bioenergy crops vary in space and time. Such variation, coupled with an increased reliance on agriculture for energy production, underscores the need for long-term environmental monitor- ing sites in major agro-ecoregions