Carbon sequestration by switchgrass

Increasing levels of carbon dioxide (CO2), which is partly due to use of fossil fuel, is primarily responsible for global climate warming. Producing and using switchgrass for bioenergy can help reduce atmospheric CO2 buildup by partly replacing use of fossil fuels and by carbon (C) sequestration. Switchgrass (Panicum virgatum L) is a potential bioenergy crop suited to the southeastern U.S. The objective of this study was to determine the effects of agricultural management practices on C sequestration by switchgrass. Field experiments were designed so that differences in row spacing, nitrogen (N) rate, switchgrass cultivar, harvest frequency, and soil type on C sequestration would be evaluated. Soil C dynamic studies indicated that soil C mineralization, microbial biomass C, and C turnover tended to increase with time after switchgrass establishment in Norfolk sandy sod. These changes were more apparent in 0 to 15 cm than 15 to 30 cm of the sandy loam soil. Ten years of continuous switchgrass resulted in higher soil C level than nearby fallow soils, but several years of continuous grass may be need before increases are measurable. Results from this study imply that management practices can impact soil C sequestration with switchgrass, such as several years for humification by conversion of the root accumulation to the stable soil C pool. The effect of N was to increase N but not C concentration of roots, which imply that any increases in C sequestration by switchgrass would be due to increases in root biomass. Switchgrass roots were more dense in Pacolet clay soil than the other soils used in this study. Carbon storage in switchgrass, shoots increased as row width and N rate increased. Carbon storage in shoots and roots generally increased with time after switchgrass establishment, and rate of increase of C storage in root was higher than that in shoot. Carbon partitioning analyses showed that C storage was soil C > root C > shoot C. The root/shoot ratio of C storage was 2.2, and this implied that C partitioning to roots plays a key role in C sequestration by switchgrass. Carbon storage in the overall switchgrass-soil system showed an upward trend after switchgrass establishment.