Switchgrass and soil carbon sequestration response to ammonium nitrate, manure, and harvest frequency on conservation reserve program land

Conservation Reserve Program (CRP) land on which perennial warm-season grasses are grown could be a resource for bioenergy feedstock production and C sequestration. A 4-yr field experiment was conducted to determine the response of switchgrass (Panicum virgatum L.) and soil C sequestration to N fertility and harvest frequency on switchgrass-dominated CRP land in eastern South Dakota. Soil at the site is an Egan silty clay loam. Three N rates (0, 112, and 224 kg ha(-1)) were applied as NH4NO3 (NH4NO3-N) and cattle (Bos taurus L.) manure (manure-N). Switchgrass was harvested at anthesis every year (EY) or alternate years (AY) from 2001 to 2004. Soil samples were collected before starting the experiment (fall 2000) and after 4 yr (fall 2004) to determine C sequestration. Averaged across N rate, the proportion of switchgrass was higher with manure-N (64.7%) than NH4NO3-N (46.8%). Total (switchgrass plus other herbaceous material) biomass production tended to be higher when harvested EY (average 5.0 Mg ha(-1) yr(-1)) compared with AY (average 4.0 Mg ha(-1) yr(-1)). However, by 2004, the proportion of switchgrass was 75% higher in plots harvested AY compared with those harvested EY. The concentration of structural components was greater in biomass harvested AY, whereas total N and ash tended to be lower. Total-N and ash concentrations in biomass were higher with NH4NO3-N than manure-N. Soil C was sequestered at a rate of 2.4 +/- 0.9 and 4.0 +/- 1.0 Mg C ha(-1) yr(-1) at the 0- to 90-cm depth with NH4NO3-N and manure-N, respectively. There were no changes in soil organic C without N fertilization. Manure could be used as an alternate N source for switchgrass biomass production on CRP land with an added benefit of increased C sequestration.