Switchgrass and Soil Carbon Sequestration Response to Ammonium Nitrate, Manure, and Harvest Frequency on Conservation Reserve Program Land

Abstract

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.