Nitrogen and Carbon of Switchgrass, Loblolly Pine, and Cottonwood Biofuel Production Systems in the Southeast United States

Abstract

Switchgrass has favorable characteristics as a biofuel feedstock, and it may be feasibly grown on sites currently forested or retired from agricultural production. It is ecologically important to understand the impacts of establishing and managing switchgrass on site nutrient cycling. Four studies conducted in the southeast United States tested switchgrass as a bioenergy crop grown in monoculture, in alley cropping systems, and as an alternative to conventional agricultural rotations. Each trial included comparisons of switchgrass with the prevailing land uses. In each trial, nitrogen mineralization (Nmin), nitrification (Nnit), microbial biomass carbon (Cmic), microbial activity, and labile carbon were measured because of their sensitivity to management practices that alter soil nutrient cycling. In all studies of loblolly pine and switchgrass, switchgrass was apparently the dominant substrate source for C mic , and N min increased when juvenile loblolly pine was isolated with herbicide as the sole vegetation. However, Cmic and Nmin of loblolly pine-switchgrass alley cropping systems were similar to those observed in conventional juvenile loblolly pine plantation conditions. In a trial of converting former agricultural fields into switchgrass or cottonwood biofuel plantations, this conversion reduced Nnit, and therefore the potential for nitrogen losses compared with those for a soybean-sorghum rotation typical for such sites.