Soil Organic Matter from Southern Pine Biofuel Feedstocks Under Different Soil Types and Management Systems.

The impacts of land use and management on soil carbon (C) loss or accumulation can influence the net C footprint of biofuel feedstocks. The use of pine plantations to produce biofuels, using pre-commercial thinnings instead of commercial timber, has the potential to deliver sustainable biofuels. However, the removal of additional biomass from these plantations could reduce C inputs belowground and therefore overall ecosystem C storage. There is a need to understand the impacts of this change in management on soil C to determine the overall life cycle analyses. This study analyzes soil C stocks to estimate changes in soil C stocks, as a function of soil type and different southern pine plantation management systems. We hypothesized that plantations with heavier pre-commercial thinning and a more intensive silvicultural regime will have lower soil C stocks than plantations with less intensive overall management, especially in plantations with well drained soils compared to poorly drained soils. We specifically expected to see these differences in soil fractions that are not mineral-associated (i.e. light fraction and particulate organic matter). In order to test these hypotheses, we sampled soils from 12 different southern pine plantation sites, representing 4 different soil types. Within each pine plantation site we took ~1 m deep soil cores at 6 different experimental plots, representing 2 silvicultural regimes across 3 levels of pre-commercial thinning each. Each soil core was separated by depth (0-15, 15-30, 30-50, 50-100cm) and the 0-15cm depth was fractionated into 4 fractions (dissolved, light, particulate, and mineral-associated). These fractions are meaningful because they inform about how much soil organic matter is readily bioavailable and how much is stabilized. We will present our initial soil C stock assessments from this study.