Carbon Dynamics of Agroforest Systems in the Lower Mississippi Alluvial Valley.

Agroforest management systems that integrate switchgrass and trees have the potential to yield high cellulosic biomass production along with a suite of other ecosystem services.  When managed as biofuel feedstocks, switchgrass (Panicum virgatum L.) and eastern cottonwood (Populus deltoides L.) have high potential for sequestering carbon (C) due to their high growth rates, minimal resource requirements, and ability to grow on marginal lands.  In 2009, we established a long-term agroforest study at three locations in the Lower Mississippi Alluvial Valley in order to compare C flux and sequestration rates associated with switchgrass, and eastern cottonwood relative to those in conventional row cropping systems (i.e., soybean-sorghum rotation) grown on marginal soils.  The three years since establishment, we have quantified net total above- and below-ground biomass C, seasonal soil CO₂ flux, as well as C, N, and labile C in soil.  In the third year, aboveground biomass production of the switchgrass and cottonwood was as great as 7.20 and 3.23 dry tons/acre, respectively—each considerably higher than the conventional soybean row crop (1.8 dry tons/acre).  Spring CO₂ flux rates were greater in the conventional row and switchgrass crops than in the cottonwood, suggesting that cottonwood systems may be better able to lower soil CO₂ emissions.  An assessment of these cropping systems on plant-soil carbon dynamics will be presented as a synthesis of spring and summer 2012 soil CO₂ flux rates and their relation to soil and biomass C, and nitrogen pools to better understand the long-term impact of these cropping systems on ecosystem services.