Soil Carbon Dynamics During the Transition to Cellulosic Ethanol Feedstock Production.

Residue removal of bio-energy crops has potential for dramatic consequences for soil processes. This study quantifies effects on soil organic carbon dynamics driven by differences in feedstock crops, net primary productivity (NPP), and residue removal. Switchgrass (Panicum virgatum L.), wheat (Triticum aestivum L.), and corn (Zea mays L.) surface residues grown at three NPP were left or completely removed. High, medium, and low NPP was achieved by irrigation, fertilizer, and seeding rate management. Yield data, soil organic carbon mineralization potential, acid hydrolysis, total carbon, and isotopic carbon standard method analyses were used to identify dynamics of organic soil carbon. The study site was at Prosser, WA, on a Warden silt loam (Coarse-silty, mixed, superactive, mesic Xeric Haplocambids). In 2008, the first year transition to cellulosic ethanol feedstock production there were differences between crop and residue removal at NPP levels. Within crop low NPP production accounted for greater mineralizable carbon for switchgrass and corn but not in wheat. Switchgrass had overall greater contribution to mineralizable carbon. Direct carbon contributions by switchgrass are verified by enrichment in carbon 13 over that of wheat soil. Increased yield associated with high NPP also translates into greater soil organic carbon addition. Switchgrass production has the potential for soil organic carbon enrichment and higher yields than corn or wheat in all NPP production systems.