Switchgrass Influences On Soil Carbon Pools along a Productivity Gradient in Pacific Northwest, USA.

Switchgrass (Panicum virgatum L.) has been identified as an herbaceous energy crop for the US. Significant impact of growing biofuel crops will be the sequestration or release of soil carbon (C). Influence of switchgrass on belowground C dynamics was compared with continuous wheat and wheat-mustard rotation along a productivity gradient, developed with varied water and nitrogen to achieve target yields in an experiment near Pendleton, Oregon.  In 2008, the strip-split plot experiment consisted of three productivity regimes (high, medium and low) at main plot, four sub plots of switchgrass, polyculture of five warm season grasses, continuous wheat, wheat-mustard, and sub-sub plots of residue treatments (removed/retained) with four replications randomized at sub-plot level. Switchgrass biomass increased from 5.64, 8.65, and 12.59 Mg ha^-1 in order of low, medium, and high productivity regimes. Soil C varied with crop and residue treatment within high and medium productivity regimes at the 0-15 cm depth increment. Soil under continuous wheat had greater C (26.2 Mg ha^-1) than switchgrass (21.0 Mg ha^-1) when residue was retained in the high productivity regime. Residue removal had a strong effect on soil C under continuous wheat in high and medium productivity regimes. Our initial results indicate a strong interaction among productivity level, residue removal, and crop biomass.  Long-term trials are needed at multiple locations before switchgrass can be introduced in the Pacific Northwest as biofuel feedstock.