Big Bluestem and Switchgrass Feedstock Harvest Timing: Nitrous Oxide Response to Feedstock Harvest Timing.

Switchgrass (Panicum virgatum L.) and big bluestem (Andropogon gerdardii Vitman) are potential bioenergy feedstocks. Feedstock storage limitations, labor constraints for harvest, and environmental benefits provided by perennials are rationales for developing localized perennial feedstock as an alternative or in conjunction with annual feedstocks (i.e., crop residues).   The objectives of this study are to compare the feedstock quantity and quality of switchgrass and big bluestem harvested in the fall or delayed until the following spring.  Furthermore to assess the impact of harvest on nitrous oxide emission. It was hypothesized that biomass yield may decline, but the feedstock quality would increase due to leaching of minerals.  Yields differed among years, but not by harvest timing; both grasses averaged 5.7 Mg ha^-1 yr^-1.   Cumulative nitrous oxide emission was 14 to 40% greater when feedstock was harvested in the spring compared to fall harvest for big bluestem and switchgrass, respectively.  The majority of emission was associated with ephemeral hot moments.  The potentially harvestable material did not decline between the two sample time, potassium and chloride content in the feedstock declined over the winter.