Assessing the Nitrous Oxide Mole Fraction of Soils From Biofuel and Corn-Soybean Fields.

Measurement of nitrous oxide (N₂O) and dinitrogen (N₂) during the denitrification process in soils helps estimate the contribution of soil N₂O emission to global warming. However, little is known about how biofuel production might alter soil organic matter and microbial activity, leading to differences in N gas emissions. We conducted a laboratory incubation of soils from three biofuel trial sites located in southern (Alfisol), central (Mollisol) and northern Illinois (Mollisol), which were established in 2002. Soils from unfertilized plots of established biofuel crops of Miscanthus and switchgrass were compared to fertilized corn-soybean plots. Nitrous oxide and N₂ production were measured using the acetylene inhibition technique to allow calculation of the N₂O mole fraction. Miscanthus and switchgrass soils had higher total denitrification compared to corn-soybean soils in our laboratory assay. The mean N₂O mole fraction (N₂O:(N₂O+N₂)) was significantly higher for corn-soybean (0.83) than that for Miscanthus and switchgrass (0.48 and 0.45, respectively) at the Alfisol site, but there was no significant difference among crops for the Mollisol sites. The preliminary results indicated that row crops might produce greater portion of N₂O compared to the biofuel crops in the denitrification process and that soil type could be one of the factors controlling the mole fraction of N₂O. Increases in soil organic matter content and changes in quality from biofuel production may be responsible for changing the N₂O mole fraction during denitrification. These data are needed to help improve estimation of the contribution of soil N₂O emission to global warming as cropping systems change.