Assessing the nitrous oxide mole fraction of soils from perennial biofuel and corn–soybean fields

Little is known about how long-term biofuel production might alter soil nitrogen (N) gas emissions. We conducted a laboratory incubation of surface soils (0–16 cm) from perennial biofuel trial plots (established 2002) at sites in Northern (Dekalb, Mollisols), Central (Urbana, Mollisols), and Southern (Dixon Springs, Alfisols) Illinois, USA. Soils from unfertilized plots of Miscanthus (Miscanthus × giganteus) and switchgrass (Panicum virgatum) were compared to fertilized corn–soybean plots during early spring and again in mid-summer. Fresh soils were packed into jars at a bulk density of 1.2 g cm−3 and adjusted to a water-filled pore space of 85%. We added about 10 mg NO3-N kg−1 dry soil in each sample, incubated for 24 h, and collected gas samples at 0, 1, 2, and 4 h to measure production of N2O and N2 using a C2H2 inhibition technique, which allowed calculation of the N2O mole fraction (N2O:(N2O + N2)). The mean N2O mole fraction (MF) was significantly higher for the cropped plot (0.83 and 0.99) than that for Miscanthus (0.48 and 0.31) and switchgrass (0.45 and 0.22) plots at the Southern site in spring and summer, respectively. There were no significant differences in N2O MF among treatment plots for the Central and Northern sites. Exchangeable soil nitrate concentrations best explained the N2O MFs for all treatments in both seasons, and production of perennial biofuel feedstock crops did not exhibit an apparent influence on N2O MFs. It appeared that soil type combined with fertilizer additions were the major factors controlling the MF of N2O in our fields, and was much more important than the crop grown or any new soil C added.