Assessing the N 2 O mole fraction of soils from biofuel and corn-soybean fields Krishna Woli 1, Mark B. David 1, Corey A. Mitchell 1, Candice M. Smith 2, and Robert G. Darmody 1 1 Natural Resources and Environmental Sciences, Univ. of Illinois at Urbana-Champaign, Urbana, IL 2 Institute for Genomic Biology, Univ. of Illinois at Urbana-Champaign, Urbana, IL Southern (Dixon Springs) site corn-corn rotation soil: Grantsburg, fine-silty, mixed, active, mesic Oxyaquic Fragiudalf Central (Urbana) site corn-soybean rotation soil: Drummer, fine-silty, mixed, superactive, mesic Typic Endoaquoll Northern (DeKalb) site corn-corn rotation soil: Flanagan, fine, smectitic, mesic Aquic Argiudoll INTRODUCTION Measurement of nitrous oxide (N 2 O) and dinitrogen (N 2 ) during the denitrification process in soils helps estimate the contribution of soil N 2 O emissions to global warming. The lower the N 2 O mole fraction (N 2 O:( N 2 O+N 2 )), the less impact reduction of nitrate has on 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 and N 2 O mole fractions. STUDY SITES AND METHODS Spring Summer Mean N 2 O mole fraction (N 2 O:(N 2 O+N 2 ) Spring Summer N gas production (mg N kg soil -1 day -1 ) There was no variation in soil C concentrations due to treatment at the Northern site, whereas both Miscanthus and switchgrass plots at the Central and Southern sites had increased C concentrations compared to cropped soils. Soil C concentration was not as strongly related to the N 2 O mole fraction as we expected. The availability of nitrate for microbial reduction seemed to affect the N 2 O mole fraction. Both N 2 O production and the mole fraction for the Alfisol site (Southern) was significantly lower in biofuel crop soils compared to that in row crop soils. However, this pattern was not observed for soils from the Mollisol sites (Northern and Central). Therefore, soil type appeared to be one of the major factors controlling the mole fraction of N 2 O in response to biofuel production. Overall, our results from three sites and two seasons show a varied response of the N 2 O mole fraction dependent primarily on soil type. We expected that soil C concentrations would be increased from 7 years of biofuel crop production, given the high biomass production of Miscanthus and switchgrass (Heaton et al., 2008). However, we only found a substantial increase in soil C concentrations at the Central site, with a smaller increase at Southern and no increase at Northern sites. Although we didn’t make any measurements, we did expect that the quality of some of the surface soil C had changed due to biofuel production at all sites. RESULTS & DISCUSSION Conclusions SpringSummer Spring Summer Spring Summer N 2 O mole fraction (N 2 O:(N 2 O+N 2 )) References Exchangeable soil N concentration (mg kg soil -1 ) NH 4 -N NO 3 -NInorg. N NH 4 -N NO 3 -NInorg. N N gas production (mg kg soil -1 day -1 ) N2ON2ON2N2 N 2 O+N 2 Illinois Bergsma T.T., G.P. Robertson, and N.E. Ostrom (2002) Influence of soil moisture and land use history on denitrification end-products. Environ. Qual. 31: Heaton E.A., Dohleman F.G., Long S.P. (2008) Meeting US biofuel goals with less land: the potential of Miscanthus. Global Change Biology 14: MiscanthusSwitchgrassCropped Northern Central Southern Northern Central Southern Nitrate was added to each soil to ensure that it was not limiting for denitrification. When sampled however, soils were quite variable in exchangeable nitrate concentrations, although nitrate never was totally depleted in any treatment. As expected, fertilized corn had the highest concentrations of nitrate, and denitrification clearly reduced concentrations in many of the treatments. It is difficult to explain the low nitrate concentrations at the Southern site in the biofuel plots during the summer, but other aspects of the N cycle may have been affecting concentrations here and in other treatments (e.g., immobilization, mineralization). Research objective: Determine the effect of cultivating bioenergy crops on soil N 2 O mole fractions. We packed each soil to a bulk density of 1.2 g cm -3 in 1 L Mason jars by accounting for the moisture content of the field-moist soil in weighing out the soil to fill a specific volume. Three sets of jars were prepared for each soil sample: (i) treated with C 2 H 2 (to provide the sum of N 2 O and N 2 by inhibiting conversion of N 2 O to N 2 ) (ii) without C 2 H 2 (as a control that provided N 2 O only), and (iii) for analyzing soil exchangeable N concentrations. We added enough deionized water (with added nitrate) to each sample to reach a target volumetric water content of 85% water filled pore space, and to provide an exchangeable nitrate concentration of about 10 mg NO 3 -N kg -1 dry soil. The soil samples were then incubated overnight with the tops open after adding the N solution. We then closed the jars and added enough C 2 H 2 to one set of jars to make a 10% atmosphere for a 4 hr incubation period. Headspace gas samples were then collected at 0, 1, 2, and 4 hr of incubation using a syringe and were transferred to 10 ml vials. After two hr of incubation, 1 set of jars were destructively sampled for exchangeable NO 3 -N and NH 4 -N concentrations. Gas samples were analyzed using a Shimadzu gas chromatograph (GC-2014) with an ECD detector. The N gas flux was calculated using regression coefficients obtained from plotting N 2 O concentrations against sampling time, which was followed by calculation of the N 2 O mole fraction. N 2 flux was calculated as the difference in N 2 O production between C 2 H 2 -amended and control jars. Oven-dried soil samples were ground and analyzed for total C and N using an elemental analyzer (EAS 4010, Costech). AAA A A AA A A AAA A A A A A A A AA AA A B B A A A A A B B A A A A A A A A A A A A BB BB B B B B B Layout of jars for laboratory denitrification experiment. Switchgrass and Miscanthus trial plots at Urbana. B A AB A B A A A A A A A A A B A A B A B AA A A A B A A A AAA AA B B B A B A B BB A A A A A A B AA A A A AA A A B B A A B B A A A A A A Miscanthus Switchgrass Cropped Miscanthus Switchgrass Cropped Miscanthus Switchgrass Cropped NorthernCentralSouthern CarbonNitrogenC:N Ratio CarbonNitrogenC:N Ratio SpringSummer Soil total C-N concentration (g kg soil -1 ) and C:N ratio Northern Central Southern A A A AAA AAA AA A AAA A AA AA B A A B A A A AA B A A B AB B AAA A BB A B C AAA Miscanthus Switchgrass Cropped ABC AAA N2ON2ON2N2 N 2 O+N 2 N2N2 N2ON2O A laboratory incubation of soils from three biofuel trial sites located at Northern (DeKalb), Central (Urbana), and Southern (Dixon Springs) Illinois was conducted in early spring and during mid-summer of Soils from unfertilized plots of established biofuel crops of Miscanthus x giganteus and switchgrass that were established in 2002 were compared to fertilized cropped plots. The three plant types (Miscanthus, switchgrass, and cropped - either corn or soybean) were replicated four times, for a total of 12 plots per site. Five soil cores at different locations from a 0-16 cm depth (Bergsma et al., 2002) were taken in each plot and combined for one composite sample per plot. Soil samples were sieved field-moist (<4 mm) for all incubations, with sub-samples oven-dried for determination of soil moisture contents. All incubations were conducted the following day in the laboratory using the field- moist and sieved samples at a room temperature of ~ 20˚C. Contrary to our expectation, total soil C concentration and thus the microbial activity in soil did not exhibit an apparent influence on N 2 O mole fractions. Specially low in soil C concentrations and high in N 2 O mole fractions for cropped soil in Southern and Central sites made this correlation weak. On the other hand, the exchangeable soil nitrate concentration turned out was related to the proportion of N 2 O. Total soil C concentration (g kg soil -1 ) Exchangeable soil NO 3 -N concentration (mg kg -1 soil) Total denitrification measured in soils collected during the summer was two to three times lower than that in the spring. This significantly lower N 2 O production in summer in all treatment plots can be attributed to the depletion of nitrate due to crop uptake during the crop-growing season. We hypothesized that the absence of tillage and fertilizer application on bioenergy crops would increase the labile carbon, resulting in reduction of N 2 O to N 2, and therefore, the N 2 O mole fraction would be lower compared to that of corn cultivation with tillage and fertilizer application. Our hypothesis turned out to be true only in case of Southern site in both seasons. No variation in N 2 O mole fraction in all treatment plots at the Central site and a slight variation at the Northern site could be attributed to the fact that both of these soils had thick, nutrient-enriched surface soil (Mollisols) possessing similar concentrations of exchangeable soil nitrate available for the denitrification process in all three treatments plots. We assumed that the cropped plot would produce the highest N 2 O production compared to biofuel crops at each of the three sites. However, this result occurred only at the Southern site in both seasons and at the Northern site in spring. Higher N 2 O production in biofuel soils compared to that in cropped soil at the Central site was consistent with the result of higher exchangeable soil nitrate concentration. y = x x R 2 = 0.87 ***