Greenhouse gas emissions from production of Miscanthus x giganteus on a Mollisol

Biofuels have a great potential to alleviate our dependence on non-renewable fossil fuel products; however the beneficial effects of substituting biomass for fossil fuel is reduced if the biofuel crops also emit large amounts of greenhouse gases (GHGs), such as carbon dioxide (CO2) and nitrous oxide (N2O) during their production. Most crops require nitrogen (N) fertilization to maximize productivity, but the amount of N fertilizer Miscanthus x giganteus might need is currently not known. Because M. x giganteus is highly efficient in its N use, it is a great potential energy crop because it has had large yields even at low N inputs from fertilizers (including no additions of N fertilizer). Therefore, it is critical to determine the response of M. x giganteus to N fertilizer rates and determine the effect of fertilization on GHG emissions. The main objective of this study was to examine the effect of N fertilization rates (0, 60 and 120 kg N ha-1 of urea) on GHG emissions from production of M. x giganteus on a central Illinois Mollisol. The study had twelve, 10x10 m plots organized in four replicate rows with each of the three N fertilizer treatments placed randomly in each row. Gas samples to determine N2O and CO2 fluxes were taken near noon throughout the year (March-November) when soil temperatures were warm enough to support microbial activity. In addition, soil moisture and soil temperature were continuously measured, and soils were regularly sampled for inorganic N to make specific inferences about abiotic factors affecting the GHG emissions. Furthermore, inorganic N leaching was assessed using resin lysimeters buried at 50 cm in each plot; the lysimeters placed in the soil in April of each year and excavated the following April. M. x giganteus biomass was measured on each plot, along with N and C concentrations in the harvested material. At the end of 2009 cumulative N2O and CO2 emissions did not have a significant response due to fertilization. However, at the end of 2010, cumulative N2O emissions significantly increased with fertilizer additions (0.35, 0.77, and 2.91 kg N ha-1 for the 0, 60, and 120 kg N ha-1 fertilizer treatments, respectively). Carbon dioxide emissions did not respond to fertilization in 2010. Larger NO3- concentrations were significantly related with larger N2O emissions. Greater temperature and greater soil moisture at 10 cm were significantly related to larger N2O emissions. Higher temperature at 10 cm was significantly related to larger CO2 emissions; conversely, soil moisture was not related to CO2. In 2010, several large precipitation events occurred following fertilization, leading to greater N2O emissions due to greater soil moisture. This study shows the potential for large N2O releases in fertilized M. x giganteus when rates were greater than 60 kg N ha-1, but this response is dependent on precipitation and resulting soil moisture status following fertilizer application. Soil CO2 emissions were unaffected by N fertilization. During the first year of the study, NO3- leaching was not significantly affected by fertilization, but by the second year, the treatment plots had significantly different NO3- leaching at 50 cm soil depth (8.9, 15.3, and 28.9 kg N ha-1 yr-1, for the 0, 60, and 120 kg N ha-1 fertilizer treatments, respectively). Yield data the first year of the study showed no significant difference among treatments and were quite small, likely due small to the previous crop failure during the establishment year and replanting (1.1, 4.1, and 4.0 Mg ha-1, for the 0, 60, and 120 kg N ha-1 treatments, respectively). After the second year, biomass was much larger for all the treatments, but was still not significantly different due to fertilization with N (14.9, 15.8, and 17.0 Mg ha-1, for the 0, 60, and 120 kg N ha-1 treatments, respectively). The amount of N removed from harvesting the biomass was significantly larger with additional fertilizer in year 2; therefore, fertilization removed more N while yielding relatively the same biomass. Overall, fertilization of M. x giganteus can lead to important N2O releases, which reduces the overall favorable GHG balance; increased fluxes of inorganic N (primarily NO3-) through the soil profile; and increases in harvested N without a significant increase in biomass