Ruan, L. 2014. Impacts of biofuel crops on greenhouse gas emissions from agricultural ecosystems. Dissertation, Michigan State University, East Lansing, MI.

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Biofuels are intended to improve future energy and climate security. However, greenhouse gases (GHGs) can be produced from soil during biofuel cultivation and thus offset the biofuel climate mitigation effect. In this dissertation, I examine three major knowledge gaps with respect to GHG fluxes in biofuel landscapes: fertilization, tillage practices and climate change feedbacks. In Chapter 2, I measured nitrous oxide (N2O), methane (CH4), and nitrate (NO3-) fluxes along a switchgrass (Panicum virgatum) nitrogen (N) input gradient for three years to reveal an exponential increase in annual N2O emissions that each year post-establishment became stronger (R2>0.9, P<0.001) even as the biomass N response diminished. Nitrate leaching also increased exponentially across the N gradient. Net greenhouse gas benefits were consequently curtailed up to ~50% at N fertilizer rates above the crop requirement. Fertilization above the crop requirement substantially challenges the climate mitigation benefit of switchgrass and presumably other cellulosic biofuel crops. In Chapters 3 and 4, I measured GHG balances in portions of three 9-21 ha USDA Conservation Reserve Program (CRP) grassland fields in Michigan converted to conventional tillage (CT) or NT soybean production, and compared these fields to a fourth field left in grassland for reference in 2009. For the initial 201-day conversion period, average daily N2 O emissions were significantly different in the order: CT > NT > reference. Similarly, soil CO2 emissions in CT were 1.2 times those in NT and 3.1 times those in the unconverted CRP reference field. All treatments were minor sinks for CH4 with no significant differences among treatments. Including foregone mitigation, I conclude that NT management can reduce GHG costs by ~60% compared to CT during initial CRP conversion. In chapter 4, I studied Conservation Reserve Program (CRP) grasslands converted to corn, prairie, and switchgrass following an initial soybean year with and without conventional tillage (CT) in 2010 and 2011. Over two years I found that after conversion to CT the soils emitted twice as much N2O and 20% more CO2 compared to no-till (NT) conversion. Net ecosystem exchange of CO2 was strongly positive for all CT treatments and negligible (prairie) or negative (corn and switchgrass) for NT treatments. CH4 oxidation was unaffected by treatment. Overall, my results show that, by reducing N2O and CO2 emissions, NT practices can reduce the global warming impact of grassland conversion compared to CT. In chapter 5, I investigated the winter response of N2O emissions to changes in snowdepth in a corn-soybean-wheat cropping system for three winters using an automated chamber system. In treatments where snow was removed, N2O emissions were 69% higher than in ambient controls and 95% higher than in double-snow treatments (P<0.001). The proportion of annual N2O emissions represented by wintertime fluxes consequently increased by ~46%. Higher fluxes coincided with a greater number of freeze-thaw cycles that destroyed macroaggregates and increased inorganic nitrogen availability. Future winters with less snow can thus be expected to accelerate N2O emissions from agriculture, creating the potential for a positive climate change feedback.

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