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Fluid Journal : Late Spring 2013
4 The Fluid Journal Late Spring 2013 Summary: Addressing concerns about future food supply and climate change requires management practices that maximize productivity per unit of arable land while reducing negative environmental impact. On-farm data were evaluated to assess energy balance and greenhouse gas (GHG) emissions of irrigated maize in Nebraska that received large nitrogen (N) fertilizer (183 kg of N/ha-1) and irrigation water inputs (272 mm or 2,720 m3 ha-1). Although energy inputs were larger than those reported for US maize systems in previous studies, irrigated maize in central Nebraska achieved higher grain and net energy yields (13.2 Mg/ha-1 and 159 GJ/ha-1 , respectively) and lower GHG-emission intensity (231 kg of CO2e/Mg-1 of grain). Large variation in energy inputs and GHG emissions across irrigated fields in the present study resulted from differences in applied irrigation water amount and imbalances between applied N inputs and crop N demand, indicating potential to further improve environmental performance through better management of these inputs. Observed variation in N-use efficiency, at any level of applied N inputs, suggests that an N-balance approach may be more appropriate for estimating soil N2O emissions than the intergovernmental Panel on Climate Change approach based on a fixed proportion of applied N. High-yield Maize and Small Global Warming Intensity The Fluid Journal • Official Journal of the Fluid Fertilizer Foundation • Late Spring 2013 • Vol. 21, No. 3, Issue # 81 High yield cropping systems require fossil-fuel inputs to substitute human and animal labor and to maximize the capture and conversion of solar radiation into crop biomass. Inputs to agricultural systems that require fossil fuel in their manufacturing process include fertilizer, seed, pesticides, and machinery. Fossil fuel also is required for application of inputs as well as field operations, irrigation pumping, and grain drying. Fossil-fuel inputs can be expressed in terms of their embodied energy, that is, the energy required for their synthesis, packaging, transport, and use in a crop production field. Because fossil fuel combustion results in GHG emissions, energy inputs also can be expressed in terms of global warming potential (GWP). Although GWP can be expressed per unit of crop production area, it also can be expressed per unit of grain yield (GWP intensity; GWPi), which recognizes the potential for indirect land use change and associated GWP from clearing of carbon-rich natural ecosystems for crop production. Although it has been speculated that the efficiency with which applied inputs that result in increased yield can be greater in intensively managed high-yield cropping systems than in their low-input low-yield counterparts, because of optimization of growing conditions in the former, this hypothesis has not been evaluated in actual cropping systems where farmer’s yields approach yield potential. The U.S. Corn Belt, including parts of the Great Plains in South Dakota, North Dakota, Nebraska, and Kansas, accounts for 33 percent of global maize production. Of total U.S. maize, approximately 13 percent is produced with irrigation on approximately 3.2 Mha with the majority grown in Nebraska. Energy-use efficiency of maize in the U.S. Corn Belt has increased steadily in recent decades as a result of (1) rising grain yield without increases in amounts of applied N fertilizer and applied irrigation, (2) widespread adoption of conservation tillage practices and center-pivot systems to replace less efficient gravity irrigation, and (3) increasing efficiency in manufacturing of agricultural inputs. Field experiments on irrigated maize Maximizing productivity per unit of arable land while reducing negative environmental impact is goal. Patricio Grassini and Kenneth G. Cassman have shown that achieving high yields and high efficiencies, together with relatively low GWP, is possible when applied inputs are precisely managed in time and space, but the extent to which farmers can achieve precise management is not known. Likewise, there is a general notion that input-use efficiency of high- yield cropping systems is low, resulting in negative energy balances, high GWP, and degradation of soil and water quality. In part, such perceptions are based on previous studies that had several deficiencies, including: • Obsolete embodied energy and GHG emissions factors for agricultural inputs DOWNLOAD PDF
Early Spring 2013