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Fluid Journal : Late Spring 2013
5 The Fluid Journal Late Spring 2013 • Obsolete values for grain yield and actual crop management practices with regard to N fertilizer rates, irrigation, and tillage • Use of metrics that do not weight energy inputs or GWP in relation to yield level • Lack of clarity on methods used to estimate energy inputs or GHG emissions and system boundaries. Hence, accurate and transparent estimates of on-farm energy balance and GWP for irrigated maize in the US Corn Belt are not available. Management practices influence energy balance and GWP by amounts and efficiencies of applied inputs and yield level. Given concerns about the cost of energy and climate change, agriculture is challenged by the need to identify management systems that maximize productivity with high energy-use efficiency and low GWP. Addressing this challenge using a structured experimental approach, however, requires factorial experiments performed over many years at multiple locations. Because this approach is very costly and there are few opportunities for long-term funding to support such efforts, most research on energy balance and GWP of agricultural systems has relied on data from aggregate agricultural statistics or data gathered from a relatively small number of selected farms. An alternative is to use farmer-reported databases, collected over a large population of field-years, to perform direct analysis of on-farm energy balance and GWP, and to use the variation in management practices within these data to identify those that give high yields, high input use efficiencies, and low GWPi. The central hypothesis of this work is that it is possible for farmers to achieve a large positive energy balance with relatively low GWPi in high-input, high-yield maize systems. To test this hypothesis, farmer-reported data collected from the Tri-Basin Natural Resources District (NRD) in central Nebraska were used to: • Quantify energy balance and GWP of irrigated maize • Compare these parameters against previously published values for maize systems • Identify and quantify the impact of energy-saving and GWP-reducing management tactics that could achieve these reductions without yield loss. Overview N2O emissions. Separate estimates of soil N2O emissions were calculated by following two methods: • The "N-input-driven approach" developed by the Intergovernmental Panel on Climate Change (IPCC; ref 23) • The "N-surplus-driven approach recently proposed by van Groenigen, et al. The N input approach assumes that N2O emissions represent a constant proportion of applied N inputs plus N in crop residues, which does not account for tremendous variability in the efficiency with which applied N is used by the crop across fields, crops, and regions. In contrast, van Groenigen et al. provide strong evidence that N2O emissions can be more accurately estimated from the magnitude of N surplus, which is defined as the difference between N inputs and crop N uptake. In this study, applied N inputs were calculated as the sum of applied N fertilizer, N-NO3- in applied irrigation water, Figure 1: Soil N2O emissions of irrigated maize against applied N inputs (A) and N surplus (B). Average (±SE) N2O emissions, N inputs, and N surplus (medians in parenthesis) are shown. B inset shows the relationship between N surplus and applied N inputs.
Early Spring 2013