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Fluid Journal : Late Spring 2013
Dr. Patricio Grassini is Research Assistant Professor, Agronomy and Dr.Kenneth Cassman is Heuremann Professor of Agronomy Chair in the Department of Agronomy and Horticulture, University of Nebraska, Lincoln, NE . Table 2. Potential impact of adoption of best management practices on energy use and global warming potential in irrigated maize in Tri-Basin NRD. Scenario Total energy* GJ Total GWP,* Mg of CO2e Actual baseline** 28,758 2,745 Potential*** 22,018 2,180 Difference**** -6 .741 (-25%) -566 (-21%) See Materials and Methods for details on calculation of energy use and GWP under each scenario. *Values are per 1,000 ha of irrigated maize in Tri-Basin NRD. **Based on actual frequency of fields under each type of irrigation system, tillage method, crop rotation, and source of energy for irrigation pumping. ***Based on full adoption of improved plant performance rating (90%), use of electrical power for irrigation water pumping, pivot irrigation, limited irrigation, and optimal N management in current irrigated maize land area that is not already under these management practices. **** Absolute and relative (in parentheses) difference in energy use and GWP under the potential scenario compared with actual baseline. use and GWP, respectively, with very little reduction in crop yield (4% reduction under limited irrigation). It is noteworthy that the greatest opportunity to reduce GHG emissions appears to be from fine- tuning N management practices aiming to reduce N surplus rather than reducing average N fertilizer rate. This proposition follows from the fact that, although many fields required higher or lower N fertilizer rates to achieve a zero N surplus (Figure 1), the estimated average N rate for optimal N management is similar to the current average fertilizer N rate (178 vs. 183kgofNha-1 , respectively). Summing up Increased demand for food and fuel with limited reserves of arable land will require further intensification of existing cropping systems. At issue is whether it is possible to achieve an ecological intensification that gives both high yields and reduced environmental burden. Results from our study clearly document that high yield and high input-use efficiencies, together with low GWP, are not conflicting goals in well-managed commercial-scale production fields. Although energy inputs and GWP per unit of land area were much greater in irrigated production compared with published values based mostly on rain-fed maize production, associated NEY and GWPi of irrigated systems were substantially greater and lower respectively. Hence, advantages of lower-input, lower-yielding maize systems vanish when metrics are scaled by grain yield or net energy output. For this reason, assessments of energy efficiency and GWP metrics are most relevant when corrected for yield rather than on a land- area basis. Our results also showed large discrepancies between two methods for estimating N2O emissions from applied N inputs. Because the current standard IPCC N-input method does not account for large variation in NUE observed across farmers’ fields, due to differences in yield level and competence in fertilizer management, estimated N2O emissions in high-yield, high NUE irrigated maize fields in the Tri-Basin NRD were much higher by using the IPCC input method than by estimating by the N-surplus approach. Hence, the IPCC method to estimate N2O emissions based on a fixed proportion of applied N inputs is likely to over-estimate N losses from well-managed, high-yield, high-input systems such as irrigated maize in Nebraska. Moreover, the N input approach cannot support incentives for investment in technologies to reduce N losses and thereby achieve better N balance without sacrificing yield. In a broad context, irrigated maize production in Nebraska can be taken as a benchmark for other current and future irrigated cropping systems because it achieves remarkably high and stable grain yields, high efficiencies in use of solar radiation, N, and water, plus has a large positive energy balance and low GWPi. 2013 Fluid Technology Roundup this coming fall in Omaha, NE. Keep tuned for more information
Early Spring 2013