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Title:   Author  item
                                            Thorp, Kellyitem
                                            Malone, Robert

                                                         - *Rob*item
                                            Jaynes, Dan
  Submitted to: Transactions of the ASABE
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 5/15/2007
                                    Publication  Date: 12/1/2007
                                    Citation: Thorp, K.R., Malone, R.W., Jaynes, D.B. 2007. Simulating long-term effects of nitrogen fertilizer application rates on corn yield and nitrogen dynamics. Transactions of the ASABE. 50(4):1287-1303.Interpretive Summary: Loss of nitrate-nitrogen (NO3-N) from agricultural fields to water resources in the Midwestern United States has negatively impacted water quality throughout the region for many years. Some cities have spent millions of dollars to install facilities to remove NO3-N from drinking water. In addition, the effects of NO3-N losses have had economic and ecological impacts as far south as the Gulf of Mexico where water quality issues from excessive NO3-N levels have damaged the natural ecosystem and disrupted the fishing industry. To solve this problem, changes must occur in the way agricultural fields in the Midwestern United States are managed, but these changes must be made in a way that preserves the productivity and economic viability of agriculture in the region. Given the wide range of soils and climate across the Midwest, the initial development and analysis of new management practices is most economically performed using computer simulations. In this research, the ARS Root Zone Water Quality Model (RZWQM), coupled with the CERES and CROPGRO crop growth models, was applied to simulate ten years of observed yield and water quality data from a production field in central Iowa.  Results showed that the calibrated model could accurately reproduce the effects of different nitrogen (N) fertilizer application rates on corn yield and NO3-N losses in subsurface drainage. Evaluation of the model's accuracy and flexibility is of great benefit to researchers, giving them increased confidence in the model's applicability to a wide range of soil, climate, and agricultural practices. Using the evaluated model, we also showed that the use of more efficient N application rates can partially help to reduce losses of NO3-N from agricultural fields while maintaining acceptable levels of crop productivity. However, other changes in the way agricultural fields are managed will also be required to ensure NO3-N losses are reduced to levels acceptable for preserving the environment.
                                Technical Abstract: Thoroughly tested agricultural systems models can be used to quantify the long-term effects of crop management practices under conditions where measurements are lacking. In a field near Story City, Iowa, ten years (1996-2005) of measured data were collected from plots receiving low, medium, and high (57-67, 114-135, and 172-202 kg N ha-1) nitrogen (N) fertilizer application rates during corn (Zea mays L.) years. Using this data, the Root Zone Water Quality Model linked with the CERES and CROPGRO plant growth models (RZWQM-DSSAT) was evaluated for simulating the various N application rates to corn. The evaluated model was then used with a sequence of historical weather data (1961-2005) to quantify the long-term effects of different N rates on corn yield and nitrogen dynamics for this agricultural system. Simulated and measured dry-weight corn yields, averaged over plots and years, were 7452 and 7343 kg ha-1 for the low N rate, 8982 and 9224 kg ha-1 for the medium N rate, and 9143 and 9484 kg ha-1 for the high N rate.  Simulated and measured flow-weighted average nitrate concentrations (FWANC) in drainage water were 10.6 and 10.3 mg L-1 for the low N rate, 13.4 and 13.2 mg L-1 for the medium N rate, and 18.0 and 19.1 mg L-1 for the high N rate. The simulated N rate for optimum corn yield over the long-term was between 100 and 150 kg N ha-1. Currently, the owner-operator of the farm applies 180 kg N ha-1 to corn in nearby production fields. Reducing long-term N rates from 180 to 130 kg N ha-1 corresponded to an 18% simulated long-term reduction in N mass lost to water resources. Median annual FWANC in subsurface drainage water was decreased from 19.5 to 16.4 mg N L-1 with this change in management. Current goals for diminishing the hypoxic zone in the Gulf of Mexico call for N loss reductions of 30% and greater. Thus, long-term simulations suggest that at least half of this N loss reduction goal could be met by reducing N application rates to the production optimum. However, additional changes in management will be necessary to completely satisfy N loss reduction goals while maintaining acceptable crop production for the soil and meteorological conditions of this study. The results suggest that after calibration and thorough testing, RZWQM-DSSAT can be used to quantify the long-term effects of different N application rates on corn production and subsurface drainage FWANC.


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