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Title:   Author  item
                                            QI, ZHIMING - Iowa State Universityitem
                                            HELMERS, MATT - Iowa State Universityitem
                                            Malone, Robert

                                                         - *Rob*item
                                            Thorp, Kelly
  Submitted to: Transactions of the ASABE
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 9/1/2011
                                    Publication  Date: 9/1/2011
                                    Citation: Qi, Z., Helmers, M., Malone, R.W., Thorp, K.R. 2011. Simulating long-term impacts of winter rye cover crop on hydrologic cycling and nitrogen dynamics for a corn-soybean crop system. Transactions of the ASABE. 54:1575-1588.Interpretive Summary: Planting winter cover crops into corn-soybean rotation is a promising approach for reducing subsurface drainage and nitrate loss. However, the long-term impact of this practice needs investigation. We evaluated the performance of the Root Zone Water Quality Model-Decision Support System for Agrotechnology Transfer (RZWQM-DSSAT) model against five years of comprehensive field data near Gilmore City, Iowa. The model adequately simulated crop growth of corn, soybean, and rye, the subsurface drainage, soil water storage (0-60cm), and NO3-N loss. Long-term simulations showed that adding rye as a winter cover crop reduced annual subsurface drainage and NO3-N loss by 13% (3.3 cm) and 10% (4.9 kg N ha-1), respectively, and increased annual evapotranspiration (ET) by 6% (3.3 cm). Results suggest that introducing winter rye cover crop to conventional corn-soybean rotation is a promising approach to reduce N loss from Iowa’s subsurface drained agricultural systems. However, simulated annual nitrate concentration under long term winter cover crop was not reduced possibly because of increased net mineralization, which is in contrast to short term simulations and observations previously reported in the literature. Also nitrogen assimilation of rye cover crop was overestimated for the validation plots under the relatively cold weather conditions that occurred during some of the field observation period. Therefore, further research is needed to refine the simulation of net mineralization in cover crop systems and rye N uptake using RZWQM-DSSAT under a wider range of weather conditions. This research will help model developers, model users, and agricultural scientists more accurately estimate the benefits and limitations of winter cover crops to improve water quality in tile drained agriculture.
                                Technical Abstract: Planting winter cover crops into corn-soybean rotation is a promising approach for reducing subsurface drainage and nitrate loss. However, the long-term impact of this practice needs investigation. We evaluated the performance of the RZWQM-DSSAT model against five years of comprehensive field data near Gilmore City, Iowa that included soil water content and used this model to study the long-term (1970-2009) hydrologic and nitrogen cycling effects of a winter cover crop within a corn-soybean rotation. The model adequately simulated crop growth of corn, soybean, and rye, the subsurface drainage, soil water storage (0-60cm), and NO3-N loss. The average grain yields of corn and soybean were predicted within 15% error for the treatments and controls. The time series data of total above ground biomass and leaf area index of corn, soybean, and rye were simulated with coefficient of determination (r2) and index of agreement (IoA) values generally greater or close to 0.75. Annual drainage was predicted with EF (Nash-Sutcliffe efficiency), r2, and IoA greater than 0.70 and simulated soil water storage was within 5% error from observed annual average. Both average annual NO3-N loss and concentration was simulated within 10% error. Long-term simulations showed that adding rye as a winter cover crop reduced annual subsurface drainage and NO3-N loss by 13% (3.3 cm) and 10% (4.9 kg N ha-1), respectively, and increased annual ET by 6% (3.3 cm). Results suggest that introducing winter rye cover crop to conventional corn-soybean rotation is a promising approach to reduce N loss from Iowa’s subsurface drained agricultural systems. However, simulated annual nitrate concentration under long term winter cover crop was not reduced possibly because of increased net mineralization, which is in contrast to short term simulations and observations previously reported in the literature. Also nitrogen assimilation of rye cover crop was overestimated for the validation plots under the relatively cold weather conditions that occurred during some of the field observation period. Therefore, further research is needed to refine the simulation of net mineralization in cover crop systems and rye N uptake using RZWQM-DSSAT under a wider range of weather conditions.


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