Corn Response to Nitrogen Fertilization in a Soil with High Residual Nitrogen

High levels of residual NO3-N are present in the soils of the Arkansas River Valley in Colorado where alfalfa (Medicago sativa L.), grains, fruits, and vegetable crops are produced. This study evaluated the use of continuous corn (Zea mays L.) to reduce residual N levels in a furrow-irrigated, silty clay soil. Fertilizer N needed to maintain optimum corn yields following watermelon [Citrullus lanatus (Thunb.) Matsum. and Nakai], and its impacts on NO3-N leaching potential were also evaluated. Treatments evaluated from 2000 through 2003 included two N sources (urea and Polyon) and six fertilizer N rates. Corn grain yields were not significantly increased by N fertilization the first year following watermelon but increased with increasing residual soil NO3-N levels the second year without additional N fertilization and increased by N fertilization in the third and fourth years. Nitrogen source did not significantly affect corn grain yields, residual soil NO3-N, or N fertilizer use efficiency (NFUE). Nitrogen use efficiency generally decreased with increasing level of available N. Average NFUE based on grain N removal over 4 yr was 55% at the lowest fertilizer N rate and 30% at the highest N rate. Excluding the first corn crop, grain yields and gross economic returns less N costs were maximized with about 265 and 258 kg ha-1 of available N (soil plus fertilizer N), respectively. Soil residual NO3-N levels declined following the second, third, and fourth corn crops. Reducing N application rates (based on credits for residual soil NO3-N and previous crop) to the first corn crop produced in rotation with fruit or vegetable crops in the Arkansas River Valley will improve profitability and reduce root zone residual soil NO3-N levels and NO3-N leaching potential. Continuous corn production for 2 to 3 yr with conservative N fertilization rates can reduce residual soil NO3-N levels in the Arkansas River Valley.