Influence of Irrigation Nitrate Crediting On Cotton Production and Residual Soil Nitrate in the Texas Rolling Plains.

Nitrate contamination of the Seymour Aquifer is a major water quality concern. In seven counties in the Texas Rolling Plains, over 50% of observations exceeded 10 mg L^-1 NO₃-N, the maximum contaminant level for drinking water. Sources of groundwater nitrates in agriculture production regions include mineralization of soil organic matter, animal waste, leakage from sewage systems, and fertilizer N. Field experiments were conducted on irrigated cotton to determine the effect of variable nitrogen application rates, with and without accounting for high NO₃-N in well water, on lint yield, N uptake, and soil N accumulation. Fertilizer N and P were surface-applied and incorporated on three irrigation treatments: center pivot, subsurface drip (SDI), and conventional furrow. When nitrate in well water was credited toward crop N demand, fertilizer rates were effectively reduced by 39% (center-pivot and SDI) and 77% (furrow). In 2010, there were no yield responses among fertilized plots. Post-harvest soil N accumulation was greater at the uncredited fertilizer rate vs. the credited N rate by 23% and 36% in pivot and SDI, respectively. In 2011, with up to 175 kg NO₃ ha^-1 applied through irrigation during an exceptional drought, there was no response to fertilizer applied N. To maximize profit and reduce environmental impacts of NO₃-N leaching into groundwater, nitrogen management considerations in the Rolling Plains should adjust for the amount of N applied through irrigation water, as well as available nitrogen in the soil profile.