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

Recent studies have shown that crediting nitrates in irrigation water towards nitrogen (N) fertilizer rates has the potential to reduce leaching losses, thereby inhibiting NO₃^- movement into groundwater and initiating substantial fertilizer savings. This study, located in the Texas Rolling Plains, was initiated to determine the relationships between N crediting in irrigated cotton and accumulation of N in plant tissue and soil. Five fertility treatments are as follows: unfertilized control, N application based on soil testing, N based on soil testing plus P, N based on soil and water testing, N based on soil and water testing plus P. Fertilizer N was surface-applied and incorporated on each of three irrigation treatments: center-pivot, SDI, and furrow. Fertilizer rates credited for NO₃^- were reduced by about 42% in center-pivot and SDI treatments, and by 76% in furrow treatment. In 2010, lint yields reflected differences in fertilizer rates in fertilized treatments relative to unfertilized treatments, but not among non-controls. Nitrogen uptake similarly did not vary significantly in non-controls. No plant response was observed above 145 kg ha^-1 combined N levels (fertilizer + irrigation + soil N to the 36” depth) in pivot and SDI, and 80 kg ha^-1 in furrow irrigated fields. Mean soil N accumulation was 30% greater in non-credited N applications relative to corresponding N credited applications in all irrigation treatments. This study suggests that fertilizer application rates can be refined to credit for irrigation NO₃^-, resulting in lower soil NO₃^- accumulation without compromising yield.