Effects of Irrigation Nitrate Crediting On Cotton Production in the Texas Rolling Plains.

Groundwater nitrate contamination in the Texas Rolling Plains is a major water quality concern. Fertilizer N is scrutinized as a non-point source of contamination. From 2010 – 2012, field experiments were conducted on to determine the effect of crediting NO₃-N applied through irrigation on cotton fertilizer N requirement, lint yield, and soil nitrate accumulation. Subsurface drip (SDI) and center-pivot (LESA) irrigated fields were fertilized for a yield goal of 1615 kg lint ha^-1, correcting for preplant soil test nitrate to 60 cm. In 2010 and 2011, by adjusting for NO₃-N in soil and in 30 cm irrigation water, fertilizer N rates were reduced by 47% to 57%. Greatest lint yields (1,530 kg lint ha^-1) occurred in LESA 2010. Irrigation nitrate crediting did not compromise lint yield in 2010 or 2011. Residual soil NO₃-N at the 15 – 90 cm depth was reduced by about 40% in SDI and LESA 2010 and 22% in SDI 2011. 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, in addition to that available in the soil profile.