Improving Nitrogen Fertilizer Use Efficiency in Subsurface Drip‐Irrigated Cotton in the Desert Southwest

Declining water availability in the American Southwest continues to generate interest in efficient subsurface drip irrigation (SDI) for cotton ( Gossypium hirsutum L.) production. Fertigating urea ammonium nitrate (UAN) at low rates with high frequency is an important advantage of SDI. However, N fertilizer management guidelines specific to SDI cotton are lacking. A 3‐yr study was conducted on a Casa Grande sandy loam soil in Maricopa, AZ, to test a pre‐plant soil profile NO 3 test algorithm and a canopy reflectance approach to manage in‐season N fertilizer for SDI cotton. Treatments included soil test‐based N management, reflectance‐based N management, and zero‐N at 100% evapotranspiration irrigation replacement. A second irrigation level of 70% evapotranspiration replacement included just the soil test‐based N and zero‐N treatments. The five treatments were replicated three times. Soil test–based N treatments received from 172 to 224 kg N ha −1 , and reflectance‐based N amounts were 112 to 158 kg N ha −1 . Nitrogen recovery efficiency (RE) of UAN‐N was high, with 24 fertigations during 6 wk between first square and mid bloom ranging from 58 to 93%. The isotope dilution method estimated similar RE in 2017. Residual post‐harvest soil NO 3 –N was notable only with 70% irrigation. Lint and seed yields were significantly reduced with the 70% irrigation treatment compared with 100% irrigation. The key result of this study is that reflectance‐based N management saved 17 to 112 kg N ha −1 without reducing lint yields compared with the soil test–based N treatment.