Nitrogen Management for Subsurface Drip Irrigated Cotton: Ammonium Thiosulfate, Timing, and Canopy Reflectance

In subsurface drip irrigation (SDI) systems, water constraints to cotton ( Gossypium hirsutum L.) production are greatly minimized and N management becomes the main priority. Injecting N fertilizer into SDI systems should, in theory, be as efficient as the irrigation delivery system itself. The objective of this study was to test duration of N injections, addition of the nitrification inhibitor ammonium thiosulfate (ATS), and using canopy spectral reflectance to guide N injections in a 2‐yr study of SDI cotton in Lubbock, TX. For a 1400 kg ha −1 expected lint yield, the N fertilizer rate was 170 kg N ha −1 minus the 0‐ to 0.6‐m soil NO 3 –N and an estimate of irrigation water NO 3 (20 and 30 kg N ha −1 for 2005 and 2006). We tested injection of urea–NH 4 NO 3 (UAN) between first square and early bloom, and first square and peak bloom. There was no effect of timing or ATS on lint yield. Reflectance‐based N management up to peak bloom resulted in savings of 28 and 17 kg UAN‐N ha −1 for 2005 and 2006, respectively, compared with soil‐test‐based N fertigation. Lint yields were similar between reflectance‐based UAN management (1814 kg ha −1 ) and UAN injected up to peak bloom based on soil test NO 3 (1880 kg ha −1 ). Nitrogen fertilizer recovery at first open boll was not affected by treatment but was high (62–75%). Flux of N 2 O + N 2 was low (800–2100 g N ha −1 ) in both years and was not different between fertilized and unfertilized plots. Residual 0‐ to 1.2‐m soil profile NO 3 after 2 yr was not affected by N fertigation treatments. Nitrogen fertilizer injections in SDI cotton are utilized very efficiently, and spectral‐reflectance‐based approaches have potential to reduce N fertilizer inputs.