Nitrogen fertilizer management for subsurface drip-irrigated cotton

Subsurface drip irrigation (SDI) systems for cotton production greatly minimize water constraints, and nitrogen (N) management becomes the main priority. Injecting N fertilizer into SDI systems should in theory be as efficient as is the irrigation delivery system. However, previous research by the Texas A&M University – Texas Agricultural Experiment Station Soil Fertility Program at Lubbock, TX has indicated that plant uptake of N fertilizer in SDI cotton is < 50 % of injected N. The research aims to improve N fertilizer management in SDI cotton. The research was conducted at Halfway, 2004 and Lubbock 2005. We tested two fluid N fertilizer sources: urea ammonium nitrate (UAN) vis a vis UAN plus ammonium thiosulfate (ATS). The ATS source theoretically acts as an N stabilizer minimizing nitrification and denitrification losses. We also aimed to fine-tune the timing of N fertigation by comparing the fertilizer injection of both the sources of fertilizers from 15 June to 15 July and from 15 June to 30 July in 2004 at Halfway. At Lubbock we injected N fertilizer from 20 June to 22 July and from 20 June to 12 August. We also tested spectral reflectance-based timing of N fertigation from 20 June to 12 August at Lubbock, 2005. Our results showed that lint yields with UAN plus ATS did not differ from UAN alone. Timing of N fertilizer injection did not affect lint yields. Lint yield with the reflectance-based treatment was not significantly different from yields with the other N-fertilized treatments. The most encouraging result of this study was the savings of 28 kg N ha-1 that was realized with the reflectance-based treatment (N rate was 72 kg N ha-1 ) relative to the other N-fertilized treatments, which received 102 kg N ha-1 Recovery efficiency of injected N averaged 63%. In this study we also made direct measurements of denitrification using vented chambers and an acetylene inhibition method. We compared three treatments, control, soil NO3- test-based UAN injection, and spectral reflectance-based UAN injection. Three chamber measurements were made in each plot. Samples were analyzed for N2O on a GC fitted with a PID detector. Denitrification fluxes in the SDI system were measurable, but not agronomically significant.