Improving Nitrogen Fertilizer Management in Sprinkler-Irrigated Cotton

Aside from water, N is the main constraint to cotton production in the western USA. Because of existing canal infrastructure, flood irrigation techniques are commonly used in Arizona cotton production, despite relatively low N recovery (15 to 34%) in these systems. With declining water resources, there is increased interest in center-pivot or linear-move overhead sprinkler systems, but N management research for cotton in these systems is lacking in the far western US. The overall objective of this study is to improve and update N fertilizer recommendations for sprinkler-irrigated cotton in Arizona, based on a 36-inch nitrate-N soil test. Specific objectives are to 1) compare soil test-based N fertilizer management with two canopy reflectance-based approaches, 2) compare urea ammonium nitrate (UAN) to UAN plus nitrification and urease inhibitors (i.e., Agrotain Plus®), and 3) construct N balances for sprinkler-irrigated cotton.

Irrigation was applied 2 to 4 times a week with FAO crop coefficients and 85% evapotranspiration (ET) replacement. Canopy reflectance was measured weekly from first square to first open boll. Surface flux of nitrous oxide (N₂O) was measured weekly for 10 weeks during the season using vented chambers and gas chromatography. The N requirement in the soil test-based N treatment was estimated based on total N need of 50 lb/bale, or 200 lb N/A for 4 bale/A cotton. Pre-plant soil profile nitrate and total N delivered in irrigation water was subtracted from the total (200 lb) to arrive at the soil test-based N fertilizer rate of 117 lb/A. Reflectance strategy N-1 was set at 50% of this. The 1.3 x soil test N fertilizer rate was 152 lb N/A, and the reflectance strategy N-2 was 50% of this. Nitrogen fertilizer was applied in three equal splits at pinhead square stage, one week before first bloom, and mid bloom.

On average, lint yield was significantly higher with N fertilizer than without (1,860 vs. 1,570 lb lint/A), but there was no difference in yield among N treatments, nor was there significant difference in agronomic N use efficiency (lb lint/lb N fertilizer). Nitrous oxide emissions increased with N rate for the majority of measurement dates. Agrotain Plus® did not significantly affect emissions. The emission factor (EF), or percent of N fertilizer emitted as nitrous oxide, ranged from 0.2 to 0.65%, which is well below the IPCC Tier 1 EF of 1%. The NDRE index consistently reflected N deficiency much earlier than amber normalized difference vegetation index (NDVI). This project will continue in 2016.