Fate of Nitrogen Fertilizer in Surface-Irrigated Cotton: Fertigation Vs. Knifing.

In a two year field study, we compared fertigation of N fertilizer with knifing in surface-irrigated cotton in central Arizona. Surface-run fertigation has the advantage of being applicable later in the season than knifing with a ground applicator, and with lower costs.  We also compared soil-test based N management with reflectance-based managment. Nitrogen fertilizer response was observed, but was not different among N treatments. Nitrogen recovery efficiency was low (maximum 30 %), but similar to the authors' previous N fertilizer research on furrow-irrigated cotton in Texas. The internal N use efficiency was greater than expected at 1.3 kg lint/kg N in plant (40 lb N/bale). Emissions of N₂O were very low (maximum 0.2 % loss of N fertilizer), but were still elevated (2 – 4X) compared to the zero-N plots. Nitrous oxide emission events were short, ending 3 days after an irrigation.  Soil transects of  exchangeable ammonium and nitrate following fertilization showed the unexpected finding that ferigation was as uniform as knifing N fertilizer.  NItrate leaching with the the 100 mm irrigation following knifing and with the fertigation event was minimal.  However, soil profile sampling in the spring of 2013 indicated that NO₃ leaching with the whole season's 800 mm of irrigation was substantial in the soil test based knife treatment. About 40 kg NO₃-N ha (30 % of added N fertilizer in 2012) was found between 1.2 and 1.8 m. The ferigation treatment of 148 kg N/ha showed a lower amount of leaching by seasons' end.  Reflectance-based N management in 2012 saved 50 % of N fertilizer of the full soil-test based dose, without a reduction in yields.