Improving Nitrogen Fertilizer Management in Surface-Irrigated Cotton

We propose conducting research in central Arizona to compare the common regional practice of N fertigation with knifed N injections in surface-irrigated cotton (i.e., level furrow irrigation). Additionally, we will compare reflectance-based nitrogen fertilizer management with soil test-based management. Furrow and other surface irrigation methods are still the most common irrigation mode for cotton in Arizona and worldwide.


Justification

Following water, N fertilizer is the main constraint to cotton production in the western USA (Morrow and Krieg, 1990). Canal infrastructure of irrigation water in Arizona means basin, flood, and furrow irrigation are still the pre-dominant choices of irrigation methods. Navarro et al. (1997) in Arizona, and Booker et al. (2007) and Bronson et al. (2007) in Texas reported that recovery efficiency ground-based N applications in furrow-irrigated cotton ranged from only 15 to 34 %. Fertigation of liquid UAN fertilizer is commonly practiced in Arizona and other western US States. In this practice the liquid N fertilizer is typically dribbled into the irrigation canal and then transported to the field through the surface irrigation water stream. Ammonia-N losses are probably high with this type of fertigation. Furthermore, non-uniformity of irrigation would result in highly variable N fertilizer applications down the furrow and nitrate leaching would occur in areas of the field that receive excess water during fertigations. Knife applications of UAN in 3-inch deep bands along the furrow should improve the uniformity of N versus water run UAN. However, comparison of these two N methods has not been adequately investigated.

In the western US, weekly petiole NO₃ sampling and analysis is the recommended approach to monitor in-season cotton plant N status. However, petiole sampling is laborious and laboratory turn-around is time-consuming. Additionally, petiole NO₃ analysis can be highly variable (Bronson et al. 2001). Canopy reflectance, on the other hand is a rapid, non-destructive method to assess in-season cotton N status (Chua et al., 2003; Bronson et al, 2003). Canopy reflectance-based N management in subsurface drip systems in Texas resulted in reduced N fertilizer use, without hurting lint yields (Yabaji et al., 2009). In that research, N fertilizer was initially applied at half the rate of a regional soil test based recommendation. When normalized difference vegetative index (NDVI, a common remote sensing vegetative index) in the reflectance treatment fell below NDVI of the soil test/adequately fertilized plot, N fertigation was increased. This simple “sufficiency index” approach has not been tested in the western US in surface-irrigated cotton.

We propose comparing the regional practice of N fertigation in surface-irrigated cotton with knife injections. Additionally, we will compare reflectance-based N fertilizer management with soil test-based management. The study we be conducted in Maricopa, AZ on a Casa Grande sandy loam.


Objectives

1. Compare urea ammonium nitrate (UAN) fertigation with knife applications of UAN for a farm-scale surface-irrigated field, furrowed for cotton.
2. Compare soil test-based N fertilizer management with canopy reflectance-based N management in surface-irrigated cotton.
3. Construct N balances for surface-irrigated cotton, i.e. quantify total N uptake, recovery N use efficiency, NO₃ leaching, and denitrification losses.

Methodology

In February, 2011, pre-plant soil sampling to 180 cm for NO₃ will be done on four samples per plot. Cotton will be planted in April, 2011 in plots that will be 8, 1-m rows wide by 180 m. At harvest, soil sampling to 180 cm for nitrate will on eight samples per plot. Nitrogen treatments will include:

Nitrogen treatment	Fertilization mode	Notes
Zero-N
Soil test-based N	Knife-blanket rate	In two splits, four-leaf and first bloom
Soil test-based N	Fertigate	In four irrigations, from first square to mid-bloom
Reflectance-based N	Knife-blanket rate	In two splits, four-leaf and first bloom
Reflectance-based N	Knife-variable rate	In two splits, four-leaf and first bloom
Reflectance-based N	Fertigate	In four irrigations, from first square to mid-bloom

The experimental design will be completely randomized block, with three replicates.

Surface flux of N₂O will be measured weekly for 10 weeks during the season using vented chambers and gas chromatography. Biomass and total N uptake will be determined plants on 2 m of row at first open boll. Nitrogen recovery efficiency, physiological N use efficiency and agronomic use efficiency will be calculated. Lint and mature seed yields will be hand harvested on 0.0004 ha areas. Mature cotton seed N will be determined and the percentage of seed N to total N uptake calculated. Micronaire and other fiber quality attributes will be determined on lint and the relationships of these to N fertilizer rate estimated. Soil sampling for extractable NO₃-N from 0 to 180 cm will be done after harvest to assess residual and NO₃ and leached NO₃ (90 – 180 cm profile NO­₃). Post-harvest soil sampling will on eight samples per plot to assess the spatial variation of leached NO₃ across the plot.