DISTRIBUTION OF RESIDUAL NITRATE-NITROGEN IN LONG-TERM FERTILIZATION STUDIES OF AN ALFISOL CROPPED TO COTTON

Nitrogen fertilizer use in cotton (Gossypium hirsutum L.) production h as recently come under scrutiny as a potential source of NO5 contamina tion of streams and groundwater. This study was conducted to evaluate the distribution of NO3--N In soil cropped to continuous cotton and de termine the fertilization and irrigation conditions that might exacerb ate the accumulation of NO3--N. Long-term N fertilization studies were conducted in five side-by-side irrigation blocks at the Southeast Bra nch Experiment Station at Rohwer, AR, on an Hebert silt loam (fine-sil ty, mixed, thermic Aeric Ochraqualf) soil. Soil samples were taken in the early spring (1994) to a depth of 1.5 m in 15-cm increments from t hree replicates of each N-treatment from four irrigated and one drylan d block and analyzed for NO3--N. Soil cropped to dryland cotton showed excessive accumulation (up to 101.2 mg NO3--N kg(-1)) of NO3--N when treated with N rates of 67 kg N ha(-1) and greater, Soils cropped to i rrigated cotton did not accumulate as much NO3--N as dryland cotton. S oil NO3--N was unaffected by N treatments at any depth in two of three center pivot irrigated blocks. Slight increases In soil NO3--N were o bserved under furrow-irrigated conditions with the greatest N treatmen ts (134 and 168 kg N ha(-1)). Dryland cotton was more likely to have a ccumulations of NO3--N in the soil profile than irrigated cotton. This was probably due to reduced plant uptake of fertilizer N under nonirr igated production conditions.