J. Scott McConnell1, Paul B. Francis2, and C. Robert Stark2. (1) Western Illinois Univ, Institute for Environmental Studies, 1 University Circle, Tillman Hall 301, Macomb, IL 61455-1390, (2) Univ of Arkansas, PO Box 3508, Monticello, AR 71656
Nitrogen (N) fertilizer use in cotton (Gossypium hirsutum L.) production has come under scrutiny as a potential source of nitrate (NO3-) contamination of streams and ground water. This study utilized the McConnell-Mitchell Plots to determine the distribution of NO3--N in soil cropped to continuous cotton. Two irrigation methods, furrow flow (FI) and high-frequency center pivot (HFCP), were compared to a dry land (DL) control. Nitrogen treatments were tested within each irrigation block and ranged from 0 to 168.0 kg N ha-1 in 33.6-kg N ha-1 increments. Nitrogen treatments were discontinued after 18 years from 2000 through 2003. Soil samples were taken in the early spring (2000 and 2004) to a depth of 1.5 m and analyzed for NO3--N. Distribution of soil NO3--N in the FI block was due to sample depth and N treatment in 2000 and 2004. Differences in soil NO3--N in the FI block after suspending N treatments for four years were similar to those found in 2000, although the soil NO3--N was generally depleted in 2004 compared to 2000. The distribution of soil NO3--N in the DL block was dependent on the interaction of sample depth with N treatment in 2000 and 2004. Soil NO3--N was minimal in the three lowest N treatments (0-, 33.6-, and 67.2-kg N ha-1) in 2000. Greatest amounts of soil NO3--N were found in conjunction with the 134.4- and 168.0-kg N ha-1 treatments. The distribution of soil NO3--N in the HFCP irrigated block was dependent on the interaction of sample depth with N treatment in 2000 and 2004. Differences in soil NO3--N tended to be too small to be of discernable or practical importance.