Nitrogen fertilizer movement in the soil as influenced by nitrogen rate and timing in irrigated wheat

Nitrogen fertilizer is a potential contaminant of groundwater supplies. The purpose of this study was to determine the influence of recommended N fertilizer rate and timing on N movement in the soil during the growing season. Durum wheat [Triticum turgidum L. subsp. durum (Desf.) Husn.] was grown at Maricopa, AZ, during the 1991 and 1992 growing seasons. A N rate study was conducted at two sites on a sandy loam soil [coarse loamy, mixed (calcareous), hyperthermic. Typic Natrargid (reclaimed)] and clay loam soil [fine loamy, mixed (calcareous), hyperthermic, Typic Torrifluvent] using ¹⁵N-labeled (NH₄)₂SO₄ and Br^- tracer. Three N rates that ranged from 5.4 to 10.1 g N m^-2 for the less than recommended rate, 18.5 to 22.5 g N m^-2 for the recommended rate and 28.0 to 37.8 g N m^- for the greater than recommended rate were applied in split applications. The experimental design was a randomized complete block with six replications and three N rates. A N timing study was conducted on the sandy loam soil at the recommended N rate where ¹⁵N and Br^- were applied at only one of the application times and nonlabeled N fertilizer was applied at the other times. The experimental design for the N timing study was a randomized complete block with six replications and four (1992) or five (1991) application times. Surface flood irrigation was applied in excess of soil water depletion (top 1.5 m), varying with year, soil type, and N rate. After harvest, the soil was sampled to a depth of 2.4 m and analyzed for ¹⁵N and Br^-. Nitrogen rate had no influence on ¹⁵N fertilizer or Br^- movement in the soil. Nitrogen rate increased the N content of the surface soil, but most of this N was not in NO₃ form. In most cases, the median depth of movement of recovered ¹⁵N for all N rates was 0.23 m compared to 1.13 m for Br^-. Timing of applications did not influence N fertilizer movement. Bromide overestimated the depth of ¹⁵N movement recovered in the soil possibly due to plant uptake and immobilization of N in the surface soil. We found that for irrigated wheat in Arizona, most of the N fertilizer recovered in the top 2.4 m of soil was in the surface soil, regardless of N fertilizer practices.