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 fert ilizer rate and timing on N movement in the soil during the growing season. Durum wheat [Triticum turgidum L. subsp. dururn (Desf.) Husn.] was grown a t Maricopa, AZ, during the 1991 and 1992 growing seasons. A N rate study wa s conducted at two sites on a sandy loam soil [coarse loamy, mixed (calcare ous), hyperthermic, Typic Natrargid (reclaimed)] and clay loam soil [fine l oamy, mixed (calcareous), hyperthermic, Typic Torrifluvent] using N-15-labe led (NH4)(2)SO4 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 th e recommended rate and 28.0 to 37.8 g N m(-2) for the greater than recommen ded rate were applied in split applications. The experimental design was a randomized complete black with six replications and three N rates. A N timi ng study was conducted on the sandy loam soil at the recommended N rate whe re N-15 and Br- were applied at only one of the application times and nonla beled N fertilizer was applied at the other times. The experimental design for the N timing study was a randomized complete block with six replication s and four (1992) or five (1991) application times. Surface flood irrigatio n was applied in excess of soil water depletion (top 1.5 m), varying with g ear, soil type, and N rate. After harvest, the soil was sampled to a depth of 2.4 m and analyzed for N-15 and Br-. Nitrogen rate had no influence on N -15 fertilizer or Br- movement in the soil. Nitrogen rate increased the N c ontent of the surface soil, but most of this N was not in NO3 form. In most cases, the median depth of movement of recovered N-15 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 N-15 movement re covered 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 th e N fertilizer recovered in the top 24 m of soil was in the surface soil, r egardless of N fertilizer practices.