Two long-term N-rate experiments with corn (Zea mays L.) which had been conducted in northwestern Iowa on the Moody and Galva silty clay loam soils for 17 and 15 years, respectively, were studied to determine the recovery of applied N (i) as NO3-N at various soil depths, (ii) as organic or fixed N, and (iii) as N removed in the harvested corn grain. The annual N rates applied as NH4NO3 were 0, 56, 112, and 168 kg N/ha on the Moody soil and 0, 90, and 134 kg N/ha on the Galva soil. Nitrogen recovery in the fertilized plots was determined by subtracting the amounts for the unfertilized treatment from those for the fertilized treatments.
The maximum amount of N recovered as nitrate in the profiles was 1,307 kg/ha in the Moody soil as contrasted to only 231 kg/ha in the Galva soil. The maximum depth of accumulation, however, was between 1.2 and 1.5 m in both soils, and there was no indication of nitrate leaching below about 2.4 to 2.7 m in either soil.
The percentage of N recovered as nitrate with increasing N applications increased from 18% to 48% in the Moody experiment compared with only 7% to 12% in the Galva; conversely, the percentage N recovery in the grain decreased from 38% to 11% for the Moody, and from 40% to 28% in the Galva, with the increasing N applications.
At rates of N required to obtain maximum yield in the Moody and Galva soils, the percentage N recoveries in the grain were similar, 38% and 40%, respectively. Likewise, the percentage recoveries were 18% and 7% as NO3-N, 18% and 22% as organic N, and 74% and 68% total, respectively. Thus, at maximum yields, the percentage of the applied N not accounted for was 26% and 32% of that applied to Moody and Galva soils, respectively. At the highest rate of application (168 and 134 kg/ha, respectively) the losses increased to 37% and 46% for the respective soils. The unaccounted for N is considered to have been lost largely through denitrification. These data emphasize (i) the close relationship that exists between efficient N utilization and possible nitrate pollution, and (ii) the importance of establishing and applying rates of N no greater than required to obtain maximum yield.