Nitrate Movement and its Distribution in the Soil Profile of Differentially Fertilized Corn Watersheds

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Abstract
Nitrate movement within the 6.1‐m soil profile of a watershed in southwestern Iowa, fertilized at 448 kg N/ha per yr, resulted in a 720‐kg/ha increase in NO3‐N below the corn root zone during the 3‐year study. The NO3‐N concentration of the baseflow from the watershed increased during this period, indicating that some of the leached NO3‐N reached the ground water. The watershed fertilized at the recommended N rate (168 kg/ha per yr) did not increase the quantity of NO3‐N below the corn root zone; however, some NO3‐N leaching did occur.Between April 1971 and April 1974, the accumulation of NO3‐N in the profile of the excessively fertilized watershed moved from the 1.0‐ to 3.1‐m depth. Three‐fourths of NO3‐N movement occurred between June 1972 and April 1973 when 80 cm of precipitation caused 21 cm of percolation below the corn root zone (183 cm). The watershed fertilized with 168 kg N/ha per yr did not show any zones of significant NO3‐N accumulation in the soil profile at any of the sampling dates.The depth to the water table on the lowest contour sampled on the excessively fertilized watershed was 4.6 and 4.9 m in April 1973 and April 1974, respectively. The average NO3‐N concentration for these two sampling dates at the water table depth increased from 3.7 to 12.9 ppm. The average NO3‐N concentration at the water table depth on the normally fertilized watershed was 2.0 and 4.5 ppm in April 1973 and April 1974, respectively, with a water table depth of 3.0 m.
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