Nitrate‐Nitrogen in Tile Drainage as Affected by Fertilization

Abstract The volumes and NO 3 ‐N contents of tile drainage from two plots grown to corn ( Zea Mays L.) in rotation with either oats ( Avena sativa L.) or soybeans ( Glycine max (L.) Merr.) for 1974–1978 were measured to determine the effect of differential N fertilization on NO 3 ‐N leaching losses. The plots, grown to corn and fertilized in even‐numbered years only, were fertilized with N at rates of 100 and 250 kg/ha in 1974 and 90 and 240 kg/ha in 1976. In 1978, both plots were fertilized with 90 kg N/ha. The NO 3 ‐N contents of tile drainage from these plots had been established in a previous 4‐year study in which both plots received 112 kg N/ha in 1970 and again in 1972. These data provided a means of comparison on both an absolute and a relative basis for this second phase of a “before and after” study. Although the ratios of NO 3 ‐N concentrations in daily samples from the two plots in phase one were constant and near unity, after differential fertilization, concentrations for the higher fertility plot exceeded those of the lower fertility plot for extended periods by a factor of two after the 1974 fertilization, and by a factor of four after the 1976 fertilization. In 1974, there was a delay of about 2 months (100 mm of flow) before the effect of surface fertilization was observed in the tile drainage; a similar delay was observed in 1976. Although relative NO 3 ‐N concentrations for the higher fertility plot eventually decreased with time after the last differential fertilization, grab samples taken 3 years later still showed the effect of the higher level of fertilization. Overall, concentrations and losses for the plot receiving 90–100 kg N/ha every‐other year averaged 20 µ g/ml and 27 kg/ha; for the plot receiving the most fertilizer, the values were 40 µ g/ml and 48 kg/ha. For the 9 years of record, annual flow volumes averaged 132 mm, which represents a significant contribution to stream flows in central Iowa.