NITRATE-NITROGEN IN TILE DRAINAGE AS AFFECTED BY FERTILIZATION

The volumes and NO3-N contents of tile drainage from 2 plots grown to corn (Zea mays) in rotation with oats (Avena sativa) or soybeans (Glycine max) for 1974-1978, were measured to determine the effect of differential N fertilization on NO3-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 NO3-N contents of tile drainage from these plots had been established in a previous 4 yr study in which both plots received 112 kg N/ha in 1970 and again in 1972. These data provided a means of comparison on an absolute and a relative basis. Although the ratios of NO3-N concentrations in daily samples from the 2 plots in phase 1 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 2 after the 1974 fertilization and by a factor of 4 after the 1976 fertilization. In 1974, there was a delay of .apprx. 2 mo. (100 mm flow) before the effect of surface fertilization was observed in the tile drainage; a similar delay was observed in 1976. Although relative NO3-N concentrations for the higher fertility plot decreased with time after the last differential fertilization, grab samples taken 3 yr later still showed the effect of the higher level of fertilization. Concentrations and losses for the plot receiving 90-100 kg N/ha every other year averaged 20 .mu.g/ml and 27 kg/ha; for the plot receiving the most fertilizer, the values were 40 .mu.g/ml and 48 kg/ha. For the 9 yr, annual flow volumes averaged 132 mm, which represented a significant contribution to stream flows in central Iowa [USA].