Effects of differentiated applications of fertilizer N on leaching losses and distribution of inorganic N in the soil

Summary Nitrogen fertilizer was applied to field plots at rates of 0, 50, 100, 150 and 200 N kg/ha yr, in order to determine the effects of differentiated N applications on drainage water and groundwater quality. Water samples, collected monthly or bimonthly from 1974 to 1983, were analysed for inorganic and total N content. In order to see the impact of residual N on leaching losses, soil samples were collected to a depth of 2 m in the N0, N100 and N200 plots, usually in September and April. Leaching of nitrate was moderate to the N100 level but increased substantially with increasing fertilization, up to 91 N kg/(ha-yr) for the highest application rate (N200), during the wet year of 1980/81. The losses were greatest during the fall, mainly due to high levels of N remaining in the soil after harvest combined with high precipitation. The N content of the groundwater did not show any significant correlation to the fertilization intensity. A buildup of inorganic N in the soil occurred only when excessive amounts of fertilizer were applied (N200), while the contents of the N0 and N100 treatments fluctuated around states of balance, approximately 45 and 70 N kg/ha respectively. Spring rape followed by winter wheat showed a great ability to reduce N contents in the tile effluent from highly fertilized plots (N150 and N200), even though the plots had received excessive amounts of fertilizer for several years. Results of this experiment in central Sweden demonstrate the importance of applying nitrogen fertilizer in balance with crop needs and of maintaining a growing crop cover as much of the time as possible in order to minimize water pollution.