CHANGES IN TOTAL INORGANIC PROFILE NITROGEN IN LONG-TERM RYE–WHEAT–RYEGRASS FORAGE PRODUCTION SYSTEM*

ABSTRACT Inorganic soil profile nitrogen (N) levels can increase with application rates greater than those necessary for maximum cereal grain yields. Forage systems are different than grain production systems in that harvest is generally prior to anthesis and gaseous plant N loss is not allowed to occur. This promotes removal of more total N from the system and results in higher nitrogen use efficiencies (NUE).Nitrogen rates representing increased profile N accumulation as well as those below this threshold were evaluated from a long-term rye–wheat–ryegrass production experiment. Nitrogen rates from 1979–1993 were 0, 56, 84, 112, 168, and 224 kg N ha−1.Rates were doubled in 1994 in an effort to add N at a rate above which no increase in forage production would be expected.Wheat was eliminated from the winter seeding mix in 1994, as it was an extremely small portion of the total harvested forage. Deep soil cores (0–366 cm) were taken during the early summer of 1996 from plots with a history of continuous fertilization and forage production since 1979.Cores were split into 15 cm (0–60 cm) or 30 cm increments (60–366 cm) and analyzed for NO3–N, NH4–N, and pH.Total inorganic N accumulations were calculated by adding NO3–N and NH4–N.Surface accumulations of NH4–N were significant with annual N rates of 224 kg N ha−1 or more. No differences in NH4–N were noted at lower depths, thus movement through the profile was not observed. Nitrate–N at the highest N rate was significantly higher than check levels down to 270 cm increasing the risk of groundwater contamination.It should be noted that the maximum N rate was increased to 448 kg ha−1 in 1994 in an attempt to determine a level above which no yield response would be noted. No increase in forage production has been consistently noted with rates over 224 kg N ha−1 and at this level the only adverse effect is increased NO3–N in the surface 90 cm. These long-term experimental results support the conclusion that nitrogen fertilizer additions at recommended rates do not increase the risk of NO3–N leaching.