Postfertilization Changes in Concentration of Nitrate Nitrogen in Kentucky Bluegrass and Tall Fescue Herbage1

Abstract

AbstractA continuing deterrent to increased use of N fertilizers on humid‐zone grasslands is the possibility of deleterious nutritive effects resulting from ingestion by animals of herbage high in nitrate. Studies were initiated to obtain data on the pattern of NO3‐N accumulation and disappearance in Kentucky bluegrass (Poa pratensis L.) and tall fescue (Festuca arundinacea Schreb.) herbage following spring fertilization with four levels of N.Identical experiments of split‐plot design were conducted on adjacent areas of the two grasses. Ammonium nitrate was applied to supply 50, 100, 150, or 200 kg/ha of N. The N treatments were replicated hi time, with a spacing of 3 weeks between replications. Herbage samples were collected at weekly intervals for a period of 6 weeks following fertilization. Dry matter yields and concentration of total N and NO3‐N in the herbage were determined.The pattern of accumulation and disappearance of NO3‐N appeared to be similar for the two grasses. Accumulation peaks were almost always reached within 14 days following fertilization, after which the values gradually declined. Length of time required for dissipation of the effect of N application on NO3‐N levels was directly related to quantity of fertilizer used but also appeared to be dependent on the species of grass. In two of the three growth periods, for example, the concentrations of NO3‐N in Kentucky bluegrass receiving 200 kg/ha of N decreased to values below 2,000 ppm within 4 weeks after fertilization. In contrast, tall fescue herbage consistently contained 2,000 ppm or more of NO3‐N 6 weeks after fertilization at the 200 kg/ha rate.Coefficients of variation for ppm of NO3‐N were 22.5% for Kentucky bluegrass and 21.1% for tall fescue; the C.V. for percent total N was 4.4% in bluegrass and 4.3% in fescue. Peak NO3‐N levels and maximum total‐N values occurred at essentially the same time.Some implications of the findings are discussed with respect to utilization of N‐fertilized grasslands and the use of herbage NO3‐N levels for predicting response of grasslands to N fertilization.