Relationship between ammonium and nitrate in wheat plant tissue and estimated nitrogen loss

Nitrogen (N) is one of the most important elements in the nutrition of higher plants and one of the most costly inputs in the production of winter wheat in the Great Plains. Nitrogen ranks second only to precipitation as the most frequent yield limiting factor, and even when N is not the yield limiting factor, wheat is less than 50% efficient at utilizing applied N fertilizer. If N supplied to the crop is not utilized efficiently, it may be lost from the cropping system to the surrounding environment. The objective of this study was to evaluate the relationship between NH4-N and NO3-N in wheat tissue and estimated plant N loss. Two experimental sites for this study were selected as subplots located within existing plots in two long-term winter wheat experiments at Stillwater (experiment 222) and Lahoma (experiment 502), Oklahoma. Wheat forage samples were collected at Feekes growth stage five (leaf sheath strongly erected) and Feekes growth stage 10.5 (flowering complete to top of ear). Samples were dried, ground, and analyzed for total N, NH4-N, and NO3-N. The relationship between total N, NH4-N, and NO3-N at both growth stages and estimated plant nitrogen loss (plant N uptake at flowering minus total N uptake in the grain plus straw) were evaluated. No relationship was found to exist between forage NH4-N and NO3-N and estimated plant N loss. Due to cool and moist climatic conditions during late spring in both years, estimated N losses were small from anthesis to maturity using the method described. Plant tissue NO3-N at Feekes five was correlated with total N accumulation in the plant at flowering and with grain N uptake at experiment 502 in both years.