Nitrogen use efficiency and nitrogen budget for conservation tilled wheat

The understanding of nitrogen (N) cycling in the soil-plant-atmosphere is necessary to maximize N use efficiency and to develop N budgets for wheat (Triticum aestivum L.) production. The objectives of this study were to determine the relative uptake rates of residual soil N and fertilizer N in conservation tilled winter wheat. These data were combined with soil mineralization and aerial NH3 flux data to present a N budget for the soil-plant-atmosphere system. Fertilizer N uptake was determined using ammonium nitrate (15NH415NO3) tagged with 3.78 atomic % 15N. The buried polyethylene bag technique was used to determine N mineralization. Fertilizer N uptake and N mineralization rates were determined four times during the spring growing season. Amounts of NH4 and NO3 in the top 0- to 300-mm soil layers were determined biweekly. Fertilizer N levels in the surface to 75-mm soil layers decreased rapidly due to plant uptake and immobilization. Of the N fertilizer utilized by the plants, 61% was absorbed in the first 28 d after application. During early vegetative growth stages (Feekes stages 3-5), fertilizer N uptake was 1.33 kg ha-1 d-1. During the elongation state (Feekes stages 5-8), however, fertilizer N was immobilized and uptake of fertilizer N ceased. This resulted in a period of soil N insufficiency which was associated with atmospheric NH3 influx to the plants. After 4 to 5 wk, mineralization of fertilizer N became apparent and fertilizer N uptake rates increased until harvest. Influx of atmospheric N was small but total NH3 efflux from the soil-plant system was 15.5 kg N ha-1. About 21% of the spring applied fertilizer N was lost through NH3 volatilization and losses of this magnitude need to be considered in N balance studies. Placement of fertilizer N below the surface soil layer may decrease immobilization and increase plant uptake of N.