Nitrogen Use Efficiency and Nitrogen Budget for Conservation Tilled Wheat

Abstract

Abstract 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 NH 3 flux data to present a N budget for the soil‐plant‐atmosphere system. Fertilizer N uptake was determined using ammonium nitrate ( 15 NH 4 15 NO 3 ) tagged with 3.78 atomic % 15 N. 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 NH 4 and NO 3 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 stage (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 NH 3 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 NH 3 efflux from the soil‐plant system was 15.5 kg N ha −1 . About 21% of the spring applied fertilizer N was lost through NH 3 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.