FERTILIZER PLACEMENT AND TILLAGE EFFECTS OF NITROGEN ASSIMILATION BY WHEAT

Little information is available concerning tillage effects on seasonal N accumulation and their effect on wheat yield. Field studies were conducted to determine the effects of fertilizer N placement and tillage practices on the reduction and assimilation of N in winter wheat (Triticum aestivum L.) during 1984 through 1987 on Renfrow silt loam (fine, mixed, thermic, Uderic, Paleustolls) near El Reno, OK. Fertilizer N (50 kg ha-1) and P (60 kg ha-1) including 4 kg N ha-1 and 11 kg P ha-1, with the seed was applied in the fall. In the spring, an additional 50 to 100 kg N ha-1 as ammonium nitrate was either broadcast or placed in narrow bands on the soil surface. Nitrate reductase (NR) activity, NO3-N, and crude protein (CP) content of wheat plants were determined periodically during spring growth until anthesis, and final grain and straw yield and their CP contents were determined. The NR activity in early spring was slightly greater during early spring in plants grown under no-till when compared to conventional till in 1985 and 1986, whereas the reverse was observed in 1987. Increased NR activity was paralleled by an increase in vegetative CP. Placement of N in a narrow band on the soil surface in the spring improved NR activity levels, but the effect of N placement had little effect on CP concentration. Grain yield followed NR activity levels and was 36% higher in no-fill in 1985, similar in 1986, but 51% lower in 1987 than conventionally tilled plots. Placement of N in narrow bands increased N-use efficiency by increasing N reduction and assimilation; but had little effect on the yield of grain and straw in both tillage methods.