Recovery of fertilizer nitrogen applied to burley tobacco

The high cash value of burley tobacco (Nicotiana tabacum L.) may lead to ineffective excess fertilization of the crop. The effects of N management on fertilizer N use efficiency by burley tobacco and recovery of residual fertilizer N by a winter cover crop were determined. Tobacco was grown in 1993 at two locations with 0 to 336 kg N ha-1 broadcast applied before transplanting or 168 kg N ha-1 sidedressed at onset of rapid growth (approximately 5 wk after transplanting). Seasonal recoveries of 15N-depleted fertilizer N were determined in aboveground tissues of the tobacco crop. The amount of labeled fertilizer N recovered at harvest (29 to 124 kg N ha-1) increased with increasing amounts of N applied, but fertilizer N use efficiency was constant at 36.6 +/- 0.9% for broadcast applications. Sidedress N enhanced labeled fertilizer N recovery (43 to 54% of applied N, depending on location), even though total aboveground dry matter was equal to or 12% less than that of plants grown with the same amount of N applied by broadcast. Recovery of residual labeled fertilizer N in the cover crop ranged from 2 to 7% of applied N and was greatest for the highest amount of N applied. Fertilizer N recoveries estimated by the difference method equaled those of the 15N method at 5 wk after transplanting. At 13 wk after transplanting, the difference method, particularly for applications of 84 and 168 kg N ha-1, overestimated fertilizer N recoveries relative to the 15N method, and is unlikely to provide an accurate measure of fertilizer N use by burley tobacco across a wide range of N applications. Reducing the amount of broadcast applied fertilizer to give a 10% decrease in air-cured leaf yield would decrease the unaccounted fertilizer N by 80 to 115 kg N ha-1 depending on location. As compared with broadcast application of fertilizer N before transplanting, drill banding the N at about 5 wk after transplanting would equal or further decrease unaccounted amounts of fertilizer N.