NITROGEN USE BY TALL FESCUE AND SWITCHGRASS ON ACIDIC SOILS OF VARYING WATER HOLDING CAPACITY

Increased use of bunchgrasses on acidic soils of the humid Northeastern USA requires more detailed knowledge on the fate of fertilizer N for economic and environmental reasons. A 3-yr field investigation using N-15-depleted fertilizer was undertaken to determine the effect of soil water holding capacity (WHC) on the use of soil and fertilizer N by cool-season (tall fescue, Festuca arundinacea Schreb.) and warm-season (switchgrass, Panicum virgatum L.) forages. Three sites of varying soil depth provided WHC of 5, 15, and 25 cm. Fertilizer N was applied at 0, 90, and 180 kg ha-1 (split application for tall fescue). Three-year production averages for Cut 1 switchgrass (mid-July) were two- to three-fold greater than for tall fescue (mid-June) on all sites receiving N, and four-fold greater on the two shallower sites without N. Cut 2 (late-October) production was nearly equal for both species, with tall fescue averaging 50%, and switchgrass 10%, of Cut 1. Nitrogen concentration of both species generally increased with N rate but not with WHC. Total N uptake (TNU) increased with N rate for both cuts of tall fescue, but only for Cut 1 for switchgrass, on all sites. Percentage of TNU derived from fertilizer N ranged from 23 to 47% for tall fescue and 14 to 39% for switchgrass. Tall fescue and switchgrass recovered from 23 to 31% and 25 to 33%, respectively, of the fertilizer N applied to the two deeper sites. Only on the shallow Klinesville site at the 90 kg N level was there a significant difference in fertilizer-N recovery, where switchgrass recovered 31% compared to tall fescue at 19%. Although these results demonstrate relatively low recoveries by both species, the more efficient use of fertilizer N by switchgrass, both in terms of production and recovery, recommend its increased usage at low N rates in the Appalachian Region.