Nitrogen Use by Tall Fescue and Switchgrass on Acidic Soils of Varying Water Holding Capacity

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Abstract
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 15N‐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.

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