Release of Available Nitrogen after Incorporation of a Legume Cover Crop in Concord Grape

Legume cover crops can be used to provide nitrogen (N) to organically produced Concord ( Vitis labruscana Bailey) grape. The cover crop must be incorporated at a time such that subsequent N mineralization is synchronous with plant demand to maximize the amount of N available to the grape plant. The objectives of this research were to 1) evaluate the effectiveness of hairy vetch ( Vicia villosa subsp. villosa L.) and yellow sweet clover [ Melilotus officinalis (L.) Lam.] in providing N to organically grown Concord grape, 2) examine the synchronization of N release from mineralization after incorporation of cover crops with plant N demand, and 3) compare soluble, more readily available sources of N to legume cover crops in providing N to grape. This work was conducted on two Concord vineyards, one commercial (COM) and one research (RES) vineyard. Both vineyards were overhead sprinkler-irrigated and plots were established in a Latin square design with four or six replicates of each treatment. Treatments consisted of hairy vetch and yellow sweet clover planted in either the spring or fall, 112 kg·ha −1 N added as either urea or blood meal, and a 0 kg·ha −1 N control. Soils were sampled weekly (0 to 30 cm) from budbreak to cover crop plot treatment establishment and were analyzed for soluble (NO 3 -N and NH 4 -N) N. Soluble N release in the plots was monitored with ion exchange membranes (plant root simulators). Grapes were harvested and evaluated for yield and °Brix. Legume and fertilizer treatments resulted in increased N availability from grape bloom until veraison. As a result of rapid nitrification, NH 4 -N was less useful than NO 3 -N in determining N mineralization patterns. Available N peaks as high as 40 mg·kg −1 NO 3 -N were well timed with the critical N demand period for Concord grape. Soluble N sources (urea and blood meal) peaked higher than plant sources. No differences were detected between legume treatments. Legume covers did, however, supply more available N per unit of biomass to the soil than a small grain cover. Yield and o Brix varied by year but not by treatment, suggesting that the cover crop or plant and soil N reserves provided sufficient available N to the grape through the study period.