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

Abstract

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 (NO3-N and NH4-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, NH4-N was less useful than NO3-N in determining N mineralization patterns. Available N peaks as high as 40 mg·kg−1 NO3-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 oBrix 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.