Winter Cover Crop Seeding Rate and Variety Affects during Eight Years of Organic Vegetables: II. Cover Crop Nitrogen Accumulation

Winter cover crops (CC) can improve nutrient use efficiency by scavenging residual soil N. Shoot nitrogen accumulation (NA) of rye (Secale cereale L.), legume-rye, and mustard was determined in December to February or March during the first 8 yr of the Salinas Organic Cropping Systems (SOCS) trial focused on high-value crops in Salinas, CA. By seed weight, legume-rye included 10% rye, 35% faba bean (Vicia faba L.), 25% pea (Pisum sativum L.), 15% common vetch (V sativa L.), and 15% purple vetch (V. bengha-lensis L.); mustard included 61% Sinapis alba L., and 39% Brassica juncea Czern. Cover crops were fall planted at 1x and 3x seeding rates (SR); 1x SR were 90 (rye), 11 (mustard), and 140 (legume-rye) kg ha(-1). Vegetables followed CC annually. Early-season NA was greatest in mustard. Nitrogen accumulation increased more gradually through the season in legume-rye than in other CC. Final NA (kg ha(-1)) was lower in rye (110) and mustard (114), than legume-rye (151), and varied by year. During December, SR increased NA in legume-rye by 41% but not for the other CC. Legumes contributed 36% of final NA in legume-rye, presumably from N scavenging and biological fixation. Nitrogen accumulation was highly correlated with shoot dry matter of legume-rye but not of rye or mustard. Seed costs per kg of NA were more than two times higher for legume-rye than rye and mustard. We conclude that high SR are necessary to hasten early season NA and minimize N leaching potential in legume-rye mixtures.