Winter Cover Crop Seeding Rate and Variety Effects during Eight Years of Organic Vegetables: III. Cover Crop Residue Quality and Nitrogen Mineralization

Winter cover crops (CCs) can improve nutrient-use efficiency in tillage-intensive cropping systems. Shoot residue quality and soil mineral N following incorporation of rye (Secale cereale L.), legume-rye, and mustard CCs was determined in December to February or March during the first 8 yr of the Salinas Organic Cropping Systems trial in Salinas, CA. Legume-rye included Vicia faba L., V. sativa L., V. benghalensis L., Pisum sativum L., and rye; mustard included Sinapis alba L. and Brassica juncea Czern. Cover crops were planted in the fall at standard and three times higher seeding rates (SRs) before vegetables annually. Significant CC x year interactions occurred for C and N concentrations and C/N ratios of CC shoots. In general, C concentrations were higher in rye and legume-rye, N concentrations were higher in mustard and legume-rye, and C/N ratios were higher in rye. During the season, C concentrations and C/N ratios tended to increase, whereas N concentrations decreased. Compared with rye and mustard, legume-rye residue quality changed least during each season. Increasing the SR reduced N concentrations and increased C/N ratios; however, the effect varied with time and by residue. Following CC incorporation, soil mineral N varied between years and CC and was typically highest following legume-rye or mustard and lowest without CC. Rainfall aft er CC incorporation reduced soil N one year, suggesting that leaching occurred. We conclude that mustard and legume-rye produce higher quality residue that will decompose more rapidly and minimize tillage challenges for subsequent vegetables but may be more prone to post-incorporation N leaching.