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 × 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 after 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.