ARS | Publication request: Seeding rate and year effects on rye, legume-rye, and mustard winter cover crops during 8 years of organic vegetables in California. 3. Residue quality and nitrogen mineralization

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 26, 2012
Publication Date: N/A

Interpretive Summary: Winter cover crops are a best management practice to nutrient-use efficiency in high-input vegetable production systems. Shoot residue quality and soil mineral N following incorporation of rye, legume-rye, and mustard cover crops was determined during 8 winter periods in a long term trial focused on high value organic crop production in Salinas, California. Cover crops were fall-planted at standard and three times higher seeding rates prior to vegetables annually. In general, cover crop carbon concentrations were higher in rye and legume-rye, nitrogen concentrations were higher in mustard and legume-rye, and carbon to nitrogen ratios were higher in rye. Cover crop residue quality tended to decline over the winter, however, this was more apparent for rye and mustard residues than legume-rye residue. Increasing the cover crop seeding rate caused a relatively minor reduction in cover crop residue quality early in the season in some cases. Soil mineral N following cover crop incorporation varied considerably between years mainly due to differences in spring rainfall. This study indicates that nitrogen leaching from cover crop residues may be a problem during rainy spring conditions. Furthermore, the higher carbon to nitrogen ratios of mustard and legume-rye may cause these residues to decompose more rapidly and create fewer challenges with residue management in subsequent vegetables production. The information from this study will help growers select the most appropriate cover crops to rotate with vegetables and make management that allow them to integrate cover crops into rotations more often.

Technical Abstract: Winter cover crops (CC) can improve nutrient-use efficiency in tillage-intensive systems. Shoot residue quality and soil mineral N following incorporation of rye (Secale cereale L.), legume-rye, and mustard CC was determined in December to February or March during the first 8 yr of the Salinas Organic Cropping Systems trial focused on high-value crops in Salinas, CA. Legume-rye included faba bean (Vicia faba L.), pea (Pisum sativum L.), vetches (V. sativa L., V. benghalensis L.) and rye; mustard included Sinapis alba L., and Brassica juncea Czern. Cover crops were fall-planted at standard and three times higher seeding rates (SR) prior to vegetables annually. Significant CC x year interactions occurred for C and N concentrations, and C:N ratios. However, 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. Over the season, C concentrations and C:N ratios tended to increase while N concentrations decreased. Legume-rye residue quality changed least over each season. Seeding rate affected N concentrations and C:N ratios, however, the effect varied over time and by residue and may be practically insignificant. 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 will decompose more rapidly and minimize tillage challenges for subsequent vegetables but may be more prone to post-incorporation N leaching.