Winter cover crops in a vegetable cropping system: Impacts on nitrate leaching, soil water, crop yield, pests and management costs

Plant-soil relationships in the surface soil layer affect other processes in agroecosystems, including crop productivity, nitrate leaching and plant-pest interactions. This study investigated the effect of altering surface soil dynamics, using a winter cover crop rotation, on biotic and abiotic characteristics of the soil profile. Two cover crop treatments, phacelia and Merced rye (Phacelia tanacetifolia cv. 'Phaci', and Secale cereale cv. 'Merced'), with a fallow control, were planted in November after harvest of a broccoli crop on a commercial farm site, and were incorporated using reduced tillage techniques the following March. Changes in plant and soil N pools throughout the profile were described, emphasizing nitrate (NO3--N) leaching during winter, and N availability during the subsequent broccoli crop. Changes in other aspects of the ecosystem, such as plant-pest interactions and plant disease incidence, were monitored after cover crop incorporation. The on-farm economic costs of cover cropping were calculated. There was a 65-70% reduction in nitrate leaching from the cover-cropped plots compared with the fallow control during winter, because plant roots in the surface soil removed N and water that would have otherwise been lost from the profile. Incorporation caused sudden large surges in inorganic N pools, net mineralizable N, and microbial biomass N and C in the surface soil, which subsided within 6 weeks, by the time the broccoli crop was planted, but which did result in increased yield at harvest in the phacelia cover-cropped treatment. No insect or disease problems which threatened the cash crops were introduced or increased as a result of the cover crops. The economic analysis indicated that the costs of cover cropping were minor compared with conventional winter management of fallowed fields, and compared with the cost of producing broccoli. The cover crops therefore provided a clear advantage during winter by significantly reducing nitrate leaching, but the effects of one cover crop rotation on subsequent nutrient dynamics in the surface oil were mostly short-lived and possibly masked by large fertilizer applications.