Integrating Components of a Managed Ecosystem: Cover Crops
Cover crops are an excellent example of an innovative strategy to integrate multiple components of a managed ecosystem. These noncrop plant species, such as vetch and clover, are grown as ground cover to manage pests, provide nitrogen for subsequent crops, increase soil organic matter, and reduce soil erosion (National Research Council, 1989b). Because cover crops increase ecosystem biodiversity which, in turn, affects multiple biological interactions involving pest management, soil fertility, and plant nutrition, ecosystem interactions should be carefully considered when integrating a cover crop into a pest-management strategy. Since the long-term impacts of cover crops are not well known, additional research can provide a greater understanding of their role in crop production (Hanna et al., 1995).
Cover crops can provide habitat and a food source for biological-control organisms. California vineyard managers plant clover and other legume ground covers to attract beneficial wasps and spiders; their abundance is associated with decreases in leafhopper pests (Hanna et al., 1995; National Research Council, 1989a). Some cover crops produce allelopathic compounds that can suppress plant parasitic nematodes. Ground covers also can delay weed emergence, giving a competitive edge to the primary crop.
However, interactions of a cover crop with other ecosystem components can lead to undesirable effects; a cover crop species that is optimal for biological control of arthropod pests may not be competitive with a troublesome weed (Ingels, 1995). Thus, considerable knowledge of ecosystem processes is necessary to successfully manage pests in cover-cropping systems.
Soil Fertility and Plant Nutrition
Cover crops can increase soil fertility and plant nutrition; legumes such as vetch and clovers exhibit powerful nitrogen-supplying capabilities. Symbiotic bacteria initiate nodules on the roots of legumes, which transform atmospheric nitrogen into a useful nitrogen source for plant growth (nitrogen fixation). This complex nitrogen-transformation process is influenced by numerous factors including soil microorganisms, cover crop species, tillage, and water (National Research Council, 1989b).
Since cover crops may also remove nutrients from the soil, nutrient status of a primary crop needs to be monitored. Thus, growers need to weigh the benefits and disadvantages of using cover crops to increase soil fertility and plant nutrition.