Assessing Biodiversity in Summer Cover Crop Rotations for Beneficial Arthropod Conservation

Abstract

Intensive vegetable crop practices can have detrimental effects on soil health, draining soil of organic matter reserves and necessitating nitrogen (N) inputs. These amendments can support crop productivity but may have considerable environmental consequences including overfertilization, water eutrophication, and reduce fauna (Agostini et al., 2010). In addition, many vegetable crop rotations are characterized by high plant density but low beneficial arthropod and pollinator habitat diversity (Carreck and Williams, 2001). Habitat simplification may cause declines in beneficial arthropods, which can lead to insufficient pollination services and increased pest pressure (Hogg et al., 2011; Wilson et al., 2018). Out of numerous arthropod services, pollination, pest control, dung burial and wildlife nutrition have been estimated to value at least $57 billion USD (Forister et al., 2019). Because of sheer amount of agricultural land, comprising over 40% of all U.S. land, agricultural ecosystems present valuable opportunity in maintaining biodiversity (USDA, 2012). Constant forage availability from early spring to late summer is particularly imperative for the survival of social insects with long-lived colonies, such as bees (Carreck and Williams, 2001). The use of cover crops can expand the seasonal availability of floral resources in agroecosystems (Hooks et al. 1998; Landis et al. 2005). In a two-part study, this research investigated the effects of summer-planted cover crops on soil nutrient contributions and beneficial arthropod density, diversity, richness and evenness. Part one consisted of a one-year (2019) field trial study where nine treatments, each containing one to seven species of cover crops, were evaluated for flowering, aboveground biomass production and nitrogen (N) content, soil-N contribution after biomass incorporation, and beneficial arthropod visitation. A seven-species mix composed of oat (Avena sativa L.), field pea (Pisum sativum subsp. Arvense L.) and 5 clover species (Trifolium spp.) added the largest amount of biomass (8747 kg/ha). Likewise, this mix contributed the most organic N (265.6 kg N/ha) and maximum soil -N subsequently increased after biomass incorporation (10.9 mg -N/kg of soil). Part two consisted of a two-year (2020-2021) study evaluating the ability of two rotational systems of summer cover crop species, selected from 2019 trials, to provide key multifunctional ecosystem benefits to vegetable crop production. The first system was an early cover crop planting followed by a broccoli cash crop, and the second system consisted of an early planting of spring lettuce followed by a late summer cover crop planting. In all three years, buckwheat (Fagopyrum esculentum Moench) and phacelia (Phacelia tanacetifolia Bendth.) monoculture produced most abundant floral resources. Beneficial arthropods observed included pollinators (native, honey and bumblebees), predators (syrphid flies and minute pirate bugs), and parasitoids. Increased floral diversity was associated with abundance of flies in the family Syrphidae. Phacelia monoculture was most attractive for bees in the families Apidae and Halictidae, both of which may provide pollination services. These results highlight floral visitation patterns as an indicator for beneficial arthropod community support and conservation. Summer-planted cover crops are an underexplored rotation option for organic farming systems in the Upper Midwest and may support a wide range of ecosystem services including increases in available soil N and beneficial arthropod services.