In agroecosystems, two of the most important factors for farmers are soil quality and arthropod communities, both of which play a large role in determining farm productivity. In organically managed farms, farmers rely on compost and green manures to improve soil quality, a process that may take several years to accomplish. Soil quality can affect belowground arthropods and plant growth, but less is known about how soil quality characteristics affect arthropods foraging aboveground. Farm arthropods can provide ecosystem services, such as pest control and pollination, but can also be destructive pests, thus it is important to understand which characteristics influence arthropod abundance and composition. We conducted research on three organically managed vegetable farms to examine relationships between soil quality and above ground arthropod communities. Between May-July 2011, we sampled soils and arthropods in three plots within each farm. We analyzed soil samples to determine soil respiration rates, available nutrients (N, P), microbial biomass, and total organic matter. We collected arthropods with pitfall traps, identified arthropods to order or family, and then classified them as decomposers, herbivores, or predators. We then used conditional inference trees to examine relationships between abundance of all arthropods, abundant orders, and feeding groups and soil characteristics.
Results/Conclusions
Five out of eight soil quality variables (organic matter, nitrate, phosphate, total organic carbon and total organic nitrogen) differed in the three sample plots, consistent with farmer knowledge used to established plots. Values for all of these variables were higher in plots that farmers perceived to have high soil quality. We collected a total of 2040 arthropods. The most abundant orders were Aranae (spiders), Coleoptera (beetles), Hymenoptera (only ants included), Hemiptera (bugs), and Orthoptera (grasshoppers and crickets). Arthropod abundance increased with weed cover in plots. Abundance of spiders decreased with leaf litter cover. Abundance of beetles increased in plots with more ammonium, and decreased where vegetation was taller than 55cm. Ant abundance increased in plots with high levels of phosphates. Abundance of bugs was greater in plots with less total nitrogen. Abundance of grasshoppers decreased with greater amounts total organic carbon. These data suggest that some soil characteristics may directly or indirectly affect the abundance of arthropods within organically-managed agroecosystems.