Abstract
Across the globe, agricultural systems are diversifying to address environmental change and enhance sustainability. The Inland Pacific Northwest’s (IPNW) dryland wheat-based cropping systems reflect this trend. Improving soil health is a significant motivator for farmers to adopt diversified agricultural practices. Soil health is a holistic concept that integrates the chemical, physical, and biological attributes of soil that enable it to perform functions relevant to agriculture and other contexts that support human well-being. Knowledge of how agricultural diversification affects soil health, specifically the biological component of soil health, in IPNW agroecosystems is limited. This dissertation investigates how agricultural diversification practices affect soil arthropod communities, a critical but often overlooked component of biological soil health. One potential diversification practice is rotational diversification. I explored how rotational diversification in dryland cropping systems shaped soil arthropod communities and investigated relationships between the Soil Biological Quality index (QBS-ar), which uses soil arthropods as bioindicators of soil health, and other physiochemical and biological soil health indicators. Specific crops used in diversified rotations modified arthropod communities to differ in biodiversity and their functional capacities. The relationships between QBS-ar and other soil health measures were complex and varied. Results suggest QBS-ar may indicate changes in soil health over short periods and could help detect changes in soil quality early in transitionary periods, such as the implementation of diversified agricultural practices.
Cover cropping is another diversification strategy, but adoption is limited in the IPNW due to a lack of regionally specific information. Cover crops can potentially improve multiple aspects of soil health, but how cover crop polycultures vs. monocultures affect soil ecology and functioning is still being determined. To address this, I compared how cover crop diversity affected soil arthropod biodiversity and determined the legacy effects of cover crops on arthropod-mediated decomposition and plant performance in a subsequent wheat cash crop. On average, polyculture cover crops outperformed their constituent monocultures in the ability to promote soil arthropod biodiversity. Cover crops exhibited legacy effects on soil arthropods in a subsequent wheat cash crop, notably with greater diversity in wheat following polyculture cover crops than following monocultures and fallow. These cover crop legacy effects on soil arthropod communities had functional implications, contributing to greater litter decomposition rates and cash crop productivity (biomass and yield).
Using a complementary on-farm study in collaboration with IPNW farmers experimenting with cover crops, I found that cover crop fields supported greater soil arthropod abundance, richness, and diversity and had more connected and complex co-association interaction networks than traditional spring crop fields. There were, however, no differences in soil arthropod abundance or biodiversity between low and high mix richness cover crops, but interaction networks and community structures were altered.
Soil arthropod communities in agricultural soils regulate processes that affect plant growth and above-belowground interactions. Most research into these effects has been conducted in natural systems, but there are important consequences for agroecosystems and sustainable plant production and protection strategies. I explored how soil arthropod community-level effects influenced crop growth and above-belowground interactions using a study system consisting of wheat and an aboveground aphid herbivore in a greenhouse microcosm experiment. Wheat grown in soils with arthropod communities had greater root and shoot biomass, greater soil nitrate concentrations, and altered root architecture than wheat without soil arthropod communities. Plant damage by aphids was lowest on wheat grown in soils with arthropods. Wheat grown in soils with arthropods had increased stress- and defense-related phytohormone levels in response to aphid herbivory. In contrast, phytohormones of wheat plants grown in soils without arthropods did not differ with aphid presence. Sustainable plant production and protection strategies should consider the community-level effects of soil organisms on above-belowground interactions in agroecosystems.
The interactions within soil arthropod communities that drive soil arthropod-mediated ecosystem services that affect above-belowground interactions are often mediated by chemicals. I explored how soil properties, such as moisture, pH, and adsorption, affect the transport and perception of semiochemicals. The soil environment introduces nested complexities resulting from heterogeneity at multiple scales that must be appreciated to decipher how soil arthropods negotiate these environments.
A thorough understanding of the effects of agricultural diversification practices on soil arthropod biodiversity, community composition, and functioning is required to enhance soil health and support the development of sustainable agriculture in the IPNW. Counteracting biodiversity loss in soils is a major challenge and opportunity for farmers and land managers across the globe. The work presented in this dissertation shows the potential for agricultural soils to play an essential role in promoting and conserving soil arthropod communities and their functions using ecologically based management strategies.