Soils, together with the vegetation they support, constitute the largest terrestrial carbon reservoir. However, human-driven land use change has caused significant carbon depletions in soils worldwide, particularly in managed ecosystems. As rising emissions threaten to exacerbate an already worsening climate crisis, strategies to replenish soil carbon stocks are imperative to mitigate further global warming. This is the main motivation for expanding second-generation bioenergy crop production in North America, with potentially substantial carbon gains possible in bioenergy cropping systems established on degraded soils. Yet in the face of uncertainties regarding the factors regulating carbon turnover and stabilization across diverse bioenergy cropping systems, the true potential for these systems to accrue carbon at meaningful rates remains unresolved. As the soil carbon cycle is directly controlled by microbial and plant processes, research has largely focused on investigating the physiological, climatic, and physiochemical factors regulating them in bioenergy cropping systems. However, very little is known about the microarthropods, small yet highly abundant and diverse soil fauna ubiquitous across ecosystems, these systems harbor. Microarthropod activity has long been known to have an important role in organic matter decomposition, yet they directly contribute relatively little to net soil carbon gains and losses due to their low biomass and metabolism. However, microarthropods can strongly influence microbes via a variety of mechanisms, including via direct microbivory, promoting nutrient availability, and altering organic matter quantity and chemistry. Thus, microarthropods can indirectly affect soil carbon dynamics by regulating microbial community structure, activity, and access to organic matter. The strength and direction of these microarthropod-microbe effects on carbon accrual, while presently unclear, will largely depend on the microarthropods and microbial communities involved, both of which are strongly dependent upon aboveground land use. In this dissertation, I approached this broad yet important knowledge by addressing the following key uncertainties: 1) what microarthropods are currently present in bioenergy cropping systems, 2) how do cropping system attributes affect their community structure, and 3) given the multitude of interactions and mechanisms operating simultaneously, what is the net impact of microarthropod-microbe interactions on soil carbon dynamics in these systems? In Chapter 1, I reviewed the literature on soil fauna effects on soil carbon cycling, narrowing my focus on these dynamics in context of perennial grass bioenergy cropping systems, and identifying where research is most needed to someday incorporate faunal activity in soil carbon models. In Chapter 2, I addressed the first and second major uncertainties by surveying microarthropod communities from bioenergy cropping systems ranging from an annual monoculture to a perennial polyculture. Over the span of two years, I found that perennial cropping systems consistently supported greater total microarthropod and mite abundances compared to the annual system. Having characterized the microarthropod communities in these systems, I addressed the third major uncertainty by conducting two greenhouse mesocosm experiments to evaluate the potential effects of these communities on key predictors of soil carbon accrual and stability. In Chapter 3, I investigated the effects of microarthropods from either a perennial or annual monoculture on microbial carbon use efficiency using a 18O-water tracer method. In Chapter 4, I narrowed my focus to perennial monoculture (switchgrass) communities to assess the relative effects of microarthropods and nematodes, alone and in combination, on nitrogen mineralization and switchgrass productivity. While I did not find conclusive evidence to suggest an effect on carbon use efficiency, I did find that microarthropods in combination with nematodes stimulated nitrogen mineralization from litter and subsequent assimilation into switchgrass roots, though only nematodes individually retained this positive effect. I conclude in Chapter 5 by reviewing the key takeaways from this research, discussing the broader implications of my findings as well as some of the methodological challenges associated soil fauna research I encountered, and suggesting future studies to address remaining questions. Despite the daunting uncertainties that remain, continued research into the complex interactions between microarthropods, microbes, and bioenergy crops will doubtlessly be important to better understand their potential contributions to soil carbon accrual and storage in bioenergy cropping systems.
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