Agriculture faces pressure to supply increased food, fiber, and fuel, while provisioning ecosystem services. Intensive agriculture has contributed to altered global carbon (C) and nitrogen (N) cycles, and is the leading source of nitrous oxide (N2O), a powerful greenhouse gas. New approaches are needed to improve soil nutrient management while sustaining agricultural productivity. In natural systems a strong relationship exists between aboveground diversity and belowground ecosystem controls on nutrients. While increasing the number of crops in rotation in agroecosystems can have multiple benefits, the linkages between rotational diversity, nutrient cycling pathways, and the soil ecosystem remain poorly understood. My dissertation addresses the effects of increasing crop rotational complexity on soil C and N cycles and the soil ecosystem process that regulate them. I extend this work by testing ecosystem-based measures of soil health on Michigan farms.Long term cropping system experiments are ideal sites to examine how rotational legacy shapes belowground processes. Chapters 1, 2, and 3, focus on a cropping biodiversity experiment in place for 10 years at the initiation of my study. The gradient ranges from continuous summer annuals – corn (Zea mays L.) and soybean (Glycine max.) – to rotations that also include a winter annual (wheat; Triticum aestivum L.), to complex rotations with overwintering cover crops. This experiment has not received any external inputs such as fertilizer or pest control agents, so that changes across the gradient are narrowed to the effect of crop rotational complexity. In Chapter 1 I examine how labile C and N pools, soil enzyme activities, and soil respiration respond to increasing rotational complexity. I found that pools of potentially mineralizable C were nearly twice as high on fields with a history of cover crops, compared to those without. Rotations with a legacy of cover crops sustained higher enzyme activities, significantly higher soil respiration and accumulated significantly higher total soil organic matter.In chapter 2, I tested the relationship between increasing the number of crops in rotation and the species diversity of soil bacteria. I found no significant difference in species diversity of bacteria, but a shift in the community between rotations with and without cover crops. Taxa responsible for this shift were mainly from the Acidobacteria and the Proteobacteria which are characterized by contrasting growth and energy use strategies. I focused on denitrification in Chapter 3, a process carried out by soil microbes that produces N2O. Rotations with cover crops had significantly higher mean N2O flux over two growing seasons. Enzyme assays showed that denitrification was more efficient on these rotations, and rotations with cover crops also had a significantly higher proportion of genes in the N2O-production pathway that derived from ammonia oxidizing bacteria.Finally, in Chapter 4 I tested soil health on fields that Michigan farmers had designated as having either good or poor soil quality. Testing captured soil variability on farmer fields, but interviews with farmers revealed caveats to implementing soil health testing. My work on increasing rotational complexity revealed novel microbial controls on soil C cycling and N2O flux, but ultimately implementing practices that enhance soil ecosystem function depends on human decisions about land use, crop production, and environmental outcomes.
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