Author ORCID Identifier
Year of Publication
Doctor of Philosophy (PhD)
Agriculture, Food and Environment
Plant and Soil Sciences
Dr. Rebecca L. McCulley
The diversity of living species in an agroecosystem affects both natural resources and agricultural production efficiency. The rise of managed plant monocultures has allowed agricultural yields to increase over time. However, simplifying agroecosystems affects the capacity of the system to sustainably provide clean air, water, and productive soils essential for continued food and fiber production. This dissertation manipulates species diversity in row crop and forage agroecosystems prevalent in the Eastern United States and tracks the associated environmental and agricultural production consequences.
In row crop agroecosystems, increasing plant diversity with winter cover crops has been suggested as a strategy to increase cash crop yields and enhance ecosystem processes such as soil carbon storage and crop weed suppression. I planted winter cover crop mixes in a maize-soybean rotation from 2016-2018 to assess soil and yield responses and found that the primary benefits of winter cover crop mixes included reduced soil erosion and decreased production costs. However, these benefits were only present in cover-cropped scenarios when they were coupled with conversion from conventional to no-tillage production, indicating that adoption of cover crops in conventional tillage regimes incurred greater costs without fully mitigating soil erosion concerns.
In a livestock feeding experiment, I conducted laboratory incubations with soils amended with urine from lambs fed biochanin A (BCA), a natural product derived from the forage legume red clover, a component of diverse pasture grazing systems. Biochanin A has been shown to improve cattle protein digestion and weight gain performance; however, effects of BCA or BCA breakdown products deposited on pastures in livestock waste remains unknown. Trace gas emissions from lamb urine-amended soils decreased with the addition of BCA compared to urine from animals fed no BCA, though urinary N excretion and soil microbial communities were unaffected, showing that BCA may mediate soil enzyme relationships or microbial activities that govern trace gas emissions from urine excreta patches.
Finally, I assessed the ecosystem effects of increasing overall stand symbiotic diversity at the plant-microbe level using tall fescue cultivars and their uniquely associated fungal endosymbionts. Greater cumulative levels of carbon dioxide were emitted over the growing season in the most diverse (three cultivars and three endophytes) compared to the least diverse (one cultivar and no endophyte) treatment. Symbiotic diversity level did not affect fescue biomass production, soil carbon levels, or soil microbial communities after five years of consistent management. However, greater symbiotic diversity was correlated with decreased plant species diversity and richness within stands, showing that greater symbiotic diversity improved fescue's competitive ability and restricted niche space for weedy plant species encroachment.
Taken together, these results show that increasing species diversity can be a useful strategy to improve sustainability, though benefits and tradeoffs depended on the unique characteristics and constraints within each agroecosystem. Manipulating species diversity improved economic and soil conservation in row crop systems, reduced trace gas emissions in livestock pasture systems, and supported forage stand persistence. Manipulating species diversity has potential wide-ranging impacts that may be further harnessed by producers and land managers to improve agroecosystem production efficiency, while minimizing adverse environmental consequences associated with the expanded food production systems necessary to sustain human populations in the future.
Digital Object Identifier (DOI)
Row crop studies were supported with funding from the United States Department of Agriculture (USDA) Natural Resources Conservation Service in the National Plant Materials Program. Livestock studies were supported by the USDA Agricultural Research Service (Grant no. 3210001023). Forage studies were supported by the USDA National Institute of Food and Agriculture (Grant no. 3200001164).
Jacobs, Alayna A., "Manipulating species diversity: environmental impacts in row crop, livestock, and grassland agroecosystems" (2022). Theses and Dissertations--Plant and Soil Sciences. 156.