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Environmental Systems
Ph.D. Dissertation Defense


"The Effects of Organic Matter  Amendments in California Agroecosystems"

Yocelyn Villa
Environmental Systems
University of California, Merced


Sustainable agriculture aims to meet society’s needs for food and fiber, while protecting natural resources and is primarily determined by the quality and health of soils. A management practice that directly benefits soil quality, through the enhancement of soil organic matter, is the use of organic matter amendments (OMA). These organic materials are rich in nutrients that could be recycled back to the soil and potentially have positive effects to soil conditions and properties, and is proposed as a climate change mitigation strategy. Through extensive field, laboratory studies, and advanced analytical techniques, I show the potential benefits of recycling organic matter amendments into agricultural soils in California. Three projects were established: the first project focused on how the application of two composts in almond orchards influenced soil fertility and nutrient cycling. Main findings from this chapter show that after two years of OMA application there was an improvement in soil fertility. The second chapter focused on how long-term application of biosolids in agricultural soils influences carbon content, specifically focusing on the importance of accounting for deep soil carbon in order to determine a soils climate change mitigation potential. Main findings show that application rate of biosolids is not a determining factor when it comes to carbon accounting, but management practices and deep soil accounting play a bigger role. Chapter 3 builds off Chapter 2, and investigates the persistence and location of soil C in the soil at these three sites. These different pools will provide insight on how vulnerable these reservoirs are to management induced changes which can contribute to C losses. Overall findings show that long-term amended soils contribute increase carbon content associated to unprotected pools in surface soils where it is easier for microbes to mineralize. Carbon content in deeper soils show that the carbon is associated to physically protected and mineral associated pools, indicating this carbon to be more persistent. Overall, these findings show that site conditions, measurement methodology, and management practices influence the potential of OMA to be beneficial to soil fertility and climate beneficial.   


Yocelyn B. Villa received her B.S. in Environmental Science from UC Riverside in 2014. She joined the Environmental Systems graduate program in the Fall of 2016. During her time at UC Merced, she received the USDA Hispanic Serving Institutions grant where she organized outreach programs to local elementary schools. She also served as an Environmental Systems Student Representative and mentor to first year graduate students. Her research focuses on how sustainable management practices influence soil fertility and climate mitigation strategies.   

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