Date of Award:

5-2016

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Plants, Soils, and Climate

Committee Chair(s)

Jeanette M. Norton

Committee

Jeanette M. Norton

Committee

John M. Stark

Committee

Jennifer R. Reeve

Committee

Jon Y. Takemoto

Committee

Paul R. Grossl

Abstract

Improved understanding of nitrogen cycling in agriculture is essential for increasing fertilizer use efficiency and sustainable food production. The availability and mobility of nitrogen are largely determined by the processes of nitrogen mineralization and nitrification. Nitrification is mediated by ammonia oxidizing bacteria, ammonia oxidizing archaea, and nitrite oxidizing bacteria. Our understanding of the controls on the relative activity of these nitrifying organisms in soil is limited. A wide variety of soil microorganisms produce enzymes that mineralize nitrogen from organic matter, but we have limited knowledge about links between enzyme activity and the microbes that work in soil. The main objective of this study was to determine the impacts of agricultural nitrogen management on soil microbial communities, enzyme activities, and functional genes for nitrogen mineralization and nitrification in conventional and organic farming systems. This work was supported by the United States Department of Agriculture and the Utah Agricultural Experiment Station. We established silage corn field plots in northern Utah and silage corn was grown using ammonium fertilizers or manure composts over five years with an additional certified organic field used for comparison. Soil nitrogen transformations were examined for their rates and the organisms responsible. Molecular tools were developed based on the genes for important enzymes in the mineralization and nitrification processes. Ammonium fertilizers increased the abundance of soil nitrifiers and nitrification. We found that application of organic N fertilizer, but not inorganic N fertilizer, increased the diversity of the microbial community and nitrogen mineralization. The abundance of functional genes and their corresponding enzyme activity was increased by organic N fertilizers in the organic farming system. Understanding the link between microbial communities and biogeochemical functions such as nitrification and mineralization may allow ecosystem models to incorporate microorganisms as dynamic components driving the nitrogen cycle. This may further efforts to manage soil nitrogen supply to match plant demand resulting in improved sustainability for agricultural systems.

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