Masters Thesis

Experimental Chronic Dry Atmospheric Nitrogen (N) Deposition Causes a Change in Soil Microbial Communities in Southern California Semi-Arid Shrublands

Global nitrogen (N) deposition has increased since the beginning of the Industrial Revolution due to anthropogenic influences such as the burning of fossil fuels and application of N fertilizer. Increased industrialization is expected to continue to escalate the quantity of N released to ecosystems and the atmosphere. Soil microbial communities are susceptible to this increase in N, as microbial activity can be limited by N and these communities are known to regulate biogeochemical cycles. In this study, the response of bacterial functional groups (FG) to N addition (> 50 Kg N ha-1 yr-1) was quantified in Southern California semi-arid topsoil (0-10 cm). Soil physical-chemical properties and enzymatic activities were also quantified to further understand the direct/indirect effects of N addition on bacterial FG. Experimental N addition significantly increased the abundance N-fixing, chitinolytic, and starch degrading bacteria in semi-arid soils; while increasing N-mineralizing and denitrifying bacteria in coastal sage shrub (CSS) soil. N input also decreased the abundance metal redox bacteria in CCS soil and decreased nitrifying bacteria in chaparral soil. Furthermore, N addition increased the abundance of copiotrophic bacteria (ie: Proteobacteria, Bacteroidetes, and Firicutes) and decreased the abundance of oligotrophic bacteria (ie: Acidobacteria) in semi-arid soils. Canonical correspondence analysis revealed that the majority of FG were influenced by soil pH and extractable nitrate, which were also significantly altered by N addition. Results indicated excessive N input directly and indirectly affected the composition of the soil bacterial community. Understanding the response of the microbial community to N additions is important in predicting ecosystem functionality and stability as anthropogenic N deposition increases.

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