Date of Submission

9-22-2025

Document Type

Thesis

Degree Name

Master of Science in Environmental Science

Department

Biology and Environmental Sciences

Advisor

Sharon. N. Kahara, Ph.D.

Committee Member

Nikolas Stasulli, Ph.D.

Committee Member

Justin C. Luong, Ph.D.

Keywords

Grasslands, Grazing, Soil Bacterial Community, Bacterial Diversity

MeSH

Soil Microbiology

LCSH

Grasslands, Grazing, Soil microbiology, Bacterial Diversity

Abstract

Cattle ranching has been a vital part of California's economy and culture since the 1700s. When appropriately applied, cattle grazing can be a powerful tool in ecological restoration as it can mimic natural processes, promote plant diversity, control invasive species, enhance soil health, and create suitable habitats for wildlife. However, poor ranch management can lead to negative impacts such as overgrazing, loss of native species, aquatic contamination and more. This study sought to examine the impacts of grazing management on soil health and nutrient cycling in a restored pasture in northern California. Specifically, this research focused on the effects of light grazing which remains poorly understood. An experimental approach was taken to compare soil bacterial community composition and diversity using 16S rRNA sequencing. Physiochemical factors such as soil pH, temperature, total carbon and total nitrogen content, bulk density, moisture content was also recorded to assess their influence on bacterial community diversity and abundance. The results revealed that grazing significantly altered the soil environment. Compared to ungrazed plots, grazed soils had significantly lower moisture content and pH but contained significantly higher concentrations of soil organic carbon and total nitrogen (p < 0.05). Microbial analysis showed that grazing significantly increased bacterial alpha diversity (Shannon index) at the phylum, family, and genus levels. Beta diversity analysis (PERMANOVA) confirmed that grazing systematically shifted bacterial community composition at all taxonomic levels (p :S 0.02), with the strongest effect observed at the phylum and order levels (R2 = 0.28 and 0.83, respectively). Specifically, grazed plots were characterized by a higher relative abundance of the phylum Firmicutes and a lower abundance of Actinobacteriota. Redundancy Analysis (RDA) integrated these findings, demonstrating that total nitrogen and pH were the primary environmental drivers shaping the bacterial community. Firmicutes were strongly associated with the high-nutrient conditions in grazed plots, while Actinobacteriota correlated with the higher pH found in ungrazed plots. This study demonstrates that cattle grazing creates a distinct soil habitat characterized by higher nutrient availability and lower pH. This environmental shift, in tum, selects for a more diverse and compositionally different bacterial community, favoring taxa adapted to disturbance and nutrient­ rich conditions. These findings highlight the profound and interconnected influence of grazing on both the abiotic and biotic components of grassland soils.

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