Date of Award

Spring 1-1-2011

Document Type

Thesis

Degree Name

Master of Arts (MA)

First Advisor

Diana Nemergut

Second Advisor

William Bowman

Third Advisor

Steve Schmidt

Abstract

Through litter inputs, root exudates, and the resulting changes in soil chemistry, plants directly interact with the soil microbial community. Recent research on plant-microbe interactions suggests that soil microbial community structure and function play an integral role in plant community succession through both positive and negative feedbacks; yet, plant-microbe dynamics along a successional gradient have not been well-studied. My study in the recently exposed soils of the Mendenhall Glacier forefield near Juneau, AK, USA examined the development of microbial communities in coordination with the establishment of the first plants. The Mendenhall Glacier features a perhumid climate, with moist soils throughout the year, and nearby vegetation that serves as a propagule source, facilitating relatively rapid plant colonization. I sampled soils under two different plant species (alder, Alnus sinuata and spruce, Picea sitchensis) and from unvegetated areas. All samples were gathered within a single transect of soils that had been exposed for 6 years. For each sample site soil pH, organic carbon (C), available nitrogen (N), bioavailable (Olsen) Phosphorus (P), microbial biomass C, and nitrogen fixation rates were determined. My research shows specific vegetation type differences in bacterial community structure and the general enrichment of α-Proteobacteria in vegetated soils. Soil nutrient and carbon pools did not correlate with bacterial community composition. Interestingly, although pH did not significantly vary by vegetation type, it was the only parameter that correlated with bacterial community structure. My study revealed a significant correlation between nitrogen fixation rates and bacterial community composition, a feedback with potentially important impacts for the ecology of these environments. Vegetation type explained more variation in differences in bacterial communities than pH, suggesting that plant acidification of soils only partly drive broad shifts in bacterial communities. Plant species-specific differences in bacterial community structure may also relate to the chemical composition of litter and root exudates. Additionally, plant carbon inputs in general likely enhance asymbiotic N-fixer function in these relatively new soils where nitrogen limitations may stifle bacterial growth. My study provides insights into how colonizer plants drive changes in bacterial community structure and function in a glacial forefield, altering bacterial succession and ecosystem development.

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