Online

Landsman-Gerjoi, Maxwell

Geology

2019

M.S.

Dissolved organic matter (DOM) plays a vital role in biogeochemical processes and can flux CO2 to the atmosphere when labile fractions are degraded, hence DOM degradation is increasingly studied. Some studies have suggested that fluorescence-derived substrate characteristics are useful metrics for estimating bioavailability (as prerequisite condition for biodegradability), however, recent findings on soil organic matter emphasize the importance of ecosystem scale factors such as physical separation of substrate from soil microbial communities and soil physiochemical cycles driving organic matter stability. I extend this principle to soil derived DOM and hypothesize that such environmental conditions, covariant with season, land use and landscape position, impact the composition of soil DOM and activity and abundance of the microbial community, which together govern DOM biodegradability. As a result, DOM bioavailability may not reliably be predicted using substrate characteristics alone. To test these hypotheses, I assessed aqueous soil extracts for water extractable organic carbon (WEOC) content, biodegradability, microbial biomass and fluorescence spectroscopy on water extractable organic matter (WEOM) across a range of environmental conditions in northern Vermont, USA. My results indicate that changes in environmental conditions affect composition, quantity, and biodegradability of DOM. WEOC concentrations were highest in the fall and lowest in the summer, while no significant differences were found between land covers or landscape position, however, DOM biodegradability was significantly higher in the agricultural (AG) site across seasons. Despite a shift in utilized substrate from less aromatic DOM in summer to more aromatic DOM in winter, biodegradability was similar for all seasons. The only exception were cold temperature incubations where microbial activity was depressed, and processing was halted. These results indicate that bioavailability cannot be reliably predicted based on fluorescence-based metric alone, rather, my core findings illustrate a complex picture of how environmental conditions, landscape characteristics, and substrate composition interact to drive the biodegradability of labile carbon pools in the soil environment. This thesis includes i) a background and comprehensive review of literature to inform the reader of any relevant topics, ii) a paper submitted for publication to Biogeochemistry (Chapter 2), and iii) supplemental information containing figures and tables pertinent to the paper.