Online

Hampsch, Alyson

Geology

2016

MS

Soils represent an important terrestrial carbon (C) sink, storing up to three times the amount of atmospheric C, however climate and land use changes may transform soils into C sources. River corridor (RC) soils and associated C are at risk to become mobilized by erosion such as bank failure and scour events. Once soil-derived organic C is transferred into the stream, microbial processes and photodegradation of the dissolved, labile (or bioavailable) fractions can lead to the production of CO2, which can evade and increase atmospheric CO2 levels. Because predicted increases in heavy precipitation will likely increase this type of riverine erosion, it is important to better understand the potential for the release of bioavailable C from RCs. One objective of this thesis was therefore to identify and characterize representative samples of soils from a typical Vermont RC for common land covers and simulate the production of dissolved organic matter (DOM) during riverine soil erosion. Field sites representative of typical agricultural and forested land uses were selected based on the analysis of 106 existing samples and resampled multiple times over the summer of 2015. Production of DOM from riverine erosion was simulated using aqueous soil extracts (ASE), where soil and water were shaken at fixed ratios followed by the separation of the extract. To study the characteristics of these extracts (which serve as analogue of stream water after erosion), water extractable C (WEOC) concentrations, water extractable nitrogen, fluorescence properties of DOM, and bioavailability were determined. Results indicated a common, dominantly terrestrial source material for all land covers, but C concentrations and fluorescence properties differed. High but variable amounts of soil organic C and WEOC were observed in agricultural riparian and agricultural stream bank samples, and lower concentrations in agricultural field, forest, forest riparian, and forest stream banks. WEOC bioavailability was high in all agricultural land covers and low in forested land covers. Because this study is the first in which ASE are used as analogues for stream water after riverine erosion, a second objective was to test laboratory methods used in this study for their effect on WEOC, fluorescence properties, and bioavailability. Specifically, the effects of soil drying, soil storage, and the effects of the extraction solution were tested. For this, ASE were prepared from soils that were field moist, dried, and after two years of storage. In addition, dried soils were extracted using different solutions including a salt solution, river water, and double deionized (DDI) water. Results indicated WEOC concentration and microbial humic-like fluorescence from extracts of dried soils were higher than those in extracts of field moist soils, while WEOC concentration and microbial humic-like fluorescence was highest in extracts of soils stored long term. In addition, the bioavailability of WEOC was higher in dried soils than field moist soils. The extraction solutions of DDI water and river water produced DOM with similar fluorescence properties, while the salt solution extracted a different, less humified pool of C. Overall, the ASE methods used in this study are effective in simulating stream bank erosion and subsequent C release into stream water, however the effects of drying the soils need to be considered when assessing DOM.