Date of Award

August 2019

Degree Name

Master of Science (MS)

Department

Civil and Environmental Engineering

Advisor(s)

Charles T. Driscoll

Second Advisor

Dawit Negussey

Keywords

Acid Rain Recovery, Carbon Cycling, Deacidification, Dissolved Organic Matter, Upstream Processing, Watershed

Subject Categories

Engineering

Abstract

Organic carbon makes up the molecular structure of all living organisms on Earth and is essential to understanding ecological interactions. For forest ecosystems, organic carbon pools and fluxes have been extensively quantified, while less is known about its more highly variable composition. Dissolved organic matter (DOM) is a major source of biologically available organic carbon and nitrogen and serves as an indicator of watershed carbon processing. The extent to which DOM is metabolized, before it is transported from the watershed, is highly dependent on its source molecular structure and the processes exacted upon it. Spectroscopic techniques, such as fluorescence characterization, utilize the optically active fraction of DOM to provide biochemical information such as its source, aromatic content, and molecular size. Analysis of changes to the DOM composition in watershed solutions can provide further insight to understanding its sources, dynamics and impacts on water quality.

In this study I measured DOM concentrations and optical properties from experimental watersheds at Hubbard Brook Experimental Forest, New Hampshire, USA. I report 2 years (2015-2016) of monthly soil solution and stream chemistry from watersheds 1 (W1) and 6 (W6). W6 serves as a reference, while W1 was treated with wollastonite (CaSiO3) in 1999 to replenish available calcium lost due to historic leaching from acid deposition. Soil solution samples, from the Oa, Bh, and Bs horizons, were collected monthly from zero-tension lysimeters at varying elevations within the watersheds. Stream water samples were also collected at locations corresponding to lysimeter elevations. Optical properties were determined from individual excitation-emission matrices (EEMs) scanned from each sample. Parallel Factor Analysis (PARAFAC) was conducted to decompose the fluorescence matrices into independent components. Additional Principal Component Analysis (PCA) was conducted to evaluate the relationships between DOM optical properties and quantifiable analytes.

PARAFAC results revealed three predominant DOM fractions present in soil solutions and stream water: humic, fulvic, and “blue-shift” humic components. While these fractions were not significantly different between calcium-treated and reference watersheds, additional carbon quality indicators such as molecular size and redox status suggested greater DOM processing in the calcium-treated watershed. This pattern was supported by statistically higher DOC & DON concentrations in the calcium-treated organic (Oa) soil solution. Further analysis was conducted on seasonal and elevational patterns. Organic soil solutions from the high-elevation hardwood zone experienced higher DOC and DON concentrations during fall months and elevated DIN concentrations during spring months, possibly due to fine root biomass decline. Low elevation mineral soils exhibited greater “blue-shifted” humic fractions and fluorescence indications of microbial DOM sourcing, which peaked during spring months. This pattern suggests that low-elevation mineral soils are important carbon sinks, due to greater rates of microbial DOM processing in bimodal podzols.

Access

Open Access

Available for download on Thursday, September 16, 2021

Included in

Engineering Commons

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