Author ORCID Identifier
Date Available
8-20-2027
Year of Publication
2025
Document Type
Doctoral Dissertation
Degree Name
Doctor of Engineering (DEng)
College
Engineering
Department/School/Program
Civil Engineering
Faculty
Dr. James F. Fox
Faculty
Dr. Mei Chen
Abstract
This dissertation investigated sediment and soil processes affecting carbon and nitrogen stable isotopic ratios (δ13C and δ15N) within a fluviokarst region. Elemental and isotopic datasets were collected frequently at surface and subsurface locations for both dissolved and particulate carbon and nitrogen phases. The research highlights the significance of terrestrial and autochthonous organic matter contributions to sediment and nutrient compositions, characterizing influences from hydrologic and geomorphologic features. Through a combination of long-term field isotopic measurements, laboratory experiments, and numerical modeling, the study addresses critical gaps in understanding how sediment transport dynamics, soil nutrient cycling, and organic matter degradation affect seasonal variations in stable isotopic ratios.
This dissertation includes five major contributions of sediment and soil processes affecting isotopic ratios, and each contribution constitutes a peer reviewed journal paper.
The first contribution (chapter 2) shows that carbon oxidation and nitrogen mineralization induce minor changes in δ13C and δ15N, while accrual of algae and seasonal sediment deposits can lead to substantial shifts in fluvial isotopic signatures. These findings lead to the recommendation that isotopic ratios of sediment sources be separated by at least 1‰ to mitigate concerns regarding tracer conservativeness.
The second contribution (chapter 3) reveal that sediment from watershed soil erosion exhibits higher carbon oxidation rates and lower nitrogen mineralization compared to streambed sediment. Degradation rates are significantly higher than previously thought, indicating active organic matter breakdown. Stable isotopic ratios show minor variations, aiding in modeling uncertainties.
The third contribution (chapter 4) recommends a minimum separation of 3‰ between mean isotopic ratios of sediment sources to ensure accurate source apportionment when streambed sediment is prominent, surpassing previous guidelines of 1‰. This emphasizes the importance of streambed sediment as a (non)conservative tracer in hydrologic studies.
The fourth contribution (chapter 5) reveals seasonal patterns in nitrate isotopic ratios and concentrations in Kentucky's inner bluegrass karst region, linked to soil moisture conditions. Notably, low δ15NNO3 isotopic values in wet winters contrast with high values in dry summers, indicating denitrification processes.
The fifth contribution (chapter 6) models nitrogen cycling and isotopic ratios over 72 months, revealing that soil leaching influenced by denitrification is the primary nitrogen source. Seasonal variations in δ15NNO3 indicate significant biogeochemical processes, with peaks in summer-fall and minima in winter. We recommend utility of an isotope bi-plot (i.e. denitrification line) as a graphical representation to assist in differentiating nitrogen source contributions and isotopic fractionating transformation processes.
This research aims to enhance the predictive capabilities of sediment and nutrient fingerprinting and tracer techniques through improvement in understanding of carbon and nitrogen cycling in freshwater ecosystems, ultimately contributing to better representation of controls along terrestrial-aquatic interfaces.
Digital Object Identifier (DOI)
https://doi.org/10.13023/etd.2025.424
Funding Information
This study supported by the Kentucky Senate Bill 271B Water Quality program and National Science Foundation Awards #1632888 and #1933779
Recommended Citation
Riddle, Brenden, "Sediment and soil processes controlling stable isotopic ratios in the inner Bluegrass karst region, Kentucky, USA" (2025). Theses and Dissertations--Civil Engineering. 156.
https://uknowledge.uky.edu/ce_etds/156
Included in
Civil Engineering Commons, Environmental Engineering Commons, Other Civil and Environmental Engineering Commons
