Ellen received her bachelor’s degree in Chemistry from St. Olaf College in Northfield, Minnesota, where her coursework included several classes in analytical chemistry, biology and environmental sciences. Her first exposure to soil chemistry was through a summer research internship at the University of Idaho. Her research interests involve using advanced analytical techniques to detangle the complexity of molecular interactions in soil and water systems. When she is not doing research, she enjoys rock climbing, canyoneering, hiking, gardening, cooking, and knitting.
Temperature effects on natural organic matter sorption to iron oxide coated minerals
Natural organic matter (NOM) makes up the largest terrestrial pool of carbon and is twice the size of the atmospheric carbon pool. Understanding the processes that govern NOM stabilization and degradation are crucial for accurate modeling of carbon cycling and for predicting the impacts of climate change. Sorption to mineral surfaces has been implicated as an important mechanism for long-term preservation of NOM. Iron, the fourth most abundant element in Earth’s crust, creates oxide minerals and surface coatings that act as effective sorbents of NOM. While NOM sorption to iron oxides has been extensively investigated, less is known about how changes in climate, such as an increase in temperature, will affect sorption behavior.
In this study, I am assessing the impact of temperature on the sorption and desorption behavior of various humic substance standards on iron oxide coated sand in packed columns. Liquid fractions collected from the columns are analyzed to determine NOM breakthrough and chemical differences between bound and unbound fractions of NOM. Because microbes more readily degrade certain types of organic compounds, alterations in the chemical composition of the available carbon pool due to changes in sorption behavior may affect overall NOM degradation rates.
This work is done in collaboration with Gabriel Lobo and Dr. Céline Pallud at University of California Berkeley.
Complexation and redox-buffering of Fe(II) by natural organic matter
Iron is an essential mineral nutrient for nearly all organisms, but the low solubility of ferric iron limits its bioavailability in most natural environments. At neutral pH, ferrous iron is several orders of magnitude more soluble than ferric iron, but it rapidly oxidizes in the presence of molecular oxygen. Intriguingly, Fe(II) has been discovered in association with NOM in particles collected from oxygen-rich environments favorable to Fe(II) oxidation. Laboratory studies of Fe-NOM redox chemistry, however, have yielded contradictory results as to whether NOM enhances, inhibits, or has no effect upon the rate of Fe(II) oxidation. Accordingly, molecular mechanisms by which NOM inhibits or enhances Fe(II) oxidation are not clearly understood. We used X-ray absorption spectroscopy (XAS) to determine the coordination environment of Fe(II) adsorbed to (chemically) reduced and unreduced NOM and investigated the effect of NOM complexation on Fe(II) oxidation kinetics. We found that the majority of Fe(II) added to reduced NOM participated in Fe(II)-citrate-like complexes, and reduced NOM facilitated a steady-state concentration of dissolved Fe(II) for several hours in the presence of oxygen. Our results have been published in the journal Environmental Science & Technology.
This project was completed in collaboration with Drs. Benjamin Gilbert and Peter Nico at Lawrence Berkeley National Laboratory.
Hydrogeomorphic controls on soil carbon quantity and composition in Colorado subalpine wetlands
The saturated soil environment in wetlands promotes the storage of large amounts of carbon (C), but varying hydrogeomorphic settings can lead to differences in organic matter supply, redox conditions, hydroperiod, and soil composition, all of which may influence the quantity and quality of stored C. We examined soils from slope and depressional subalpine wetlands to determine the impacts of differing hydrologic regimes on soil C dynamics. Analysis of bulk organic C composition using NMR indicated aliphatic structures dominated most of the top 40 cm of the soil profile in the depressional wetlands whereas aromatic/alkene and carbohydrate structures were predominant in the slope wetlands. Higher alkyl/O-alkyl ratios in subsurface depressional wetland soils suggested increased decomposition of soil C relative to slope wetland soils. These results suggest that different decomposition rates or processes in depressional and slope wetlands dictate variability in soil C composition.
This work is done in collaboration with Dr. Charles Rhoades of the U.S. Forest Service.
For more details about the research conducted in the Borch group please click here.
Daugherty, E., Gilbert, B., Nico, P., Borch, T. “Complexation and Redox Buffering of Iron(II) by Dissolved Organic Matter.” Environ. Sci. Technol. 2017, 51, 11096-11104. doi: 10.1021/acs.est.7b03152
For more publications in the Borch group please click here.
Dr. Benjamin Gilbert, Earth Science Division, Lawrence Berkeley National Laboratory
Dr. Peter Nico, Earth Science Division, Lawrence Berkeley National Laboratory
Dr. Céline Pallud, Department of Environmental Science, Policy and Management, University of California, Berkeley
Dr. Charles Rhoades, Rocky Mountain Research Station, United States Forest Service
Fraser Experimental Forest, United States Forest Service