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, where she studied heavy metal contamination in the Coeur d’Alene River Basin and the activity of an organic herbicide. At Colorado State, her work has focused on the role of organic matter (OM) chemistry in iron-OM interactions and wetland carbon processing. She is interested in the role of organic matter in improving soil health in agricultural systems. In her free time, she enjoys hiking, rock climbing, canyoneering, gardening, cooking, and knitting.
Ellen defended her dissertation in June and is currently seeking job opportunities in Washington State.
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 assessed the impact of temperature on the sorption and desorption behavior of various types of dissolved organic matter on iron oxide coated sand in packed columns and in batch studies. Liquid fractions collected from the columns were analyzed to determine NOM breakthrough and aromaticity of effluent. Contrary to expectations, results showed a trend of increasing sorption with temperature from 7 to 25 to 45˚C for batch studies and from 25 to 45˚C for column studies. The amount of DOM sorbed and sensitivity of sorption to temperature depended on DOM type and reaction conditions. Desorption also generally increased with temperature. The findings from this work suggest that in some soil systems, iron oxides may remove carbon substrates from the accessible carbon pool as temperature increases.
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 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 stability and composition of soil organic C. We examined soils from slope and depressional subalpine wetlands to determine the impacts of differing hydrologic regimes on soil C dynamics. Depressional wetlands were characterized by seasonally declining water tables, high clay content, and thick organic layers. Slope wetlands had relatively consistent groundwater inputs, coarser soil textures, and thinner organic layers.
Analysis of bulk organic C composition using NMR indicated the proportion of carbohydrate structures decreased with depth while the percentage of aromatic structures increased with depth in both wetland types. In depressional wetlands, aliphatic structures represented a higher fraction of surface carbon and increased with depth. The higher clay and SOC contents and long hydraulic residence times in depressional wetlands likely favor anaerobic conditions, which limit decomposition and promote the preservation of thermodynamically unfavorable aliphatic compounds. These results suggest that different decomposition processes in depressional and slope wetlands dictate variability in soil C composition, and that changes in wetland hydrology due to climate change or human disturbance could influence soil C processing.
This work is done in collaboration with Dr. Georgina McKee and 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