Soil Organic Carbon Protection in the Presence of Iron Oxides
Open Access
- Author:
- Cronk, Stephanie Sarah
- Graduate Program:
- Environmental Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- November 03, 2016
- Committee Members:
- Christopher Aaron Gorski, Thesis Advisor/Co-Advisor
William D Burgos, Committee Member
Nathaniel Richard Warner, Committee Member - Keywords:
- Climate change
peatlands
ferrihydrite
soil dissolved organic carbon
Mossbauer
TEM
STEM-EDS
coprecipitation
iron oxides - Abstract:
- Soil organic carbon in peatlands is an important terrestrial pool. Peatlands are typically considered carbon dioxide sinks because their water-saturated soils maintain anaerobic conditions that impede the microbial mineralization of organic carbon. In the face of a warming climate, environmental changes are predicted to induce more aerobic zones in peatland soils, potentially leading to rapid and large releases of carbon dioxide into the atmosphere. Recent work has shown that minerals, particularly those part of the colloidal fraction like iron oxides, may slow this process by associating with organic carbon and prevent biodegradation. This work focused on examining the biodegradation of organic carbon in iron-carbon associations in peatlands. I hypothesized that organic carbon coprecipitation with and sorption to ferrihydrite reduces the biodegradation of organic carbon, thereby minimizing carbon dioxide emissions from these environments. To test this, I isolated dissolved organic carbon and a mixed microbial community from a peatland soil to perform incubation experiments. Reactors in incubation experiments were amended with different synthesized iron-organic carbon coprecipitates and adsorption treatments over a 66 day incubation. The transformation of organic carbon throughout the incubation was tracked in two ways. To detect degradation of the soil dissolved organic carbon, changes in the concentration of the dissolved pool of organic carbon were monitored using total organic carbon analysis. The mineralization of organic carbon into carbon dioxide was tracked by headspace evolution measurements using gas chromatography alongside equilibrium calculations to account for carbon dioxide in solution. To begin understanding the mechanisms behind organic carbon transformations, the soil dissolved organic carbon, iron-carbon mineral associations, and microbial community were characterized at the beginning and end of the incubation period. The overall aromaticity, or chemical complexity, of the initial and final soil dissolved organic carbon stock in incubation treatments were characterized using specific ultraviolet absorbance. Chemical and structural changes of iron-carbon associations were determined using microscopy and 57Fe Mossbauer spectroscopy. Over the course of incubation, organic carbon mineralized to carbon dioxide in all inoculated reactors to different extents. Reactors with coprecipitate treatments evolved far less carbon dioxide than sorption treatments, though both coprecipitation and sorption treatments evolved less carbon dioxide than the positive control of soil dissolved organic carbon stock and soil inoculum. Tranmission electron microscopy (TEM), scanning transmission electron microscopy with energy dispersive X-ray spectroscopy (STEM-EDS), and 57Fe Mossbauer spectroscopy indicated structural and chemical changes to three of the four coprecipitate treatments, but no changes in the ferrihydrite sorption treatment. Specific ultraviolet absorbance measurements signaled a general trend towards more aromatic, complex carbon compounds over the course of incubation, particularly in coprecipitation treatments. Results from this work pointed to substantial organic carbon preservation in coprecipitation treatments during aerobic biodegradation. Sorption treatments also hindered biodegradation towards the end of incubation, but initial organic carbon mineralization was on pace with the positive control of the soil dissolved organic carbon stock and soil inoculum. In a peat system where dissolved organic carbon is at risk of rapid mineralization into carbon dioxide, association with the common iron oxide, ferrihydrite, may act to minimize aerobic biodegradation.