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Assessing suppression of methanogenesis in wetlands by electron accepting and polyphenolic moieties in natural organic matter

Final Report Summary - NOMSUPPRESSCH4 (Assessing suppression of methanogenesis in wetlands by electron accepting and polyphenolic moieties in natural organic matter)

Wetland ecosystems, including bogs, fens, marshes and peatlands, play an important role in the global carbon cycle. Estimates of the amount of carbon stored in wetlands range from 15 to 30 % of the total global soil carbon pool. Due to the quantity of carbon stored in these (largely) oxygen-free ecosystems and the biological and chemical processes in these wetlands that emit significant amounts of the greenhouse gases carbon dioxide and methane, understanding the movement of carbon through and out of these systems of great interest. Furthermore, due to the potency of methane as a greenhouse gas compared to carbon dioxide, understanding the how both of these gases are produced is of critical importance.
Current understanding of the microbial processes present in wetlands suggest that equal quantities of carbon dioxide and methane should be produced. However, previous studies of wetlands have shown that greater amounts of carbon dioxide are released compared to methane. While this imbalance in gaseous emissions is beneficial from a overall global warming perspective (less of the high potency methane is produced than is expected), the factors that govern this balance are still unknown. NOMsuppressCH4 seeks to ascertain whether this imbalance of carbon dioxide and methane is affected by the presence of electrochemically active organic matter molecules in the wetland material. One compelling hypothesis to explain part or all of this imbalance is the use of electrochemically active organic matter by wetland microorganisms to form carbon dioxide, bypassing the pathway used to produce methane. Specifically, it is thought that some of the organic matter can be used as a terminal electron acceptor by the wetland microorganisms, much like aerobes such as humans use oxygen as a terminal electron acceptor.
All experiments in NOMsuppressCH4 were carried using material collected from rain-fed peat bogs in central Sweden or from groundwater fed fens and rain-water fed bogs in northern Minnesota, USA. These sampling locations and samples were chosen to be representative of the variety of wetland ecosystems. The chemistries of the wetlands were characterized through both on-site measurements (including pH, electrical conductivity, and dissolved carbon dioxide and methane concentration), and laboratory measurements of standard spectroscopic, elemental analyses, cation and anion concentrations and organic matter concentration. All on-site and laboratory measurements were in the range expected for the types of wetlands sampled.

Following collection, characterization, drying and milling, the wetland materials were subjected to electrochemical analyses to measure the concentration of electron donating and accepting moieties in the organic matter. Prior to analysis, all individual samples were separated fractionated by size into larger sized (particulate) particles and smaller (dissolved) particles. In all samples, electron donating groups were in much greater abundance than electron accepting groups and were generally more abundant in samples collected at deeper depths. Although the density of both these types of electrochemically active moieties was much lower in the larger particulate particles of organic matter, the relative high abundance of these larger particulate particles means that the particulate organic matter accounts for >80% of both the electron accepting and donating capability, whereas <20% can be found in the smaller dissolved fraction.
To measure the effect of these electrochemically active moieties on gas production, other, unprocessed wetland materials were moved into airtight vials filled with oxygen-free gas. Periodically, gases were removed from these incubation vials and the concentration of carbon dioxide and methane were measured. After a stable baseline of gas production was established, several of the vials received additions of a electrochemically active chemical, AQDS. AQDS contains a chemical moiety that is similar to the moieties in organic matter that provide much of its electron accepting capability. It has been used successfully by many researchers as a surrogate for the electron accepting capability of genuine organic matter. While our original plan was to add genuine organic matter, the very low density of the measured electron accepting capacity of the wetland material made its use impractical.
For several weeks following the addition of AQDS, the carbon dioxide and methane concentrations in the vials was monitored. After several weeks of monitoring, no statistically significant difference in the rate of carbon dioxide and methane production or the ratio of the two was found. This strongly suggests that the electron accepting capability of AQDS, and by extension organic matter, do not have a significant effect on the production of carbon dioxide and methane.