13.7: Managing greenhouse gas emissions

By learning more about environmental controls on microbial greenhouse gas emissions, we will be better able to predict how emissions will change in the future in response to climate and land use changes. In addition, this knowledge can be used to develop strategies that alter microbial activity in some environments to decrease the extent to which they produce greenhouses gases.

To develop such a strategy, we can adopt the same thought process that this book recommends for developing strategies that employ microorganisms to remediate water resource contaminants (Section 14.5). Specifically, we can consider the two-way interactions between microorganisms and their environments. We must first consider what change(s) in microbial activity would decrease greenhouse gas production. Then, we need to consider the how we can alter the environment to bring about those change(s).

As an example, previous research has tested the ability of ferric iron amendments to decrease methane production in wetland soils. Microbial iron reduction can have thermodynamic and kinetic advantages over methanogenesis. Where fresh supplies of ferric iron minerals are available, therefore, iron-reducing microorganisms have the potential to outcompete methanogens for electron donors and thereby limit methane generation (Lueders and Friedrich, 2002; Roden, 2003; Roden and Wetzel, 2003).

To test this strategy, Roden (2003) added ferric iron minerals to organic carbon-limited cultures that contained wetland sediment microorganisms and slightly acidic water (initial pH 6.5 to 6.7). Ferric iron sources they tested were hydrous ferric oxide (HFO; a poorly crystalline ferric iron source), two different goethite phases, and hematite. Results showed that methane production was significantly lower in cultures that received ferric iron compared to unamended control cultures. Moreover, methane production was inhibited just as strongly by goethite and hematite as HFO when quantities of the mineral amendments were nearly equivalent in terms of mineral surface areas.

A main conclusion from the laboratory study is that ferric (oxyhydr)oxide amendments can be an effective strategy for suppressing methane production in wetland soils, but that the surface area of those phases is important to consider when determining how much mineral to add. Recall from Section 9.7 that (oxyhydr)oxide surface area influences the rate at which iron can be reduced. As such, surface area has the potential to influence the extent to which iron-reducing microorganisms can outcompete methanogens for electron donors.

In addition to surface area, pH will likely also be important to consider. Results from Marquart et al. (2019) indicate that iron reduction decreases relative to methanogenesis as pH increases in many systems, potentially in response to the decreasing free energy yield of iron reduction with increasing pH (Sections 8.3, 10.2.5). As a result, the strategy of amending soils with ferric iron to decrease methane generation may become less effective at basic pH, at least for highly crystalline amendments such as goethite and hematite, which are less thermodynamically favorable as electron acceptors than poorly crystalline phases (Postma and Jakobsen, 1996).

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