Methane consumption by a landfill cover soil under variable soil moisture and temperature conditions

Abstract

Methane (CH4) is a significant greenhouse gas (GHG) that contributes to climate warming when released into the atmosphere, with over 80 times the warming potential of carbon dioxide (CO2) over a 20-year period. Landfills are one of the largest anthropogenic sources of CH4, and hot-spots of CH4 emissions in landfill cover soils represent a large proportion of emissions that are thus a target for mitigation. These hot-spots can enrich microbes that consume CH4 and produce CO2 as a less potent GHG, via CH4 oxidation. CH4 oxidation rates are modulated by multiple environmental variables including soil moisture and temperature. Therefore, it is important to investigate the interactive effects of these factors on CH4 oxidation rates, to further understand the response of CH4 oxidation activity under changing conditions whether via seasonality or climate change.

In Chapter 2, I conducted a closed-headspace batch experiment with cover soil from a hot-spot of a former landfill to measure CH4 consumption and CO2 efflux rates associated with variations in soil moisture and temperature simultaneously. Soil samples were incubated under a factorial design of 5 soil moisture contents ranging from 11 to 47% WFPS (water-filled pore space), and 6 temperatures ranging from 1 to 35°C. At each temperature and WFPS combination, CH4 was spiked into the headspace, and headspace CH4 and CO2 concentrations were measured over 2 hours to calculate CH4 consumption and CO2 efflux rates. The maximum CO2 efflux rate was observed at the maximal WFPS and temperature conditions of this experiment (91.5±10.3 nmol h-1 g DW-1 at 47% WFPS and 35°C), while the maximum CH4 consumption rate was observed at intermediate soil moisture and temperature conditions (1.86±0.05 nmol h-1 g DW-1 at 25% WFPS and 25°C). The results from this experiment showed the preliminary optimal conditions for CH4 consumption and associated CO2 efflux within this range of tested soil moisture and temperature conditions, and served as a baseline for the experimental design of Chapter 3.

In Chapter 3, I conducted a series of closed-headspace batch incubations with cover soil from the same hot-spot site to expand on the findings from Chapter 2. The incubations assessed the CH4 consumption and CO2 efflux rates under simultaneous variations of soil moisture and temperature, with modifications including a wider range of soil moistures, and higher concentrations and subsequent injections of CH4. Soil samples were incubated under a factorial design of 5 soil moisture contents ranging from 20 to 100% WFPS, and 4 temperatures ranging from 1 to 35°C. At each temperature and moisture combination, CH4 was spiked into the headspace through multiple consecutive injections, and headspace CH4 and CO2 concentrations were measured to calculate CH4 consumption and CO2 efflux rates. The maximum CH4 consumption rate was observed at the moderate soil moisture and temperature conditions (330±12.3 nmol h-1 g DW-1 at 60% WFPS and 25°C), while the maximum CO2 efflux rate was observed at the maximal WFPS and temperature conditions used in the incubations (652±85.0 nmol h-1 g DW-1 at 100% WFPS and 35°C). A diffusion-reaction model was developed to simulate and fit the observed data to represent the effects of temperature and soil moisture on the CH4 consumption and CO2 efflux rates, predicting similar optimal conditions to the observed experimental data. Temperature sensitivity analysis (Q10) also supported the CH4 consumption being via CH4 oxidation. These results provide insight into how seasonal changes in soil moisture and temperature impact CH4 oxidation rates, and therefore also net CH4 emissions, in landfill cover soils and other environments.

Overall, the results from Chapters 2 and 3 together emphasize that the dominant controls on the optimal soil moisture for CH4 consumption are the interactive effects of moisture limitation of microbial activity and of gas (CH4 and O2) diffusion, whereas for the CO2 effluxes, the dominant controls are the interactive effects of moisture limitation of gas (O2) diffusion and solute mobility. The difference in optimal conditions for CH4 consumption and CO2 efflux rates also highlight the presence of different microbial groups underlying the various soil processes. These findings can be expanded on for further understanding of CH4 oxidation activity at hot-spots and for the development of tools for mitigation of CH4 emissions from landfills.