Principal Investigators
At a Glance
Methane (CH4) is the second most important anthropogenic climate forcer after carbon dioxide. Determining the importance and mechanisms of different anthropogenic and natural methane sources and sinks across temporal and spatial scales remains a fundamental challenge for the scientific community. Wetlands are dominant but highly variable sources of methane and are predicted to play a critical role in carbon-climate feedbacks. Methane emissions from these areas are shaped by a complex and poorly understood interplay of microbial, hydrological, and plant-associated processes that vary in time and space. The factors responsible for the greatest methane emission from wetlands remain unknown. The CMI Wetland Project aims to identify the biological and chemical mechanisms that promote methane emissions from wetlands. The goal is to improve predictions of carbon-climate feedbacks and strategies of methane mitigation. A better understanding of the factors responsible for the greatest methane emission from wetlands is crucial to bp’s actions aimed at targeting this powerful greenhouse gas and thus a vital step towards a low-emissions future.
Research Highlight
Atmospheric CH4 has risen to levels roughly 150% above preindustrial concentrations due to human activities. These levels continue to rise despite a short period of stabilization between 1999 and 2006. Wetlands are geographically and biogeochemically diverse environments that together constitute the largest and most variable sources of methane to the atmosphere. CMI Wetland Project researchers are investigating the microbial, chemical, and hydrological pathways that regulate methane emissions from diverse wetland systems that vary in biogeochemical composition and hydrologic environment.
Current research (Reji et al., in preparation) builds on prior CMI discoveries that transient oxygenation associated with hydrological variability unlocks a microbial “latch” on wetland carbon flow that ultimately makes mineral-poor, peaty wetlands drastically more methanogenic (Wilmoth et al., 2021). The researchers have pieced together fragments of genetic information from Sphagnum, a genus of peat moss. The researchers are using these microbiomes to recreate microbial genomes. This has allowed the researchers to show that transient oxygenation selects for different keystone microorganisms at multiple steps of the microbial food chain underlying peat carbon conversion into methane (Figure 2.1, Reji et al., 2022).
Transient oxygen exposure triggers a shift in microbial community succession during microbial degradation of complex aromatic peat carbon that promotes methane formation (Reji et al., 2022).
Reji et al. (in preparation) examine wetlands along a freshwater to saltwater continuum. The goal is to better constrain the effects of hydrologically driven oxygen variability on methane emissions from a greater diversity of wetlands. This includes an organic-rich Sphagnum-dominated peat in a coniferous forest, a mineral-soil marsh, and a saltmarsh. The results indicate that different wetland types respond differently to changes in oxygen levels. Unlike in Sphagnum peat from a peat bog (Wilmoth et al., 2021), methane emissions in the forest peat were largely unaffected by oxygen exposure (Figure 2.2a). A similar trend was observed for the mineral soil, or freshwater, marsh. Saltmarsh sediments, in contrast, did not release any methane even under prolonged, continuous anoxia, which refers to an absence of oxygen. Carbon dioxide emissions were generally higher (up to ~threefold) in oxygen-shifted samples across all three wetland types. This was particularly pronounced during the oxic, or oxygen-present, period in both forest peat (Figure 2.2b) and freshwater marsh. The flow of carbon following oxygen shifts was mostly directed towards carbon dioxide. This observation suggested a fundamentally different mechanism regulating the flow towards methane in these wetlands compared to that in a typical Sphagnum peat bog.
Geochemical data indicated that the forest peat and the freshwater marsh were much more resilient to short-term (one-week) oxygen exposure compared to the bog-origin Sphagnum peat. The microbial data similarly indicated no significant changes in community composition across the oxygen shift (Figure 2.2c). In contrast, community composition in oxygen-shifted forest peat was significantly different compared to anoxic controls (Figure 2.2c). In particular, key microbial taxa linked to altered methane dynamics in the bog-origin Sphagnum peat were largely absent in the forest peat. These observations suggest that microbial community composition can be a powerful indicator of wetland responses to pulse disturbances – in this case, changes in oxygen that are driven in nature by shifts in hydrology.
(a) Fold change in total methane yield between oxygen-shifted versus continuously anoxic peat. Both peat types were exposed to oxygen for one week, followed by three weeks of anoxic incubation. (b) Carbon dioxide emissions in Tree Moss peat over incubation time. Green shaded area indicates the period of oxygen exposure. (c) Relative abundances of major microbial taxa in Sphagnum and Tree Moss peat incubations. Green shading indicates the oxic period. Taxa present in both peat types are in bold letters.
Threshold disturbance level required to shift the resilience behavior (i.e., would a longer period of oxygen exposure change the carbon flow in these wetlands) were examined experimentally, exposing forest peat slurries to a much longer oxic period (i.e., four weeks) before they were made anoxic. The longer oxygen exposure did not change methane and carbon dioxide emission trajectories observed in the original experiments. This suggests that methane dynamics in the forest peat may be resilient to prolonged aerobic conditions, such as those occurring during a drought or water-table drawdown. Further ongoing investigations will compare microbial functional profiles across wetland types (using metagenomes and metatranscriptomes). The aim is to better constrain the microbial mechanisms resulting in differential response of wetlands to transient oxygen shift.
The divergent responses of the two Sphagnum peat types to oxygen exposure likely results from the interplay between complex environmental factors. This includes vegetation inputs and hydrology that varied significantly between the two peatlands. These results underscore the need to assess the resiliency of peatlands in the context of their divergent ecological settings. Such careful characterization of the environmental heterogeneity is essential for accurately scaling up laboratory observations to predictive global models of peatland methane emission trajectories.
The CMI wetland project has identified the influence of environmental conditions (e.g., oxygen, soil saturation, water table, salinity) and soil molecular form on microbial biodiversity as keys to better constrain and mitigate wetland methane emissions. The researchers urge the adoption of strategies to limit greenhouse gas emissions from natural and constructed wetlands as part of land-based climate solution initiatives in freshwater wetlands (e.g., Wilmoth et al., 2021; Calabrese et al., 2021). Ongoing collaborations with the Bourg, Stone, and Porporato groups at Princeton University (Yang et al., 2021) address how soil minerology and biophysics can be manipulated to support soils-based carbon mitigation efforts.
References
Calabrese, S., A. Garcia, J.L. Wilmoth, X. Zhang, and A. Porporato, 2021. Critical inundation level for methane emissions from wetlands. Environmental Research Letters 16:044038. (https://doi.org/10.1088/1748-9326/abedea).
Reji, L., and X. Zhang, 2022. Genome-resolved metagenomics informs functional ecology of uncultured Acidobacteria in redox oscillated Sphagnum peat. mSystems 7:e00055-22. (https://doi.org/10.1128/msystems.00055-22).
Reji, L., and X. Zhang. Effects of oxygen variation on wetland microbial ecology and biogeochemical resilience. In preparation.
Wilmoth J.L. et al., 2021. The role of oxygen in stimulating methane production by wetlands. Global Change Biology 27(22):5831-5847. (https://doi.org/10.1111/gcb.15831).
Yang, J.Q., X. Zhang, I. Bourg, and H. Stone, 2021. 4D imaging reveals mechanisms of clay-carbon protection and release. Nature Communications 12:622. (https://doi.org/10.1038/s41467-020-20798-6).