Peatlands and climate change: from threat to cure

Sustainable management of peatlands and mitigating climate change: Waterloo’s Ecohydrology Research Group helps close key knowledge gaps.

Peatlands are a valuable but vulnerable natural resource. Peat forms through the slow accumulation of partially decomposed plant biomass in water-saturated soils of wetlands across the boreal and taiga ecoregions, as well as in cool temperate and tropical locations. Peatlands are transitional environments between land and water (Figure 1): their hydrological, ecological, and biogeochemical functions support a variety of ecosystem services, including biodiversity conservation, water quality protection and climate regulation. While peatlands cover only about 3% of the continents (Figure 2), they are estimated to store on the order of 10% of all fresh water and approximately 30% of land-based organic matter.

Fig 2: Global distribution of peatlands (Creator: Levi Westerveld, 2017; https://www.grida.no/)

Northern peatlands are attracting particular attention because high latitude regions are warming faster than the global average. This creates the conditions for increasing carbon dioxide and other greenhouse gas (GHG) emissions from peat soils, further accelerating global climate change. Another consequence of global warming is the enhanced leaching of dissolved organic matter (DOM) from peatlands with far-reaching consequences for the water quality, optical properties, thermal regimes, and trophic state of receiving streams, lakes, and nearshore coastal areas.

Researchers with the Ecohydrology Research Group (ERG) are interested in understanding and predicting how northern peatlands respond to anthropogenic disturbances, in particular, land-use changes and global warming. Their research is helping to unravel the coupling of pore-scale soil processes and functions to hydrological and climate drivers acting at local to regional scales such as wetland drainage, freeze–thaw cycles, and changes in the extent and depth of snow cover. The process-level knowledge and data are then scaled up using modelling approaches, including Machine Learning algorithms, to forecast possible future trajectories of peatlands under various scenarios of environmental change.

Compared to most mineral soils, peat soils are highly structured and complex porous media, with unique physical, geochemical, thermal, and hydraulic properties (Figure 3). The structure of peat soil consists of pores that are open and connected, dead-ended or isolated. This multi-porosity nature of peat soils affects water flow and retention that, in turn, influence the fate and transport of carbon, nutrient elements, GHGs and contaminants. In particular, the larger pores (the mobile porosity) actively participate in macroscopic pore water flow and relatively fast advective chemical transport, and smaller pores (the immobile porosity) that exchange solutes with the mobile porosity through slower, short-range molecular diffusion. At ERG, we use chemical tracers, for example deuterated water (also known as heavy water), to probe the relationships between transport processes and pathways, on the one hand, and chemical transport, on the other hand.

Fig 3: Conceptual diagram illustrating the complex structure of peat, comprised of larger pores (the mobile porosity) and smaller pores (the immobile porosity), and how it evolves with increasing degree of decomposition (from top to bottom), for both sphagnum (bog) and sedge (fen) peat. The diagram also represents various important hydro(bio)geochemical processes in peat. Reproduced from McCarter et al. (2020).

The immobile regions within peat are the primary sites where microbially mediated biogeochemical transformation processes take place. These transformation processes are driven by the decomposition of the peat, which under water-saturated conditions is very slow because of the absence of molecular oxygen (O2). The microorganisms responsible for decomposing the organic compounds that make up the peat must therefore rely on less powerful oxidants than O2, or on fermentation. In addition, the supply of these alternative oxidants – but also the removal of the decomposition by-products, for example CO2 and methane (CH4) – are hampered by the slow diffusional exchanges between the immobile and mobile porosity. To better understand how the pore structure and transport properties of peat soils control their biogeochemical activity, ERG researchers combine geochemical, isotopic, and genetic analyses with X-ray tomography imaging and tracer experiments.

Undisturbed peatlands produce more plant biomass than is decomposed belowground. As a result, peat accumulation represents a net sink of atmospheric CO2. Human perturbations of peatlands, however, can shift this CO2 balance in the opposite direction. A prime example is the drainage of peatlands for their use in agriculture. As the water table is drawn down, the peat is exposed to air causing the accumulated organic material to be oxidised by O2 and generating CO2 that is emitted to the atmosphere. That is, the drained peat soils now become a source of CO2, hence enhancing the radiative forcing of the atmosphere. Even in the absence of direct land-use changes, however, climate warming may turn northern peat deposits into GHG sources because of the melting of permanently frozen grounds (or permafrost), enhanced wildfire activity and changes in snow cover. At ERG, we are currently using Artificial Intelligence techniques to derive predictive algorithms from time series data and then use global warming scenarios to project future peatland GHG emissions.

Another consequence of global warming is sea level rise. Together with colleagues from the University of Rostock in Germany, we are investigating coastal peat deposits to determine the effects of freshwater–seawater tidal mixing and exposure to rising sulphate concentrations on peat carbon dynamics. This collaborative work is motivated by the need to better predict the combined consequences of sea level rise and land subsidence from wetland drainage that are increasingly exposing nearshore peatlands to seawater intrusion. In addition, we are assessing the potential of peat to attenuate the dispersion of agricultural and municipal pollutants.

Overall, ERG’s research is advancing the quantitative understanding of the biogeochemical reactivity of peat under a broad range of environmental conditions. The resulting data and knowledge are the key to inform restoration and management practices that enhance the beneficial functions of peatlands, including their capacity to counteract rising atmospheric CO2 concentrations. From that perspective, peatlands have a great potential to contribute to nature-based solutions for climate change mitigation and adaptation.

To read more about our work, check out our eBook on multidisciplinary environmental water research activities.

References

Rafat, A., Rezanezhad, F., Quinton, W.L., Humphreys, E.R., Webster, K., and Van Cappellen P. (2021) Non-growing season carbon emissions in a northern peatland are projected to increase under global warming. Communications Earth & Environment, 2:111

Ackley, C., Tank, Hayes, K., S.E., Rezanezhad, F.,McCarter, C., and Quinton, W.L. (2021). Coupled hydrological and geochemical impacts of wildfire in peatland-dominated regions of discontinuous permafrost. Science of the Total Environment 782, 146841.

McCarter, C.P.R., Rezanezhad, F., Quinton, W.L., Gharedaghloo, B., Lennartz, B., Price, J., Connon, R., and Van Cappellen, P. (2020) Pore-scale controls on hydrological and geochemical processes in peat: Implications on interacting processesEarth-Science Reviews 207, 103227.

Rezanezhad, F., McCarter, C.P.R., and Lennartz, B. (2020). Editorial: Wetland Biogeochemistry: Response to Environmental Change. Frontiers in Environmental Science 8, 55.

Kreuzburg, M., Rezanezhad, R., Milojevic, T., Voss, M., Gosch, L., Liebner, S., Van Cappellen, P., and Rehder, G. (2020) Carbon release and transformation from coastal peat deposits controlled by submarine groundwater discharge: A column experiment studyLimnology and Oceanography 65, 1116-1135.

Gosh, L., Townsend, H., Kreuzberg, M., Janssen, M., Rezanezhad, F., and Lennartz, B. (2019) Sulfate mobility in fen peat and its impact on the release of solutes. Frontiers in Environmental Science 7, 189.

McCarter, C., Rezanezhad, F., Gharedaghloo, B., Price, J., and Van Cappellen, P. (2019) Transport of chloride and deuterated water in peat: The role of anion exclusion, diffusion, and anion adsorption in a dual porosity organic mediaJournal of Contaminant Hydrology 225, 103497.

Liu, H., Zak, D., Rezanezhad, F. and Lennartz, B. (2019). Soil degradation determines release of nitrous oxide and dissolved organic carbon from peatlands. Environmental Research Letters 14, 094009.

Rezanezhad, F, Price J.S., Quinton W.L., Lennartz B., Milojevic T., and Van Cappellen P. (2016) Structure of peat soils and implications for water storage, flow and solute transport: A review update for geochemists. Chemical Geology 429, 75-74.



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Special Report Author Details
Author: Prof. Fereidoun Rezanezhad & Prof. Philippe Van Cappellen
Organisation: Ecohydrology Research Group, Water Institute, University of Waterloo
Email: pvc@uwaterloo.ca
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