Preprints
https://doi.org/10.5194/bg-2021-259
https://doi.org/10.5194/bg-2021-259

  20 Oct 2021

20 Oct 2021

Review status: this preprint is currently under review for the journal BG.

Peat macropore networks – new insights into episodic and hotspot methane emission

Petri Kiuru1, Marjo Palviainen2, Tiia Grönholm3, Maarit Raivonen4, Lukas Kohl5,6, Vincent Gauci7,8, Iñaki Urzainki1,9, and Ari Laurén1 Petri Kiuru et al.
  • 1School of Forest Sciences, Faculty of Science and Forestry, University of Eastern Finland, P.O. Box 111, 80101 Joensuu, Finland
  • 2Department of Forest Sciences, University of Helsinki, P.O. Box 27, 00014 Helsinki, Finland
  • 3Finnish Meteorological Institute (FMI), Erik Palmenin aukio 1, 00560 Helsinki, Finland
  • 4Institute for Atmospheric and Earth System Research (INAR)/Physics, Faculty of Science, University of Helsinki, P.O. Box 68, 00014 Helsinki, Finland
  • 5Department of Agricultural Sciences, University of Helsinki, P.O. Box 56, 00014 Helsinki, Finland
  • 6Institute for Atmospheric and Earth System Research (INAR)/Forest Sciences, Faculty of Agriculture and Forestry, University of Helsinki, P.O. Box 56, 00014 Helsinki, Finland
  • 7Birmingham Institute of Forest Research (BIFoR), University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
  • 8School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
  • 9Natural Resources Institute Finland (Luke), Latokartanonkaari 9, 00790 Helsinki, Finland

Abstract. Peatlands are important natural sources of atmospheric methane (CH4) emissions. The emissions are strongly influenced by the diffusion of oxygen into the soil and of CH4 from the soil to the atmosphere. This diffusion, in turn, is controlled by the structure of macropore networks. The characterization of peat pore structure and connectivity through complex network theory approaches can give insight into how the relationship between the microscale pore space properties and CH4 emissions on a macroscopic scale is shaped. The formation of anaerobic pockets, which are local hotspots of CH4 production in unsaturated peat, can also be conceptualized through a pore network approach. In this study, we extracted interconnecting macropore networks from three-dimensional X-ray micro-computed tomography (µCT) images of peat samples and evaluated local and global connectivity metrics for the networks. We also simulated the water retention characteristics of the peat samples using a pore network modeling approach and compared the simulation results with measured water retention characteristics. The results showed large differences in peat macropore structure and pore network connectivity between vertical soil layers. The macropore space was more connected and the flow paths through the peat matrix were less tortuous near the soil surface than at deeper depths. In addition, macroporosity, structural anisotropy, and average pore throat diameter decreased with depth. Narrower and more winding air-filled diffusion channels may reduce the rate of CH4 transport as the distance from the peat layer to the soil–air interface increases. Hysteresis was found to affect the evolution of the volume of connected air-filled pore space in unsaturated peat. Thus, the formation of anaerobic pockets may occur in a smaller soil volume and methanogenesis may be slower when the peat is wetting compared to drying conditions. This hysteretic behavior should be taken into account in biogeochemical models to explain the hotspots and episodic spikes of CH4 emissions. The network analysis also suggests that both local and global network connectivity metrics, such as the network average clustering coefficient and closeness centrality, might serve as proxies for assessing the efficiency of CH4 diffusion in air-filled pore networks. However, the applicability of the network metrics was restricted to the high-porosity near-surface layer. The spatial extent and global continuity of the pore network and the spatial distribution of the pores may be reflected in different network metrics in contrasting ways.

Petri Kiuru et al.

Status: open (until 10 Dec 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Comment on bg-2021-259', Sanna Sevanto, 18 Nov 2021 reply
  • RC1: 'Comment on bg-2021-259', Anonymous Referee #1, 30 Nov 2021 reply

Petri Kiuru et al.

Petri Kiuru et al.

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Short summary
Peatlands are large sources of methane, and peat structure controls methane production and emission. We used X-ray microtomography imaging, network theory methods, and pore network modeling to describe the properties of peat macropore networks and the role of macropores in methane-related processes. We show that conditions for methane production and transport vary with depth and are affected by hysteresis, which may explain the hotspots and episodic spikes in peatland methane emissions.
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