Peat macropore networks and their conceptual implications for methane production and emission

Peatlands are globally significant modulators of biogeochemical cycles and important natural sources of methane. The emissions are strongly influenced by the diffusion of oxygen into the peat and the diffusion of methane from the peat to the atmosphere. The structure of peat macropore networks controls the gas transport. The characterization of peat pore structure and connectivity using complex network theory methods can give important conceptual insight into the relationship between the microscale pore space characteristics and methane emissions on a macroscopic scale. Both gas transfer in unsaturated peat and the evolution of the connected air-filled pore space can be conceptualized through a pore network modeling approach. Pores that become isolated from the atmosphere may eventually develop into anaerobic pockets, which are local hotspots of methane production in unsaturated peat.
We extracted macropore (diameter greater than 0.1 mm) networks from three-dimensional X-ray micro-computed tomography (micro-CT) images of peat samples collected from a boreal forested peatland and evaluated local and global connectivity metrics for the networks. We also simulated the soil-water retention curves of the peat samples using pore network modeling and compared the results with measured water retention characteristics. There were fundamental differences in macropore structure and connectivity between vertical peat layers. Macropore connectivity was higher and the flow routes through the peat matrix were less tortuous in the near-surface peat than in the deeper layers. Furthermore, the number and volume of macropores, the average width of pore throats, and the structural anisotropy of peat decreased with depth. Therefore, gas exchange with the atmosphere may be slowed down because of narrower and more tortuous air-filled diffusion channels as the distance between the peat layer and the soil-atmospheric interface increases.
The network analysis also suggests that local and global network connectivity metrics, such as the network average clustering coefficient and closeness centrality, might be proxies for gas diffusion capability in air-filled pore networks. However, the applicability of the metrics was restricted to the topmost peat layer with high porosity. The spatial extent and larger-scale connectivity of the network and the spatial distribution of the pores within the network may be reflected in different network metrics in contrasting ways.
The hysteresis of peat water content was found to affect the evolution of the interconnected air-filled pore volume in unsaturated peat. Therefore, the volume available for the formation of anaerobic pockets may be smaller and methane production may be slower in wetting conditions than in drying conditions. This hysteretic behavior might be one of the reasons behind observed hotspots and episodic spikes of methane emissions, and therefore hysteresis should be included in biogeochemical models describing methane dynamics in peat.