Rewetting for the ages – methane and carbon dioxide emissions decades after peatland restoration

To ensure that rewetting of northern peatlands serves as an effective climate buffer within the timeframes relevant to current emission scenarios and projected climate change by 2100 and beyond, these restored peatlands must not only immediately reduce net emissions compared to previous intensive land uses but also eventually revert to being net sinks of greenhouse gases. In natural wetlands, this balance hinges on the net uptake of carbon dioxide (CO₂) and peat formation, which in natural system exceed the combined net emissions of methane (CH₄) and lateral carbon export in runoff.

Although the climate mitigation benefits of rewetting have been recognized for decades, empirical evidence on the long-term impacts of rewetting on the net exchange of greenhouse gases, particularly CH₄, is lacking. Furthermore, rewetting is not a one-size-fits-all solution and results in diverse post-rewetting ecosystems depending on hydrological management, such as pond/lake formation and vegetation encroachment, each with distinct physico-chemical dynamics affecting CO₂ and CH₄ emissions differently.

To address this knowledge gap, we present data on the net surface exchange of CH₄ and CO₂ over two growing seasons in chronosequences of two rewetting trajectories: pond formation and vegetation encroachment on formerly drained peatlands. We included a clearcut, former Norway spruce plantation, and a near-natural peatland as end members of these chronosequences, with the latter representing the baseline for the peatlands we wish to restore and ultimately recreate.

Our field investigations aimed to capture spatiotemporal variability in CH₄ and CO₂ fluxes from characteristic surface types in the studied locations, including sedges, sphagnum moss, bare decomposed peat, and water surfaces in ponds and ditches. We estimated both diffusive and ebullition fluxes. Preliminary findings indicate that combined ecosystem respiration (CO₂) and CH₄ emissions in encroached rewetted systems are higher than in drained and baseline sites. For sites with ponding, net CO₂ emissions occur throughout the season but at lower rates compared to both drained and near-natural sites, while diffusive CH₄ emissions are comparable to those in encroached areas. In both rewetting trajectories, CO₂ and CH₄ emissions decrease over time, but elevated fluxes persist for at least two decades post-restoration. Including ebullitions, net CH₄ emissions in ponded system are highest and show only a weak trend towards lower fluxes over time. We also investigated whether the elevated fluxes were due to more reactive peat substrate, as proposed in a series of incubation experiments.  

Whilst our findings shed light on one little-known aspect of rewetting, there remain critical knowledge gaps regarding ecosystem net carbon balances, particularly net ecosystem productivity, including woody vegetation in encroached rewetted peatlands, net deposition of autochthonous and allochthonous carbon in ponds, and how carbon decomposition in formerly drained peat relates to biogeochemical indicators such as nitrogen (N) and phosphorus (P). We also wish to discuss our findings within a broader northern peatland context and how future studies can be designed to investigate the long-term climate impacts of rewetting.