Geomorphic control of peat age and extent in temperate floodplains

The preservation and restoration of peatlands has not only ecological goals but is also seen as an important climate mitigation tool. Peat stored in temperate floodplains are key soil organic carbon (SOC) storage hotspots but the factors controlling alluvial peat distribution, and its evolution remain poorly understood. Nevertheless, in order to properly assess the carbon storage potential of restored alluvial peatlands, forecasting alluvial peat dynamics requires robust modelling tools which in turn require a sufficient understanding of the long-term evolution that present-day peatlands have undergone. This includes detailed info on its spatial pattern, age and factors controlling its preservation.  

Here, we present recently derived field and laboratory analysis data on two alluvial sites in contrasting environmental settings in Belgium. The valleys vary in geomorphic setting, plant ecology and past trajectories of human disturbance. In terms of fluvial energy, these sites vary between high-energy systems in the upland Ardennes to low-energy systems in lowland Flanders.  

The upland floodplain site carbon storage totals 548.1 Mg C/ha, of which 43% is in the form of peat soils. The lowland floodplain site carbon storage is 1425 Mg C/ha with peat providing 80% of the SOC stock. In comparison, non-floodplain soils in Belgian have an average carbon content of only 40 to 90 Mg C/ha for agricultural and forested areas, respectively, whereas for ombrotophic peat bogs on the nearby Hautes Fagnes plateau values between 176 and 856 Mg C/ha were reported [Li et al. 2024 https://doi.org/10.1016/j.geoderma.2024.117009]. Temperate floodplain environments thus store significant amounts of carbon rivalling extensive peat bogs in non-alluvial settings.

There is a striking variation in the age of the peat and carbon between both sites. The average age of carbon in the upland site is limited to only 300 years. This is due to recent re-wetting and rapid peat reformation, combined with its relatively recent initiation (1400–3800 cal. BP): higher energy conditions lead to more lateral channel activity removing previously formed peats. SOC was concentrated in the upper horizons and decreased with depth. In contrast, in the lowland site, the average age of stored carbon is 7500 years and SOC concentration decreases towards the surface due to recent drainage (last 100-200 years). Peat development here already started in the pre-Holocene (Bølling–Allerød 14690 – 12850 cal. BP, responsible for 11% of total carbon storage), temporarily stopped in the Younger Dryas, and resumed in the Preboreal. Lateral channel activity is limited in these low energetic environments preserving older peats.

The differences between these sites have significant implications for carbon stock accounting and management. In areas storing ancient (pre-Holocene) carbon, environmental changes can release large carbon reserves which cannot be replaced, unlike agricultural or forested areas that maintain a more dynamic equilibrium. In contrast, the recently re-wetted site demonstrated the rapid carbon-capturing capability of reviving an acrotelm layer.  These detailed data will be complemented with data from nine additional sites and used to calibrate long-term alluvial peatland models required to identify management and governance options for SOC-rich floodplains.