Peatland subsidence and carbon loss from drained temperate fens

Peat-forming organic soils lose large amounts of carbon and soil volume when drained. Although surface subsidence is often taken as a proxy for the associated carbon loss, other mechanisms also cause a change in volume. To infer the reliability of subsidence for estimating carbon loss, we compared long-term subsidence rates of an 80 ha area of temperate fen that was drained 140 yr ago against estimates of subsidence based on soil bulk densities measured at four sites. Both methods correlate significantly, yield similar subsidence rates of 0.8–1.6 cm/yr and underpin the value of using profile information for inferring volumetric loss. Peat oxidation accounts for 28–64% of the loss in volume, which is equivalent to annual carbon loss rates of 2.5–5.5 t C/ha. Whereas the profile-based method is also suitable for estimating carbon loss, the wide range of oxidative contribution to the overall subsidence indicates that subsidence alone cannot provide an unbiased estimate of carbon emission factors from drained fens.