Misinterpreting carbon accumulation rates in records from near-surface peat

Abstract

Peatlands are globally important stores of carbon (C) that contain a record of how their rates of C accumulation have changed over time. Recently, near-surface peat has been used to assess the effect of current land use practices on C accumulation rates in peatlands. However, the notion that accumulation rates in recently formed peat can be compared to those from older, deeper, peat is mistaken – continued decomposition means that the majority of newly added material will not become part of the long-term C store. Palaeoecologists have known for some time that high apparent C accumulation rates in recently formed peat are an artefact and take steps to account for it. Here we show, using a model, how the artefact arises. We also demonstrate that increased C accumulation rates in near-surface peat cannot be used to infer that a peatland as a whole is accumulating more C – in fact the reverse can be true because deep peat can be modified by events hundreds of years after it was formed. Our findings highlight that care is needed when evaluating recent C addition to peatlands especially because these interpretations could be wrongly used to inform land use policy and decisions.

A regime shift from erosion to carbon accumulation in a temperate northern peatland

“…Estimates of more recent apparent rates of C accumulation in northern peatlands are often higher, for example 51–149 g C m −2 year −1 (Piilo et al., 2019); however, these higher rates are, in part, due to an artefact known as the ‘acrotelm effect’. Even under a constant climate, rates of C accumulation calculated for near‐surface peat layers produce an apparent increase (Clymo, Turunen, & Tolonen, 1998; Turner, Swindles, & Roucoux, 2014; Young et al., 2019). This is caused by the younger litter and peat near the peatland surface having undergone less decay and decomposition than deeper layers.…”

“…This is caused by the younger litter and peat near the peatland surface having undergone less decay and decomposition than deeper layers. Unfortunately, this effect is very difficult to correct for (Young et al., 2019).…”

“…Where newly formed peat is underlain by older remnant peat, as is the case at MIG2 and MIG3, the decay losses from this older peat (even if protected to some extent by a rising water-table following the switch) need to be combined with the rates of new peat addition to obtain net rates of C accumulation. However, such losses are difficult to estimate retrospectively (Young et al, 2019). At MIG1 and MIG4, it is likely that all old peat was eroded away before the new peat developed (see Section 2 and Appendix S1).…”

Mass Balances of a Drained and a Rewetted Peatland: on Former Losses and Recent Gains

“…Direct flux measurements carried out close to our sampling site in the years 2012 and 2013 showed C fluxes ranging from a net uptake of 70 to a net loss of 100 g C m −2 (CO 2 and CH 4 combined; [11]). Typical long-term rates of carbon accumulation are around 20 g C m −2 a −1 (e.g., [44]) and recent rates can amount up to~100 g C m −2 a −1 in temperate sites [45]. The sink size derived from our core data lies in the upper range of the known values, which may be attributed to vegetation restructuring after rewetting including the build-up of a thick root mat that is predominantly made up of Carex acutiformis roots.…”

Contextualizing UK moorland burning studies with geographical variables and sponsor identity

“…The critique represents part of an intense debate about UK moorland burning (Baird et al., 2019; Brown, Holden, & Palmer, 2016; Davies et al., 2016; Douglas, Buchanan, Thompson, & Wilson, 2016; Evans et al, 2019). Most recently, some studies on peat and carbon accumulation (Heinemeyer, Asena, Burn, & Jones, 2018; Marrs et al., 2019b) were suggested to have overstated conclusions due to use of incorrect methods (Young et al., 2019), and these papers have required corrections to clarify perceived competing interests (Heinemeyer et al., 2018; Marrs et al., 2019a). At the same time, as researchers are increasingly required to evidence societal impact of their work, perceived decreases in public funding mean that researchers are seeking to diversify research funding, which may include sponsors with some form of agenda.…”