Richard E. Fewster

Background: Peatlands are globally-important carbon sinks at risk of degradation from climate change and direct human impacts, including drainage and burning. Peat accumulates when there is a positive mass balance between plant productivity inputs and litter/peat decomposition losses. However, the factors influencing the rate of peat accumulation over time are still poorly understood. Methodology/Principal Findings: We examine apparent peat accumulation rates (aPAR) during the last two millennia from 28 well-dated, intact European peatlands and find a range of between 0.005 and 0.448 cm yr^-1 (mean = 0.118 cm yr^-1). Our work provides important context for the commonplace assertion that European peatlands accumulate at ~0.1 cm per year. The highest aPAR values are found in the Scandinavian and Baltic regions, in contrast to Britain, Ireland, and Continental Europe. We find that summer temperature is a significant climatic control on aPAR across our European sites. Furthermore, a significant relationship is observed between aPAR and water-table depth (reconstructed from testate-amoeba subfossils), suggesting that higher aPAR levels are often associated with wetter conditions. We also note that the highest values of aPAR are found when the water table is within 5–10 cm of the peatland surface. aPAR is generally low when water table depths are < 0 cm (standing water) or > 25 cm, which may relate to a decrease in plant productivity and increased decomposition losses, respectively. Model fitting indicates that the optimal water table depth (WTD) for maximum aPAR is ~10 cm. Conclusions/Significance: Our study suggests that, in some European peatlands, higher summer temperatures may enhance growth rates, but only if a sufficiently high water table is maintained. In addition, our findings corroborate contemporary observational and experimental studies that have suggested an average water-table depth of ~10 cm is optimal to enable rapid peat growth and therefore carbon sequestration in the long term. This has important implications for peatland restoration and rewetting strategies, in global efforts to mitigate climate change.

Northern peatlands are an important carbon store in mid to high latitudes, but become increasingly discontinuous in the higher latitudes, associated with temperature and precipitation limits on plant growth. During the last four decades, mean annual temperatures in the Arctic have increased on average by ~3 °C. Warmer temperatures and longer growing seasons likely drive increases in plant productivity throughout northern latitudes, but it is not clear whether warming has resulted in lateral spread of Arctic peatlands. Using long time-series Landsat satellite data, coupled with information gathered from fieldwork in situ, we show that Arctic peatlands have likely undergone lateral expansion over the last 40 years. On 21 transects from the edges of 16 extant peatlands in the European and Canadian Arctic (both high and low Arctic locations from 62 to 79°N), over two thirds of the peatland edges we studied showed statistically significant peak-summer greening (as Normalised Difference Vegetation Index) in the last 15 to 20 years, compared to the period 1985–1995. Peak summer moisture (as normalised Difference Moisture Index) levels remained stable or increased at most study sites. The lateral expansion of Arctic peatlands suggests they are an increasingly important natural carbon sink, at least in the near term.

Peatlands have been widely recognised as important carbon stores, ecological habitats and natural hydrological buffers. However, comparatively less attention has been given to the role of peatlands as long-term stores of pollutants, particularly toxic metals and metalloids (TMMs). Furthermore, the potential for their release is poorly understood. An improved understanding of TMM distribution and release in peatlands is critical, because climate warming risks increasing their mobilisation, through enhanced decomposition and changes to hydrological processes, with potentially significant implications for natural ecosystems and human health. The PIPES project (Pollutants In Peatlands: from sink to Source) aims to identify global “hot spots” of peatland pollutants and establish likely release mechanisms of currently inert TMMs. We use a unique combination of observational and controlled-experimental approaches to address two research questions: (1) What is the content and distribution of pollutants in global peatlands? and (2) Under what conditions, and through which pathways, are these pollutants most likely to be released? In this presentation, we share early findings from both components of the PIPES project. Firstly, we present our ongoing analysis of the distribution of TMMs in global peatlands, with a primarily focus on spatial patterns identified across our comprehensive network of sites in the UK and Ireland. We quantify the total content of TMMs using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) in peat cores compiled by a network of > 90 international collaborators. Secondly, we present preliminary results from controlled environmental simulations of TMM release in peat monoliths from subarctic Sweden. We explore both pore-water and atmospheric release under scenarios of drought, climate warming and a shallow burn. Our findings provide crucial new insights into the potential fate of pollutants in global peatlands and their implications for human health and natural ecosystems.

Human industry has contaminated peatlands through the atmospheric deposition of pollutants released by industrial processes over many centuries. Relative to other ecosystems, peatlands sequester a far greater proportion of toxic metals than their areal extent. This is especially true in industry-impacted landscapes, where toxic metals in surficial peat can be elevated well above natural concentrations. Despite peatlands acting as contaminant sinks that can maintain their carbon storage functionality under low metal concentrations, high rates of metal pollution can lead to the degradation of peatland processes that sustain carbon sequestration. For example, the loss of keystone peatland species, such as Sphagnum mosses, limits peat accumulation and long-term carbon accumulation. Therefore, in these degraded peatlands, peat forming processes are often suppressed even decades after the source of contamination has reduced or ceased.Once degraded, peatlands become susceptible to additional disturbances such as fire or erosion, which can release their toxic legacy into the environment and drinking water. Predicted future warmer and drier conditions are expected to increase wildfire prevalence on the landscape and may be further compounded by land use change. The release of previously sequestered metals arguably represents one of the largest contemporary global environmental disasters and greatest future global environmental challenge.In this paper, we present an in-depth review of existing literature on ombrotrophic peatland metal contamination from a range of disciplines. After a detailed search and screening process, data were extracted from 97 studies. 500 individual points covering 26 countries were extracted from these studies, which were published between 1973 to 2022. For each study, the depth at which maximum heavy metal concentration (Cmax) occurred was recorded, along with surface concentration and concentration at depth. Using Kernel Density Estimates, the distribution of Cmax was typically within the top 0.2m of the peat surface across all studies, though with variation in the mean depth profiles between different metals. For example, Cmax for Cd and Zn typically peaked at 0.1m below the surface with few studies showing Cmax below 0.2m. As, Cu and Pb also had mean Cmax values

Nascent peatlands represent an emerging, nature‐based carbon sink in the global climate system. A warming climate and changing precipitation regime could drive peat initiation beyond the current latitudinal and altitudinal boundaries of the peatland bioclimatic envelope, through increases in plant productivity and moisture availability, with potential implications for global radiative forcing. However, contemporaneous observations of new peat formation remain scarce. We investigate peat initiation within the deglaciating Rob Roy valley in the Southern Alps, Aotearoa/New Zealand. We find that montane peats have developed across the head of the valley since ∼1949 C.E., coinciding with regional climate warming and glacial retreat. Further, we identify a common ecological succession, characterized by a rise in brown mosses (mainly Bryum ) beginning around ∼1963 C.E. Our findings indicate the potential for wider peat expansion in increasingly warm and wet montane landscapes. However, further bioclimatic modeling is required to elucidate where future peatland developments may occur.

Peat accumulates when there is a positive mass balance between plant productivity inputs and litter/peat decomposition losses. Here we examine apparent peat accumulation rates (aPAR) during the last two millennia from 28 well-dated European peatlands and find them to range between 0.005 and 0.448 cm yr-1 (mean = 0.118 cm yr-1). Our work provides important context for the commonplace assertion that peatlands accumulate at ~1mm per year. We find that relationships between aPAR and climatic variables are generally weak – summer temperature is the only significant climatic control on aPAR across our European sites. aPAR tends to be higher when water-table depth (reconstructed from testate-amoeba subfossils) is within 5-10 cm of the peatland surface. When a Generalized Additive Model and Gaussian Response Curve are fitted to the data, both methods show that the optimal water-table depth for highest aPAR is ~10 cm. aPAR is generally lower when water table depths are 25 cm, which may relate to a decrease in plant productivity and increased decomposition losses, respectively. These findings corroborate contemporary experimental studies which examined the relationship between peatland water-table depth, or the thickness of the aerobic surface layer (the ‘acrotelm’), and the rate of peat formation. Our work suggests that for European peat bogs, an average water-table depth of ~10 cm is optimal to enable rapid peat growth and therefore carbon sequestration in the long term. This has important implications for peatland restoration and rewetting strategies, in our global efforts to mitigate climate change.

Permafrost peatlands are vulnerable to rapid structural changes under climatic warming, including vertical collapse. Peatland water budgets, and therefore peat hydraulic properties, are important determinants of vegetation and carbon fluxes. Measurements of hydraulic properties exist for only a limited number of permafrost peatland locations, primarily concentrated in North America. The impacts of thaw‐induced collapse upon properties such as horizontal saturated hydraulic conductivity (K_h), and thus lateral drainage, remain poorly understood. We made laboratory determinations of K_h from 82 peat samples from a degrading Swedish palsa mire. We fitted a linear mixed‐effects model (LMM) to establish the controls on K_h, which declined strongly with increasing depth, humification and dry bulk density. Depth exerted the strongest control on K_h in our LMM, which demonstrated strong predictive performance (r² = 0.605). Humification and dry bulk density were influential predictors, but the high collinearity of these two variables meant only one could be included reliably in our LMM. Surprisingly, peat K_h did not differ significantly between desiccating and collapsed palsas. We compared our site‐specific LMM to an existing, multi‐site model, fitted primarily to boreal and temperate peatlands. The multi‐site model made less skillful predictions (r² = 0.528) than our site‐specific model, possibly due to latitudinal differences in peat compaction, floristic composition and climate. Nonetheless, low bias means the multi‐site model may still be useful for estimating peat K_h at high latitudes. Permafrost peatlands remain underrepresented in multi‐site models of peat hydraulic properties, and measurements such as ours could be used to improve future iterations.

In northern peatlands, near‐saturated surface conditions promote valuable ecosystem services such as carbon storage and drinking water provision. Peat saturated hydraulic conductivity (K_sat) plays an important role in maintaining wet surface conditions by moderating drainage and evapotranspiration. Peat K_sat can exhibit intense spatial variability in three dimensions and can change rapidly in response to disturbance. The development of skillful predictive equations for peat K_sat and other hydraulic properties, akin to mineral soil pedotransfer functions, remains a subject of ongoing research. We report a meta‐analysis of 2,507 northern peat samples, from which we developed linear models that predict peat K_sat from other variables, including depth, dry bulk density, von Post score (degree of humification), and categorical information such as surface microform type and peatland trophic type (e.g., bog and fen). Peat K_sat decreases strongly with increasing depth, dry bulk density, and humification; and increases along the trophic gradient from bog to fen peat. Dry bulk density and humification are particularly important predictors and increase model skill greatly; our best model, which includes these variables, has a cross‐validated r² of 0.75 and little bias. A second model that includes humification but omits dry bulk density, intended for rapid field estimations of K_sat, also performs well (cross‐validated r² = 0.64). Two additional models that omit several predictors perform less well (cross‐validated r² ∼ 0.5), and exhibit greater bias, but allow K_sat to be estimated from less comprehensive data. Our models allow improved estimation of peat K_sat from simpler, cheaper measurements.