Biogeochemical indicators of peatland degradation – a case study of a temperate bog in Northern Germany

Organic soils in peatlands store a great proportion of the global soil carbon pool and can lose carbon via the atmosphere due to degradation. In Germany, most of the greenhouse gas (GHG) emissions from organic soils are attributed to sites managed as grassland. Here, we investigated a land use gradient from near-natural wetland (NW) to an extensively managed (GE) to an intensively managed grassland site (GI), all formed in the same bog complex in northern Germany. Vertical depth profiles of δ13C, δ15N, ash content, C / N ratio and bulk density as well as radiocarbon ages were studied to identify peat degradation and to calculate carbon loss. At all sites, including the near-natural site, δ13C depth profiles indicate aerobic decomposition in the upper horizons. Depth profiles of δ15N differed significantly between sites with increasing δ15N values in the top soil layers paralleling an increase in land use intensity owing to differences in peat decomposition and fertilizer application. At both grassland sites, the ash content peaked within the first centimetres. In the near-natural site, ash contents were highest in 10–60 cm depth. The ash profiles, not only at the managed grassland sites, but also at the near-natural site indicate that all sites were influenced by anthropogenic activities either currently or in the past, most likely due to drainage. Based on the enrichment of ash content and changes in bulk density, we calculated the total carbon loss from the sites since the peatland was influenced by anthropogenic activities. Carbon loss at the sites increased in the following order: NW < GE < GI. Radiocarbon ages of peat in the topsoil of GE and GI were hundreds of years, indicating the loss of younger peat material. In contrast, peat in the first centimetres of the NW was only a few decades old, indicating recent peat growth. It is likely that the NW site accumulates carbon today but was perturbed by anthropogenic activities in the past. Together, all biogeochemical parameters indicate a degradation of peat due to (i) conversion to grassland with historical drainage and (ii) land use intensification.

Palaeohydrology, fires and vegetation succession in the southern Baltic during the last 7500 years reconstructed from a raised bog based on multi-proxy data

We present the first 7500 yr long multi-proxy record from a raised bog located at the southern Baltic coast, Poland. Testate amoebae, plant macrofossils, pollen and microscopic charcoal were used to reconstruct environmental changes in Pomerania (northern Poland, Kaszuby Lakeland) from a 7-m thick peat archive of Stążki bog dated 5500 BC–AD 1250. We obtained a record of proxies representing different spatial scales: regional vegetation changed simultaneously with local vegetation, and testate amoebae showed a pattern of change similar to that of pollen and plant macrofossils. On the basis of the combined proxies, we distinguished three hydroclimatic stages: moist conditions 5500–3450 BC, drier conditions with regionally increased fires up to 600 BC, and again moist conditions from 600 BC onward. During the drier interval, a first climatic shift to wetter conditions at 1700 BC is indicated by regional pollen as the replacement of Corylus by Carpinus, and locally by, e.g., the increase of Hyalosphenia elegans and mire plants such as Sphagnum sec. Cuspidata. Furthermore, we observed a correlation since 600 BC among the re-expansion of Carpinus (after a sudden decline ca. 950 BC), increased peat accumulation, increase of Hyalosphenia species, and fewer fires, suggesting lower evapotranspiration and a stable high water table in the bog. Fagus started to expand after AD 810 gradually replacing Carpinus, which was possibly due to a gradually more oceanic climate, though we cannot exclude human impact on the forests. Peat accumulation, determined by radiocarbon dating, varied with bog surface wetness. The hydroclimatic phases found in Stążki peatland are similar to moisture changes recorded in other sites from Poland and Europe. This is the first detailed record of hydroclimatic change during the Holocene in the southern Baltic region, so it forms a reference site for further studies on other southern Baltic bogs that are in progress.

The performance of single- and multi-proxy transfer functions (testate amoebae, bryophytes, vascular plants) for reconstructing mire surface wetness and pH

Peatlands are widely exploited archives of paleoenvironmental change. We developed and compared multiple transfer functions to infer peatland depth to the water table (DWT) and pH based on testate amoeba (percentages, or presence/absence), bryophyte presence/absence, and vascular plant presence/absence data from sub-alpine peatlands in the SE Swiss Alps in order to 1) compare the performance of single-proxy vs. multi-proxy models and 2) assess the performance of presence/absence models. Bootstrapping cross-validation showing the best performing single-proxy transfer functions for both DWT and pH were those based on bryophytes. The best performing transfer functions overall for DWT were those based on combined testate amoebae percentages, bryophytes and vascular plants; and, for pH, those based on testate amoebae and bryophytes. The comparison of DWT and pH inferred from testate amoeba percentages and presence/absence data showed similar general patterns but differences in the magnitude and timing of some shifts. These results show new directions for paleoenvironmental research, 1) suggesting that it is possible to build good-performing transfer functions using presence/absence data, although with some loss of accuracy, and 2) supporting the idea that multi-proxy inference models may improve paleoecological reconstruction. The performance of multi-proxy and single-proxy transfer functions should be further compared in paleoecological data.

Impact of an abrupt cooling event on interglacial methane emissions in northern peatlands

Rapid changes in atmospheric methane (CH4), temperature and precipitation are documented by Greenland ice core data both for glacial times (the so called Dansgaard-Oeschger (D-O) events) as well as for a cooling event in the early Holocene (the 8.2 kyr event). The onsets of D-O warm events are paralleled by abrupt increases in CH4 by up to 250 ppb in a few decades. Vice versa, the 8.2 kyr event is accompanied by an intermittent decrease in CH4 of about 80 ppb over 150 yr. The abrupt CH4 changes are thought to mainly originate from source emission variations in tropical and boreal wet ecosystems, but complex process oriented bottom-up model estimates of the changes in these ecosystems during rapid climate changes are still missing. Here we present simulations of CH4 emissions from northern peatlands with the LPJ-Bern dynamic global vegetation model. The model represents CH4 production and oxidation in soils and transport by ebullition, through plant aerenchyma, and by diffusion. Parameters are tuned to represent site emission data as well as inversion-based estimates of northern wetland emissions. The model is forced with climate input data from freshwater hosing experiments using the NCAR CSM1.4 climate model to simulate an abrupt cooling event. A concentration reduction of ~10 ppb is simulated per degree K change of mean northern hemispheric surface temperature in peatlands. Peatland emissions are equally sensitive to both changes in temperature and in precipitation. If simulated changes are taken as an analogy to the 8.2 kyr event, boreal peatland emissions alone could only explain 23 of the 80 ppb decline in atmospheric methane concentration. This points to a significant contribution to source changes from low latitude and tropical wetlands to this event.

Transient simulations of the carbon and nitrogen dynamics in northern peatlands: from the Last Glacial Maximum to the 21st century

The development of northern high-latitude peatlands played an important role in the carbon (C) balance of the land biosphere since the Last Glacial Maximum (LGM). At present, carbon storage in northern peatlands is substantial and estimated to be 500 ± 100 Pg C (1 Pg C = 1015 g C). Here, we develop and apply a peatland module embedded in a dynamic global vegetation and land surface process model (LPX-Bern 1.0). The peatland module features a dynamic nitrogen cycle, a dynamic C transfer between peatland acrotelm (upper oxic layer) and catotelm (deep anoxic layer), hydrology- and temperature-dependent respiration rates, and peatland specific plant functional types. Nitrogen limitation down-regulates average modern net primary productivity over peatlands by about half. Decadal acrotelm-to-catotelm C fluxes vary between −20 and +50 g C m−2 yr−1 over the Holocene. Key model parameters are calibrated with reconstructed peat accumulation rates from peat-core data. The model reproduces the major features of the peat core data and of the observation-based modern circumpolar soil carbon distribution. Results from a set of simulations for possible evolutions of northern peat development and areal extent show that soil C stocks in modern peatlands increased by 365–550 Pg C since the LGM, of which 175–272 Pg C accumulated between 11 and 5 kyr BP. Furthermore, our simulations suggest a persistent C sequestration rate of 35–50 Pg C per 1000 yr in present-day peatlands under current climate conditions, and that this C sink could either sustain or turn towards a source by 2100 AD depending on climate trajectories as projected for different representative greenhouse gas concentration pathways.

DYPTOP: a cost-efficient TOPMODEL implementation to simulate sub-grid spatio-temporal dynamics of global wetlands and peatlands

Simulating the spatio-temporal dynamics of inundation is key to understanding the role of wetlands under past and future climate change. Earlier modelling studies have mostly relied on fixed prescribed peatland maps and inundation time series of limited temporal coverage. Here, we describe and assess the the Dynamical Peatland Model Based on TOPMODEL (DYPTOP), which predicts the extent of inundation based on a computationally efficient TOPMODEL implementation. This approach rests on an empirical, grid-cell-specific relationship between the mean soil water balance and the flooded area. DYPTOP combines the simulated inundation extent and its temporal persistency with criteria for the ecosystem water balance and the modelled peatland-specific soil carbon balance to predict the global distribution of peatlands. We apply DYPTOP in combination with the LPX-Bern DGVM and benchmark the global-scale distribution, extent, and seasonality of inundation against satellite data. DYPTOP successfully predicts the spatial distribution and extent of wetlands and major boreal and tropical peatland complexes and reveals the governing limitations to peatland occurrence across the globe. Peatlands covering large boreal lowlands are reproduced only when accounting for a positive feedback induced by the enhanced mean soil water holding capacity in peatland-dominated regions. DYPTOP is designed to minimize input data requirements, optimizes computational efficiency and allows for a modular adoption in Earth system models.

Palaeoenvironmental changes in Central Europe (NE Poland) during the last 6200 years reconstructed from a high-resolution multi-proxy peat archive

Peat deposits from an ombrotrophic bog (north-eastern Poland) were analysed to reconstruct peatland development and environmental changes. This paper presents reconstructions of hydrological changes and plant succession over the last 6000 years. The methods included the high-resolution analysis of plant macrofossils, pollen and testate amoebae, supported by radiocarbon dating. Three main phases were identified in the history of the bog and surrounding woodland vegetation: 4000–400 BC, 400 BC–AD 1700 and AD 1700–2011. Except for terrestrialisation and the fen-to-bog transition phase, the development of bog vegetation was mainly dependent on the climate until approximately AD 1700. The dominant taxon in Gązwa bog was Sphagnum fuscum/rubellum. Woodland development was significantly affected by human activity at several time periods. The most visible human activity, manifested by the decline of deciduous species, occurred ca. 350 BC, ca. AD 250, ca. AD 1350 and after AD 1700. These events correspond to phases of human settlement in the area. During 400–300 BC, the decline of deciduous trees, primarily Carpinus, coincided with an increase in indicators of human activity and fire frequency. At ca. AD 200, Carpinus and Tilia abundance decreased, corresponding to an increased importance of cereals (Secale and Triticum). Since ca. AD 1350, the impact of Teutonic settlement is apparent, and after AD 1700, deciduous forests largely disappeared.

Long-term hydrological dynamics and fire history over the last 2000 years in CE Europe reconstructed from a high-resolution peat archive

Sphagnum peatlands in the oceanic-continental transition zone of Poland are currently influenced by climatic and anthropogenic factors that lead to peat desiccation and susceptibility to fire. Little is known about the response of Sphagnum peatland testate amoebae (TA) to the combined effects of drought and fire. To understand the relationships between hydrology and fire dynamics, we used high-resolution multi-proxy palaeoecological data to reconstruct 2000 years of mire history in northern Poland. We employed a new approach for Polish peatlands – joint TA-based water table depth and charcoal-inferred fire activity reconstructions. In addition, the response of most abundant TA hydrological indicators to charcoal-inferred fire activity was assessed. The results show four hydrological stages of peatland development: moderately wet (from ∼35 BC to 800 AD), wet (from ∼800 to 1390 AD), dry (from ∼1390 to 1700 AD) and with an instable water table (from ∼1700 to 2012 AD). Fire activity has increased in the last millennium after constant human presence in the mire surroundings. Higher fire activity caused a rise in the water table, but later an abrupt drought appeared at the onset of the Little Ice Age. This dry phase is characterized by high ash contents and high charcoal-inferred fire activity. Fires preceded hydrological change and the response of TA to fire was indirect. Peatland drying and hydrological instability was connected with TA community changes from wet (dominance of Archerella flavum, Hyalosphenia papilio, Amphitrema wrightianum) to dry (dominance of Cryptodifflugia oviformis, Euglypha rotunda); however, no clear fire indicator species was found. Anthropogenic activities can increase peat fires and cause substantial hydrology changes. Our data suggest that increased human fire activity was one of the main factors that influenced peatland hydrology, though the mire response through hydrological changes towards drier conditions was delayed in relation to the surrounding vegetation changes.

Present-day vegetation and the Holocene and recent development of Egelsee-Moor, Salzburg province, Austria

This paper describes the present-day vegetation, stratigraphy and developmental history of the mire of Egelsee-Moor (Salzburg, Austria; 45°45′N, 13°8.5′E, 700 m a.s.l., 15 ha in area) since the early Late Glacial on the basis of 4 transects with 14 trial borings across the peatland. We present a vegetation map of the mire, a longitudinal section through the peat body based on six cores showing the peat types, overview macrofossil diagrams of six cores showing the local mire development and two pollen diagrams covering the Late Glacial and Holocene. The chronology of the diagrams depends on biostratigraphic dating for the Late Glacial and early Holocene and radiocarbon dating for the remaining Holocene. The northern part of the mire originated through terrestrialisation of nutrient-rich, mostly inundated fen and the southern part through paludification of wet soils. The very small lake of today was a reservoir until recently for providing water-power for timber rafting (‘Holztrift’). The mire vegetation today is a complex of forested parts (mainly planted Pinus sylvestris and Thuja occidentalis, but also spontaneous Picea abies, Betula pubescens and Frangula alnus), reed-lands (Phragmites) and litter meadows (Molinietum, Schoenetum, etc.). The central part has hummock-hollow complexes with regionally rare species of transitional mires (Drosera anglica, D. intermedia, Lycopodiella inundata, Scorpidium scorpioides, Sphagnum platyphyllum, S. subnitens). The results indicate that some of the mid-Holocene sediments may have been removed by the timber-rafting practices, and that water extraction from the hydrological catchment since 1967 has resulted in a partial shift of transitional mire to ombrotrophic bog. The latter potentially endangers the regionally rare species and was used as an argument to stop further water extraction.