Ombrotrophic Peat Bog Research Articles

Microbial abundances and carbon use under ambient temperature or experimental warming in a southern boreal peatland

Organic peat soils occupy relatively little of the global land surface area but store vast amounts of soil carbon in northern latitudes where climate is warming at a rapid pace. Warming may result in strong positive feedbacks of carbon loss and global climate change driven by microbial processes if warming alters the balance between primary productivity and decomposition. To elucidate effects of warming on the microbial communities mediating peat carbon dynamics, we explored the abundance of broad microbial groups and their source of carbon (i.e. old carbon versus more recently fixed photosynthate) using microbial lipid analysis (δ13C PLFA) of peat samples under ambient temperatures and before/after initiation of experimental peat warming (+ 2.25, + 4.5, + 6.75, and + 9 °C). This analysis occurred over a profile to 2 m depth in an undrained, ombrotrophic peat bog in northern Minnesota. We found that the total microbial biomass and individual indicator lipid abundances were stratified by depth and strongly correlated to temperature under ambient conditions. However, under experimental warming, statistically significant effects of temperature on the microbial community were sporadic and inconsistent. For example, 3 months after experimental warming the relative abundance of Gram-negative bacterial indicators across depth combined and > 50 cm depth and Gram-positive bacterial indicators at 20–50 cm depth showed significant positive relationships to temperature. At that same timepoint, however, the relative abundance of Actinobacterial indicators across depth showed a significant negative relationship to temperature. After 10 months of experimental warming, the relative abundance of fungal biomarkers was positively related to temperature in all depths combined, and the absolute abundance of anaerobic bacteria declined with increasing temperature in the 20–50 cm depth interval. The lack of observed response in the broader microbial community may suggest that at least initially, microbial community structure with peat depth in these peatlands is driven more by bulk density and soil water content than temperature. Alternatively, the lack of broad microbial community response may simply represent a lag period, with more change to come in the future. The long-term trajectory of microbial response to warming in this ecosystem then could either be direct, after this initial lag time, or indirect through other physical or biogeochemical changes in the peat profile. These initial results provide an important baseline against which to measure long-term microbial community and carbon-cycling responses to warming and elevated CO2.

Influence of the microtopography of patagonian peatbogs on the fluxes of greenhouse gasses and dissolved carbon in porewater

Peatlands play an important role in global carbon cycling, as they act as a natural sink of carbon dioxide (CO2) or as a source of methane (CH4). The influence of microtopography (lawns and hummock-hollow complexes) in the biogeochemical dynamics of dissolved organic and inorganic carbon (DOC and DIC) and dissolved CH4 and CO2 is generally miss-considered. Southernmost Patagonia has huge areas of pristine peatlands, which are still in a largely natural state with scarce anthropogenic influence. In this study we provide foundational insights into the dynamics of greenhouse gasses (GHGs) and DOC in Sphagnum dominated peatlands of Southern Patagonia, assessing the impact of microtopography on these dynamics. The stocks of dissolved GHGs and DOC in hummock-hollows complex and lawns were analysed at three depths (25, 50, and 75 cm) in four ombrotrophic peatbogs. CH4, N2O and CO2 fluxes, net ecosystem exchange (NEE) and gross primary productivity (GPP) were also studied. CO2 and CH4 fluxes were strongly affected by microtopography. Hummock-hollows showed higher CO2 fluxes, and temperature and radiation were the main drivers for respiration and GPP, respectively. In addition, in this microtopography, higher DOC concentrations were observed at the top depth. In contrast, lawns acted as a source of CH4, with higher emission rates and high dissolved GHGs concentration throughout the depth profile.

First assessment of changes in dust sources in the black forest during the Holocene: case study at Wildseemoor

A 600 cm peat sequence was extracted from the ombrotrophic peat bog Wildseemoor in the northern Black Forest, covering the last ca. 10,000 years thus allowing for identification of potential changes in dust input during the Holocene. Such information is critical to the understanding of past environmental change at the local to supra-regional scale and helps with interpreting, in particular, fire history and its links to both climate change and anthropogenic impacts. Dust composition change was investigated using X-ray fluorescence core scanning (XRF-CS) to establish bulk chemical composition of lithogenic input to the peatland, using element signatures, and in particular the ratios of Ca/Ti and Ti/Zr. Two main different sources of dust could be differentiated, a local and a distal (long-distance) dust source, which show variations through time. Distal dust input is elevated during the early Holocene (ca. 8,800 - 8,300 cal BP), as well as around 5,000 cal BP, 3,000 cal BP, 2,300 cal BP and the last ca. 200 years. Distal dust might originate from remobilized loess deposits in the Upper Rhine Graben or, periodically, more distant sources such as the Sahara or the Massif Central. Local dust input roughly correlates to peaks in charcoal influx after ca. 3,850 cal BP, potentially recording phases of increased anthropogenic influence in the surroundings of Wildseemoor. Additional research in nearby bogs (from the Black Forest and the Vosges), as well as more precise geochemical fingerprinting of the different dust sources, is needed to explore the regional extent and significance of Holocene environmental changes recorded at Wildseemoor.

Microbial activity contributes to spatial heterogeneity of wetland methane fluxes.

The emission of methane from wetlands is spatially heterogeneous, as concurrently measured surface fluxes can vary by orders of magnitude within the span of a few meters. Despite extensive study and the climatic significance of these emissions, it remains unclear what drives large, within-site variations. While geophysical factors (e.g., soil temperature) are known to correlate with methane (CH4) flux, measurable variance in these parameters often declines as spatial and temporal scales become finer. As methane emitted from wetlands is the direct, net product of microbial metabolisms which both produce and degrade CH4, it stands to reason that characterizing the spatial variability of microbial communities within a wetland-both horizontally and vertically-may help explain observed variances in flux. To that end, we surveyed microbial communities to a depth of 1 m across an ombrotrophic peat bog in Maine, USA using amplicon sequencing and gene expression techniques. Surface methane fluxes and geophysical factors were concurrently measured. Across the first meter of peat at the site, we observed significant changes in the abundance and composition of methanogenic taxa at every depth sampled, with variance in methanogen abundance explaining 70% of flux heterogeneity at a subset of plots. Among measured environmental factors, only peat depth emerged as correlated with flux, and had significant impact on the abundance and composition of methane-cycling communities. These conclusions suggest that a heightened awareness of how microbial communities are structured and spatially distributed within wetlands could offer improved insights into predicting CH4 flux dynamics. IMPORTANCE Globally, wetlands are one of the largest sources of methane (CH4), a greenhouse gas with a warming impact significantly greater than CO2. Methane produced in wetlands is the byproduct of a group of microorganisms which convert organic carbon into CH4. Despite our knowledge of how this process works, it is still unclear what drives dramatic, localized (<10 m) variance in emission rates from the surface of wetlands. While environmental conditions, like soil temperature or water table depth, correlate with methane flux when variance in these factors is large (e.g., spring vs fall), the explanatory power of these variables decline when spatial and temporal scales become smaller. As methane fluxes are the direct product of microbial activity, we profiled how the microbial community varied, both horizontally and vertically, across a peat bog in Maine, USA, finding that variance in microbial communities was likely contributing to much of the observed variance in flux.

Climatic regulation of atmospheric mercury deposition: Evidence from mercury isotopes in an alpine peat core

Abstract Mercury is a global pollutant that can undergo long-range transport in the atmosphere. While anthropogenic activities have largely increased atmospheric Hg emission and deposition since global industrialization, mechanisms governing the atmospheric Hg cycle in preindustrial periods remain unclear. Alpine peatlands receive Hg mainly from atmospheric Hg deposition and are sensitive to climatic changes, therefore alpine peat cores can be a useful archive for understanding the relationship between atmospheric Hg deposition and climate change. Here we reconstruct a 1200-year Hg deposition record based on a core drilled from an ombrotrophic peat bog in Yunnan-Guizhou Plateau, SW China. This core shows a dramatic change of Hg deposition fluxes (30.3–515 μg/m2/yr) associated with variable Hg isotopic composition (δ202Hg: –1.02‰ to –2.01‰; Δ199Hg: –0.16‰ to –0.50‰; Δ200Hg: –0.08‰ to 0.07‰). Using a Δ200Hg-based model, deposition fluxes of atmospheric Hg(0) and Hg(II) were estimated, which vary coherently with climatic indices (e.g., temperature, precipitation, and monsoon intensity). Results of this study imply that atmospheric Hg deposition in SW China is governed by two climate scenarios. In warm-house climates, low precipitation intensity and vegetation growth rates in SW China caused low atmospheric Hg(II) and Hg(0) deposition fluxes, respectively. In cold-house climates, high precipitation intensity and vegetation growth rates caused high atmospheric Hg(II) and Hg(0) deposition fluxes. This study highlights that climate changes can have a strong control on atmospheric Hg deposition.

Quantifying the Effects Sizes of Common Controls on Methane Emissions From an Ombrotrophic Peat Bog

AbstractNorthern wetlands and bogs are a large source of biogenic methane (CH4) to the atmosphere and shape climate. There is then substantial interest in quantifying the effect sizes of controls on bog CH4 emissions. Yet quantification is complicated because wetland emissions are, like the controls that regulate them, spatially and temporarily heterogeneous within individual wetland systems. To account for this heterogeneity, we established 55 measurement plots in a sampling design that had extensive spatial, and repeated temporal, coverage over one ombrotrophic bog. This coverage allowed us to robustly estimate the effect sizes of known environmental controls on CH4 emissions such as temperature, water table depth (WTD), and plant functional composition. As expected, 89% of the cumulative measured flux came from 16% of the plots, with consistently high CH4 emissions over two consecutive growing seasons. Soil temperature exhibited the strongest control on emissions, while WTD and vegetation composition had much smaller relative effects compared to temperature. Interestingly, at high temperatures, WTD was a strong determinant of whether a plot had high versus lower emissions. Despite temperature's large effect size, the controls we measured only explained ∼29% of the variance in CH4 fluxes. Overall, our data confirm observations from other wetlands that CH4 emissions are highly variable at local scales, and yet our data also reveal that effect sizes can be quantified robustly within this heterogeneity. Highly‐replicated, local designs may therefore be useful for generating data that can serve to help refine wetland CH4 emissions uncertainties.

Ecological impacts of the industrial revolution in a lowland raised peat bog near Manchester, NW England.

(1) Ombrotrophic peat bogs provide valuable records of environmental change on long timescales but are rarely preserved near the major centers of industrial activity. Holcroft Moss is a rare example of a stratigraphically intact lowland peat bog in NW England, which provides a valuable opportunity to trace industrial impacts on vegetation in a sensitive environmental archive close to the early industrializing cities of Manchester and Liverpool. (2) We reconstructed environmental changes at Holcroft Moss before and after the Industrial Revolution using a decadal-scale record of pollen, non-pollen palynomorphs, microcharcoal, peat composition (organic content and ash-free bulk density) and heavy metal content, constrained by a radiocarbon and SCP (spheroidal carbonaceous particle) chronology. We examine the relationship between abiotic and biotic environmental tracers using principal component analysis and evaluate the role of local and regional climatic and anthropogenic drivers using canonical redundancy analysis and partitioning of variation. (3) Results show significant changes in bog vegetation composition during the last 700 years. Prior to 1750 CE, climate and agro-pastoral activity (grazing and fires) were the main drivers of vegetation change. Subsequently, regional coal-fired industry contributed to major increases in atmospheric pollutants (dust, heavy metals, and acid deposition) that severely impacted vegetation, driving the decline of Sphagnum. Grasses rose to dominance in the 20th century associated especially with bog conversion and cumulative nitrogen deposition. Although atmospheric pollution significantly decreased in the post-industrial era, vegetation has not returned to pre-industrial conditions, reflecting the ongoing impact of global change drivers which pose challenges for conservation and restoration. (4) Synthesis. Paleoecological studies are needed to reveal the long-term history of vegetation degradation and to offer guidelines for restoration and conservation practices. This study reconstructs the last 700 years of a peat bog located between Manchester and Liverpool, revealing the timing and nature of vegetation changes across the trajectory of early industrialization and eventual post-industrial decline. Our study reveals the progressive dominance of regional anthropogenic forcing and highlights that the present-day vegetation does not have past analogs within the last 700 years. Conservation measures favoring the reintroduction of Sphagnum are justified in redressing the major biological legacy of the Industrial Revolution, while steps to increase Calluna should also be considered in light of its resilience to dry and fire-prone conditions.

THE COMPLEMENTARY USE OF CHARCOAL NUMBER AND MORPHOLOGY TO RECONSTRUCT FIRE HISTORY IN A LATE HOLOCENE PEAT SEQUENCE FROM NW ROMANIA

Fires influence basic ecosystem processes, including the distribution, the structure and the composition of vegetation, as well as the carbon cycle and climate at the global scale. Long-term knowledge (i.e., centuries or millennia) of fire regime history and forest fire ecology and dynamic in local environments is necessary for sustainable ecosystem management and biodiversity conservation strategies. Charcoal is a widely used proxy for reconstructing fire regime history and vegetation burning. Charcoal analysis is based on the accumulation of charred particles in depositional environments such as lakes and bogs, during and shortly after the occurrence of fire events. Ombrotrophic peat bogs are sensitive to local environmental changes and, given that the deposition of allochthonous material is exclusively atmospheric, they are ideal archives for reconstructing charcoal fluxes resulting from biomass burning. This study quantifies charcoal abundance (number) in a peat sequence extracted from Tăul Mare ombrotrophic peat bog (Văratec Massif, Lăpuş Mts, north-western Romania) aiming to reconstruct local fire history over the last 3000 years and to explore potential drivers. As a novelty, observations concerning the morphology of charred fragments are added to provide additional information on the type of material burnt (woody vegetation or herbs), fire severity and charcoal source area, thus strengthening the interpretation of the charcoal record. Results showed moderate fire activity between 3000 and 2300 cal yr BP, which slightly decreased between 2300 and 700 cal yr BP and varied considerably throughout the last 700 years. Several major fire episodes were further identified around 2500, 1700, 1300, 900 and 500 cal yr BP. Of these, the first four overlapped two phases of progressive landscape opening and extension of pastoral activities, while the latter was unprecedented in the entire history of the bog and was likely associated with the development of mining activities in the area. This study provides the first complementary use of charcoal morphologies in fire history reconstruction in Romania and adds a valuable perspective on the extent of past human impact on the mid-elevation mountain landscape.

Exploring Dissolved Organic Carbon Variations in a High Elevation Tropical Peatland Ecosystem: Cerro de la Muerte, Costa Rica

Tropical peatlands are distributed mainly in coastal lowlands; however high elevation regions exhibit a large prevalence of small and fragmented peatlands that are mostly understudied. Artificial drainage of peatlands to expand the area of cattle farming, horticulture, and urbanization is increasing carbon losses to the atmosphere and streams worldwide. Here, we present an exploratory characterization of dissolved carbon optical properties in ombrotrophic peat bogs of the Talamanca range of Costa Rica, across an altitudinal gradient (2,400–3,100 m a.s.l.) during the rainy season. Dissolved organic matter (DOM) sources and decomposition processes were evaluated in the light of dissolved organic and inorganic carbon (DOC and DIC), optical properties, and major water chemistry. DOC concentrations ranged from 0.2 up to 47.0 mg/L. DIC concentrations were below 2 mg/L and δ13CDIC values indicated a mixture between soil organic matter, CO2 in soil water, and to a lesser degree DIC derived from bacterial CO2. Absolute fluorescence intensity of humic-like peaks was 6–7 times greater than fresh-like peaks across all sites. Fluorescence peak ratios coupled with the biological and humification indexes point to a greater relative contribution of recalcitrant soil-derived DOM. Excitation/Emission matrices denoted a high prevalence of humic and fulvic acids in the peat bogs, with moderate intensities in soluble microbial by-products-like and aromatic protein regions at three sites. Our data provides a baseline to underpin tropical carbon dynamics across high elevation peatlands.

Refining fire history in the Upper Rhine Graben region - Challenges and opportunities highlighted from a case study at Wildseemoor (northern Black Forest, Germany)

A 600 cm peat succession covering the last ca. 10,000 years from Wildseemoor, a perilacustrine ombrotrophic peat bog in the northern Black Forest, offers the opportunity to study Holocene environmental evolution and fire history in the Upper Rhine region. More particularly it is used here for a pilot study 1) dealing with the influence of potential peat growth disturbance on charcoal influx interpretation, and 2) proposing a new approach to disentangle regional from local influences on charcoal influx, based on the internal variability of charcoal particle size variations. The latter suggests a fluctuating local to regional main source of charcoal influx to Wildseemoor. Two plausible age-depth models were established based on seven identified radiocarbon-dated macrofossils, showing either reduced depositional rates or breaks in deposition during the early- to mid-Holocene and (less extensively) the Middle Ages, that are not visible to the naked eye in the sediment sequence. The Holocene Climatic Optimum, a time interval of particular interest with its climate conditions analogue to future projections, falls directly into an extensive period of reduced accretion or depositional hiatus. This severely impacts the interpretation of the charcoal record for that period but highlights potential impacts of climate variations on peat growth. Regardless which age-depth model is used, phases of high charcoal influx were identified after ca. 5,000 cal BP and can most likely be linked to anthropogenic pressure on the region. The findings of this study may be used in future Holocene fire dynamics reconstructions, as they stress the necessity for high-resolution age control, but also the opportunity to exploit periods of disturbed/halting peat growth as a proxy to detect past periods of climate change. Using the internal variability of charcoal particle size to estimate local versus regional charcoal transport tendency showed promising results and may help push forward charcoal record interpretations in the future.

Persistent, multi-sourced lead contamination in Central Europe since the Bronze Age recorded in the Füramoos peat bog, Germany

Despite a multitude of lead-contamination archives, the history of anthropogenic lead (Pb) contamination in Central Europe during the Holocene has remained incompletely understood. Contributing to a regionally differentiated understanding of Pb contamination in Central Europe since the Late Glacial (14.5 kyrs ago), this paper presents a record of atmospheric Pb deposition from an ombrotrophic peat bog at Füramoos, southern Germany. We combine Pb content and isotope data with pollen data allowing the interpretation of Pb signals within a context of natural change and human activities. The data show that Pb accumulation rates during the Late Glacial and early Holocene (0.074 mg*m-2*yr-1) were considerably higher than those during the mid-Holocene (0.016 mg*m-2*yr-1). Anthropogenic Pb contamination commenced at 3600 years BP and originates primarily from the Western Mediterranean region until the demise of the Roman Empire, after which contamination sources shifted to Britain, France, and Germany. For the past 3600 years, the data document an exponential increase in Pb contamination (max 5.6 mg*m-2*yr-1) that is unhinged from natural Pb deposition and far exceeds the natural Holocene background levels. The integrated geochemical and palynological approach allows placement of Pb contamination signals into detailed historical context. Land-use changes at Füramoos are consistent with the timing of political, economic, and social developments of medieval societies that modulated the ability and demand for mining during the past millennia.

Holocene paleoclimate inferred from stable isotope (\u03b418O and \u03b413C) values in Sphagnum cellulose, Mohos peat bog, Romania

We measured stable isotopes (δ18O and δ13C) in Sphagnum cellulose that was extracted from a long peat core drilled in the ombrotrophic Mohos peat bog, Ciomadul Mountain, Romania. The 10-m-long peat profile spans the period from 11,800 cal yr BP to present. The δ18O and δ13C data indicate there were several cooling events and warm periods in the area of the Mohos peat bog during the Holocene. The 8.2-ka cold event, however, was not detected using δ18O and δ13C values. Response of the peat bog to changing environmental conditions was inferred using data on organic matter accumulation, independent of the stable isotope results. All cool periods during the Holocene, whether of short or long duration, were identified as times of reduced organic matter accumulation rate. Similarly, dry periods were also correlated with reduced accumulation rates of organic matter.

Chronic atmospheric reactive N deposition has breached the N sink capacity of a northern ombrotrophic peatbog increasing the gaseous and fluvial N losses

Peatlands play an important role in modulating the climate, mainly through sequestration of carbon dioxide into peat carbon, which depends on the availability of reactive nitrogen (Nr) to mosses. Atmospheric Nr deposition in the UK has been above the critical load for functional and structural changes to peatland mosses, thus threatening to accelerate their succession by vascular plants and increasing the possibility of Nr export to downstream ecosystems. The N balance of peatlands has received comparatively little attention, mainly due to the difficulty in measuring gaseous N losses as well as the Nr inputs due to biological nitrogen fixation (BNF). In this study we have estimated the mean annual N balance of an ombrotrophic bog (Migneint, North Wales) by measuring in situ N2 + N2O gaseous fluxes and also BNF in peat and mosses. Fluvial N export was monitored through a continuous record of DON flux, while atmospheric N deposition was modelled on a 5 × 5 km grid. The mean annual N mass balance was slightly positive (0.7 ± 4.1 kg N ha−1 y−1) and varied interannually indicating the fragile status of this bog ecosystem that has reached N saturation and is prone to becoming a net N source. Gaseous N losses were a major N output term accounting for 70% of the N inputs, mainly in the form of the inert N2 gas, thus providing partial mitigation to the adverse effects of chronic Nr enrichment. BNF was suppressed by 69%, compared to rates in pristine bogs, but was still active, contributing ~2% of the N inputs. The long-term peat N storage rate (8.4 ± 0.8 kg N ha−1 y−1) cannot be met by the measured N mass balance, showing that the bog catchment is losing more N than it can store due its saturated status.

15N study of the reactivity of atmospheric nitrogen in four mountain forest catchments (Czech Republic, central Europe)

Reactivity of atmospherically deposited nitrate (NO3−) and ammonium (NH4+) was investigated in three mountain forest catchments and one ombrotrophic peat bog located in the northern Czech Republic. The study sites are characterized by moderate to high N pollution rates that are currently decreasing. Monitoring of hydrodynamic data and catchment hydrochemistry (precipitation, soil solutions and runoff) was complemented by δ15N analyses of dissolved inorganic N (NO3−, NH4+). Measured δ15N data were used to calculate the extent of biogeochemical reactions and to construct N mass balances. Two-component models of runoff generation were developed using hydrological and δ18O–H2O data on local precipitation, runoff, and soil waters collected by lysimeters. Catchment discharge was formed by groundwater (60–80%) and storm precipitation (40–20%), with minimal time lags. Groundwater had a diluting effect on reactions proceeding in the soil zone, including nitrification and denitrification. The rates of nitrification were estimated by comparing the δ15N values of atmospheric input and soil water, while the rates of denitrification were derived from the differences in the δ15N values of soil water and stream discharge. The Ν isotope effect of mineralisation of organic N was assessed by comparing δ15N values of soil and soil extracts. Nitrogen release from the catchments was controlled by temperature-dependent seasonality of soil reactions. Atmospheric N entered the runoff directly only during a short late winter - early spring period, accounting for approximately 10% of the total N input. Important inputs of mineralised organically cycled soil N were observed at the beginning and the end of the growing season, with measurable denitrification occurring during the same time periods. The measured apparent N isotope fractionations were significantly lower than previously published values (−3 to −14‰ vs. −30‰ for nitrification, and −3 vs. −20‰ for denitrification). Lower 15N fractionations originated both from the dilution effect of groundwater on stream discharge and from the depletion of available ammonium during nitrification reactions. Denitrification proceeding during recharge of the groundwater body was estimated from the difference in the δ15N values of NO3− in precipitation and groundwater.

Mercury dynamics in the pore water of peat columns during experimental freezing and thawing

Biogeochemical processes in northern peatland ecosystems are influenced by seasonal temperature fluctuations that are changing with the climate. Methylmercury (MeHg), commonly produced in peatlands, affects downstream waters; therefore, it is important to understand how temperature transitions affect mercury (Hg) dynamics. We investigated how the freeze-thaw cycle influences belowground peat pore water total Hg (THg), MeHg, and dissolved organic carbon (DOC). Four large, intact peat columns were removed from an ombrotrophic peat bog and experimentally frozen and thawed. Pore water was sampled across seven depths in the peat columns during the freeze-thaw cycle and analyzed for THg, MeHg, and DOC concentrations. Freezing results showed increased concentrations of THg below the ice layers and limited change in MeHg concentrations. During thawing, THg concentrations significantly increased, whereas MeHg concentrations decreased. Limited bromide movement and depth decreases in THg and DOC concentrations were associated with increased bulk density and degree of humification in the peat. The experiment demonstrates the effects of the freeze-thaw cycle on Hg concentrations in northern peatlands. Changes to freeze-thaw cycles with climate change may exacerbate Hg cycling and transport processes in peatland environments.

Early to mid-Holocene hydroclimate trends in the western Carpathians of Romania

In this study, we analyse a sequence from Ic Ponor ombrotrophic peat bog in the Apuseni Mountains of NW Romania using a multi-proxy approach (testate amoebae, pollen, microcharcoal, loss on ignition, magnetic susceptibility, radiocarbon dating, lithology) to reconstruct the palaeoenvironmental conditions and the hydroclimate trends that occurred between 12,700 and 6220 cal yr BP. The sequence evolved from a lacustrine phase (between 12,700 cal yr BP and 12,000 cal yr BP) into an ombrotrophic peat bog (12,000–7600 cal yr BP), accumulated in drier climate conditions, with a subdued water table. Subsequently, wetter conditions occurred, with an elevated water table until 6220 cal yr BP, when the peat bog was flooded and peat accumulation paused, a depositional hiatus being marked by a clayey layer. Peat started to accumulate again ca. 100 years ago, under drier conditions. The peat bog records the local and regional responses to rapid climate change events that occurred at 11,400–11,250, 10,500–10,200, 9400–9200, and 8300–8250 cal yr BP. Our quantitative reconstruction of hydrological changes, as well as pre-existing ones for this region, show similar conditions to those that occurred in Central-Eastern Europe between 12,700 and 7600 cal yr BP, but differ from those that occurred between 7600 and 6220 cal yr BP, suggesting a distinct regional character of Middle Holocene climate conditions in Romania.

Environmental factors determining distribution and activity of anammox bacteria in minerotrophic fen soils.

In contrast to the pervasive occurrence of denitrification in soils, anammox (anaerobic ammonium oxidation) is a spatially restricted process that depends on specific ecological conditions. To identify the factors that constrain the distribution and activity of anammox bacteria in terrestrial environments, we investigated four different soil types along a catena with opposing ecological gradients of nitrogen and water content, from an amended pasture to an ombrotrophic bog. Anammox was detected by polymerase chain reaction (PCR) and quantitative PCR (qPCR) only in the nitrophilic wet meadow and the minerotrophic fen, in soil sections remaining water-saturated for most of the year and whose interstitial water contained inorganic nitrogen. Contrastingly, aerobic ammonia oxidizing microorganisms were present in all examined samples and outnumbered anammox bacteria usually by at least one order of magnitude. 16S rRNA gene sequencing revealed a relatively high diversity of anammox bacteria with one Ca. Brocadia cluster. Three additional clusters could not be affiliated to known anammox genera, but have been previously detected in other soil systems. Soil incubations using 15N-labeled substrates revealed that anammox processes contributed about <2% to total N2 formation, leaving nitrification and denitrification as the dominant N-removal mechanism in these soils that represent important buffer zones between agricultural land and ombrotrophic peat bogs.

Eddy covariance based surface‐atmosphere exchange and crop coefficient determination in a mountainous peatland

AbstractIn the soil–plant–atmosphere continuum, fluxes of water, energy, and carbon determine the water and carbon balance of peat bogs. We used eddy covariance (EC) measurements to study surface atmosphere exchange and its drivers above an ombrotrophic peat bog (Odersprungmoor) in the Harz Mountains, Germany, with nonideal measurement conditions during the growing season in 2013. For montane peatlands, only very few EC flux measurements exist due to site constraints, for example, surface slope, limited fetch, and frequent dew formation on open path sensors. The measured data were carefully filtered resulting in valid and representative fluxes for the bog. The evapotranspiration (ET) was further characterized by determining the adjusted crop coefficient (Kc*) for July and August and comparing it with Kc* values from 7 years of the FLUXNET site Mer Bleue bog, Ontario, Canada. While soil moisture was taken into consideration, the adjustment was nevertheless necessary as plant health and nutrient supply were not evaluated as required by FAO guidelines. Actual ET at OM was well described by the Kc* model (Kc* = 0.85, R2 = 0.85). The primary control on ET was available energy and atmospheric conditions and, marginally, the soil moisture conditions. This Kc* value is comparable to the calculated Kc* values for MB, which ranged between 0.82 and 0.86 (R2 between 0.84 and 0.97). Since these Kc* ranges are narrow for the different sites and years, we hypothesize that these values are good estimates for the true crop coefficients of Sphagnum‐dominated peat bogs.

Robust Inverse Modeling of Growing Season Net Ecosystem Exchange in a Mountainous Peatland: Influence of Distributional Assumptions on Estimated Parameters and Total Carbon Fluxes

AbstractWhile boreal lowland bogs have been extensively studied using the eddy‐covariance (EC) technique, less knowledge exists on mountainous peatlands. Hence, half‐hourly CO2 fluxes of an ombrotrophic peat bog in the Harz Mountains, Germany, were measured with the EC technique during a growing season with exceptionally dry weather spells. A common biophysical process model for net ecosystem exchange was used to describe measured CO2 fluxes and to fill data gaps. Model parameters and uncertainties were estimated by robust inverse modelling in a Bayesian framework using a population‐based Markov Chain Monte Carlo sampler. The focus of this study was on the correct statistical description of error, i.e. the differences between the measured and simulated carbon fluxes, and the influence of distributional assumptions on parameter estimates, cumulative carbon fluxes, and uncertainties. We tested the Gaussian, Laplace, and Student's t distribution as error models. The t‐distribution was identified as best error model by the deviance information criterion. Its use led to markedly different parameter estimates, a reduction of parameter uncertainty by about 40%, and, most importantly, to a 5% higher estimated cumulative CO2 uptake as compared to the commonly assumed Gaussian error distribution. As open‐path measurement systems have larger measurement error at high humidity, the standard deviation of the error was modeled as a function of measured vapor pressure deficit. Overall, this paper demonstrates the importance of critically assessing the influence of distributional assumptions on estimated model parameters and cumulative carbon fluxes between the land surface and the atmosphere.

High-resolution age modelling of peat bogs from northern Alberta, Canada, using pre- and post-bomb 14C, 210Pb and historical cryptotephra

Abstract High-resolution studies of peat profiles are frequently undertaken to investigate natural and anthropogenic disturbances over time. However, overlapping profiles of the most commonly applied age-dating techniques, including 14C and 210Pb, often show significant offsets (>decadal) and biases that can be difficult to resolve. Here we investigate variations in the chronometers and individual site histories from six ombrotrophic peat bogs in central and northern Alberta. Dates produced using pre- and post-bomb 14C, 210Pb (corroborated with 137Cs and 241Am), and cryptotephra peaks, are compared and then integrated using OxCal's P_Sequence function to produce a single Bayesian age model. Environmental histories for each site obtained using physical and chemical characteristics of the peat cores, e.g. plant macrofossils, humification, ash content and dry density, provide important constraints for the models by highlighting periods with significant changes in accumulation rate, e.g. fire events, permafrost development, and prolonged surficial drying. Despite variable environmental histories, it is possible to produce high-resolution age-depth models for each core sequence. Consistent offsets between 14C and 210Pb dates pre-1960s are seen at five of the six sites, but tephra-corrected 210Pb data can be used to produce more coherent models at three of these sites. Processes such as permafrost development and thaw, surficial drying and local fires can disrupt the normal processes by which chronological markers and environmental records are incorporated in the peat record. In consequence, applying standard dating methodologies to these records will result in even greater uncertainties and discrepancies between the different dating tools. These results show that using any single method to accurately date peat profiles where accumulation has not been uniform over time may be unreliable, but a comprehensive multi-method investigation paired with the application of Bayesian statistics can produce more robust chronologies. New cryptotephra data for the Alberta region are also reported here, including the historical Novarupta-Katmai 1912 eruption, White River Ash (East), and glass from Mt. St. Helens, Mt. Churchill, and probable Aleutian sources.