Search

The tropical peat swamp forests of South‐East Asia are being rapidly converted to agricultural plantations of oil palm and Acacia creating a significant global "hot‐spot" for CO2 emissions. However, the effect of this major perturbation has yet to be quantified in terms of global warming potential (GWP) and the Earth's radiative budget. We used a GWP analysis and an impulse‐response model of radiative forcing to quantify the climate forcing of this shift from a long‐term carbon sink to a net source of greenhouse gases (CO2 and CH4). In the GWP analysis, five tropical peatlands were sinks in terms of their CO2 equivalent fluxes while they remained undisturbed. However, their drainage and conversion to oil palm and Acacia plantations produced a dramatic shift to very strong net CO2‐equivalent sources. The induced losses of peat carbon are ~20× greater than the natural CO2 sequestration rates. In contrast, a radiative forcing model indicates that the magnitude of this shift from a net cooling to warming effect is ultimately related to the size of an individual peatland's carbon pool. The continuous accumulation of carbon in pristine tropical peatlands produced a progressively negative radiative forcing (i.e., cooling) that ranged from −2.1 to −6.7 nW/m2 per hectare peatland by 2010 CE, referenced to zero at the time of peat initiation. Peatland conversion to plantations leads to an immediate shift from negative to positive trend in radiative forcing (i.e., warming). If drainage persists, peak warming ranges from +3.3 to +8.7 nW/m2 per hectare of drained peatland. More importantly, this net warming impact on the Earth's radiation budget will persist for centuries to millennia after all the peat has been oxidized to CO2. This previously unreported and undesirable impact on the Earth's radiative balance provides a scientific rationale for conserving tropical peatlands in their pristine state. Tropical peatlands in South‐East Asia store large amounts of carbon in their peat soils. Widespread conversion of tropical peatlands to agricultural oil palm and pulp plantations entails draining water from the peat soils. These drier soil conditions lead to peat carbon being rapidly released to the atmosphere as carbon dioxide. We calculated the effect of these significant greenhouse gas emissions on the global climate and found that they will contribute to climate warming for several centuries. Reducing drainage of tropical peatlands and restoring drained peatlands will reduce this climate warming impact. [ABSTRACT FROM AUTHOR]

Northern peatlands are an important source for greenhouse gases, but their capacity to produce methane remains uncertain under changing climatic conditions. We therefore analyzed a 43 year time series of the pore-water chemistry to determine if long-term shifts in precipitation altered the vertical transport of solutes within a large peat basin in northern Minnesota. These data suggest that rates of methane production can be finely tuned to multidecadal shifts in precipitation that drive the vertical penetration of labile carbon substrates within the Glacial Lake Agassiz Peatlands. Tritium and cation profiles demonstrate that only the upper meter of these peat deposits was flushed by downwardly moving recharge from 1965 to 1983 during a Transitional Dry-to-Moist Period. However, a shift to a moister climate after 1984 drove surface waters much deeper, largely flushing the pore waters of all bogs and fens to depths of 2 m. Labile carbon compounds were transported downward from the rhizosphere to the basal peat at this time producing a substantial enrichment of methane in Δ14C with respect to the solid-phase peat from 1991 to 2008. These data indicate that labile carbon substrates can fuel deep production zones of methanogenesis that more than doubled in thickness across this large peat basin after 1984. Moreover, the entire peat profile apparently has the capacity to produce methane from labile carbon substrates depending on climate-driven modes of solute transport. Future changes in precipitation may therefore play a central role in determining the source strength of peatlands in the global methane cycle. [ABSTRACT FROM AUTHOR]

In this study, we contrast the fluorescent properties of dissolved organic matter ( DOM) in fens and bogs in a Northern Minnesota peatland using excitation emission matrix fluorescence spectroscopy with parallel factor analysis ( EEM- PARAFAC). EEM- PARAFAC identified four humic-like components and one protein-like component and the dynamics of each were evaluated based on their distribution with depth as well as across sites differing in hydrology and major biological species. The PARAFAC- EEM experiments were supported by dissolved organic carbon measurements ( DOC), optical spectroscopy ( UV-Vis), and compositional characterization by ultrahigh resolution Fourier transform ion cyclotron resonance mass spectroscopy ( FT- ICR MS). The FT- ICR MS data indicate that metabolism in peatlands reduces the molecular weights of individual components of DOM, and oxygen-rich less aromatic molecules are selectively biodegraded. Our data suggest that different hydrologic and biological conditions within the larger peat ecosystem drive molecular changes in DOM, resulting in distinctly different chemical compositions and unique fluorescent fingerprints. PARAFAC modeling of EEM data coupled with ultrahigh resolution FT- ICR MS has the potential to provide significant molecular-based information on DOM composition that will support efforts to better understand the composition, sources, and diagenetic status of DOM from different terrestrial and aquatic systems. [ABSTRACT FROM AUTHOR]

Ground water recharge is assumed to occur primarily at raised bog crests in northern peatlands, which are globally significant terrestrial carbon reservoirs. We synoptically surveyed vertical profiles of peat pore water δ18O and δ2H from a range of bog and fen landforms across the Glacial Lake Agassiz Peatlands, northern Minnesota. Contrary to our expectations, we find that local-scale recharge penetrates to not only the basal peat at topographically high bog crests but also transitional Sphagnum lawns and low-lying fen water tracks. Surface landscape characteristics appear to control the isotopic composition of the deeper pore waters (depths ≥0.5 m), which are partitioned into discrete ranges of δ18O on the basis of landform type (mean ± standard deviation for bog crests = −11.9 ± 0.4‰, lawns = −10.6 ± 0.1‰, fen water tracks = −8.8 ± 1.0‰). Fen water tracks have a shallow free-water surface that is seasonally enriched by isotope fractionating evaporation, fingerprinting recharge to underlying pore waters at depths ≥3 m. Isotope mass balance calculations indicate on average 12% of the waters we sampled from the basal peat of the fen water tracks was lost to surface evaporation, which occurred prior to advection and dispersion into the underlying formation. These new data provide direct support for the hypothesis that methane production in deeper peat strata is fuelled by the downward transport of labile carbon substrates from the surface of northern peat basins. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Multiple analyses of dissolved organic carbon (DOC) from pore waters were conducted to define the processes that govern carbon balance in peatlands: (1) source, reactivity, and transport of DOC with respect to vegetation, peat, and age of carbon substrate, (2) reactivity of DOC with respect to molecular size, and (3) lability to photoxidation of surficial DOC. We found that surface organic production fuels heterotrophic respiration at depth in advection-dominated peatlands, especially in fens. Fen DOC was Δ14C enriched relative to the surrounding fen peat, and fen respiration products were similar to this enriched DOC indicating that DOC was the main microbial substrate. Bog DOC was more variable showing either enrichment in ∆14C at depth or ∆14C values that follow peat values. This variability in bogs is probably controlled by the relative importance of vertical transport of labile carbon substrates within the peat profile versus DOC production from bog peat. These results extended our set of observations to 10 years at one bog-fen pair and add two additional bog-fen pairs to our series of observations. Anaerobic incubations of peat, rinsed free of residual DOC, produced DOC and respiration products that were strikingly similar to the peat values in a bog and two fens. This result demonstrated conclusively that downward advection is the process responsible for the presence of modern DOC found at depth in the peat column. Fen DOC has lower C/N values and up to twice as much LMW (<1 kDa) DOC as bogs due to differences in organic inputs and greater microbial processing. Fluorescence irradiation experiments showed that fen DOC is more photolabile than bog DOC. [ABSTRACT FROM AUTHOR]

We report the results of an investigation on the processes controlling heat transport in peat under a large bog in the Glacial Lake Agassiz Peatlands. For 2 years, starting in July 1998, we recorded temperature at 12 depth intervals from 0 to 400 cm within a vertical peat profile at the crest of the bog at sub-daily intervals. We also recorded air temperature 1 m above the peat surface. We calculate a peat thermal conductivity of 0.5 W m -1 °C-1 and model vertical heat transport through the peat using the SUTRA model. The model was calibrated to the first year of data, and then evaluated against the second year of collected heat data. The model results suggest that advective pore-water flow is not necessary to transport heat within the peat profile and most of the heat is transferred by thermal conduction alone in these waterlogged soils. In the spring season, a zero-curtain effect controls the transport of heat through shallow depths of the peat. Changes in local climate and the resulting changes in thermal transport still may cause non-linear feedbacks in methane emissions related to the generation of methane deeper within the peat profile as regional temperatures increase. [ABSTRACT FROM AUTHOR]

Recent research indicates that accumulation and release of biogenic gas from northern peatlands may substantially affect future climate. Sudden release of free-phase gas bubbles into the atmosphere may preclude the conversion of methane to carbon dioxide in the uppermost oxic layer of the peat, resulting in greater contribution of methane to the atmosphere than is currently estimated. The hydrology of these peatlands also affects and is affected by this process, especially when gas is released suddenly and episodically. Indirect hydrological evidence indicates that ebullitive gas releases are relatively frequent in some peatlands and time-averaged rates may be significantly greater than diffusive releases. Estimates of free-phase gas contained in peat have ranged from 0 to nearly 20% of the peat volume. Abrupt changes in the volume of gas may alter hydraulic gradients and movement of water and solutes in peat, which in turn could alter composition and fluxes of the gas. Peat surfaces also move vertically and horizontally in response to accumulation and release of free-phase gas. Future research should address the distribution, temporal variability, and relative significance of ebullition in peatlands and the consequent hydrological responses to these gas-emission events. [ABSTRACT FROM AUTHOR]

Hydraulic head was overpressured at middepth in a 4.2-m thick raised bog in the Glacial Lake Agassiz peatlands of northern Minnesota, and fluctuated in response to atmospheric pressure. Barometric efficiency (BE), determined by calculating ratios of change in hydraulic head to change in atmospheric pressure, ranged from 0.05 to 0.15 during July through November of both 1997 and 1998. The overpressuring and a BE response were caused by free-phase gas contained primarily in the center of the peat column between two or more semielastic, semiconfining layers of more competent peat. Two methods were used to determine the volume of gas bubbles contained in the peat, one using the degree of overpressuring in the middepth of the peat, and the other relating BE to specific yield of the shallow peat. The volume of gas calculated from the overpressuring method averaged 9%, assuming that the gas was distributed over a 2-m thick overpressured interval. The volume of gas using the BE method averaged 13%. Temporal changes in overpressuring and in BE indicate that the volume of gaseous-phase gas also changed with time, most likely because of rapid degassing (ebullition) that allowed sudden loss of gas to the atmosphere. Estimates of gas released during the largest ebullition events ranged from 0.3 to 0.7 mol m−2 d−1. These ebullition events may contribute a significant source of methane and carbon dioxide to the atmosphere that has so far largely gone unmeasured by gas-flux chambers or tower-mounted sensors. [ABSTRACT FROM AUTHOR]

Raised bogs represent end-products of peatland development that are generally assumed to be decoupled from the regional groundwater system. However, the development of two peat mounds in northern Minnesota was driven by three major reversals in its groundwater-flow regime. A 4000-year record of groundwater flow was reconstructed by stratigraphic studies that utilized both botanical indicators of the surface-water chemistry, and mineral indicators of the pore-water chemistry and chemical weathering regime. The mineral stratigraphy indicated that the peat mounds were initially formed under a discharge (upward flow) regime. During the succeeding moist period, these peat mounds developed into raised bogs as their water-table mounds drove local recharge (downward) flow into the mineral substratum. A major reversal to a prevailing discharge regime at 1200 BP, however, converted the smaller peat mound into a spring fen and weathered the silicate minerals within the peat profile of the larger mound. This reversal was apparently caused by the onset of extended droughts, which dissipate the water table mound within these peat landforms. [ABSTRACT FROM PUBLISHER]