Long-term impacts of drier conditions on the hydrology of northern peatlands are poorly understood. We used long-term drainage near a historic drainage ditch, separating an area from the main ...peatland, as an analogue for long-term drying in a northern temperate bog. The objective was to identify the impact of drier conditions on ecohydrological processes and groundwater flow patterns in an area now forested and an area that maintained a bog-like character. Groundwater flow patterns alternated between mostly downward flow and occasionally upward flow in the bog area and were mostly upward-orientated in the forested area, which suggested that there the flow pattern had shifted from bog- to fen-like conditions. Flow patterns were in agreement with changes in post-drainage hydraulic conductivities, storage capacity of the peat and water table levels. Compared to the bog, hydraulic conductivities in the forested area were one to three orders of magnitude lower in the uppermost 0.75 m of peat (paired t test, p < 0.05). Bulk density had increased and the water table level was lower and more strongly fluctuating in the forested area. Our findings suggest hydraulic gradients and flow patterns have changed due to increased evapotranspiration and interception with the emergence of a tree cover. The smaller size of the now-forested area relative to the remaining bog area appeared to be important for the hydrological change. With the main Mer Bleue bog as hinterland, enhanced runoff to the drainage channel had little effect on hydrologic and vegetation patterns. In the cut-off, smaller, now forested area pervasive changes in vegetation and hydrologic processes occurred. The difference in response to local drainage raises questions about tipping points with respect to the impact of drying on peatland ecosystems that need to be addressed in future research.
We present data on a transitional mire in South-Western Siberia that evolved from early thermokarst lake succession. The vegetation of the mire shows a remarkable zonation from the edges to the ...center. Vegetation, peat characteristics, pH and electric conductivity were recorded at 10 sites along a transect of 1.5 km. At two of the transect points with contrasting vegetation and succession stage (floating mat vs. birch forest) pore water peepers were inserted once for 3 weeks and pore waters of the upper 60 cm were analyzed for major anions and cations, and dissolved CO
2
and CH
4
concentrations. Pore waters substantially differed between the floating mat and the birch forest regarding base cation chemistry and pH whilst nutrient availability was comparable. Compared to literature, depth integrated productions (DIPs) of CH
4
and CO
2
were high for both sites but three times higher for the floating mat (CH
4
10.89 mmol m
−2
d
−1
, CO
2
34.19 mmol m
−2
d
−1
). Along with other reasons, the higher DOC input at this location seems to be responsible for the higher DIP by fueling higher microbial activity. We discuss driving factors for biogeochemical differences between both sites and draw conclusions on CH
4
production during mire evolution.
The arsenic contamination of aquifers has been linked to the input of dissolved organic matter (DOM). In light of this suggestion, the aim of this study was to quantify chemical effects of DOM on ...desorption and redox transformations of arsenic bound to synthetic iron oxide and natural samples from different geochemical environments (soils, shallow aquifer, lake sediment). In batch experiments, solutions containing 25–50 mg/L of two different types of DOM (purified peat humic acid and DOM from a peat drainage) were used as extractants in comparison to inorganic solutions. DOM solution was able to mobilize arsenic from all solid phases. Mobilization from iron oxides (maximum: 53.3%) was larger than from natural samples (maximum: 2.9%). The mobilization effect of extractants decreased in the order HCl > NaH
2PO
4 > DOM > NaNO
3. DOM solutions, therefore, mainly targeted weakly sorbed arsenic. Mobilization was complete within 24–36 h and DOM was sorbed during incubation indicating competition for sorption sites. The same patterns were observed for different DOM types and concentrations. Addition of DOM lead to (a) enhanced reduction (maximum 7.8%) and oxidation (6.4%) of arsenic in aqueous solution and (b) the appearance of arsenite in aqueous phase of soil samples (5.5%). As the primary mechanism for the arsenic release from solid phases we identified the competition between arsenic and organic anions for sorption sites, whereas redox reactions were probably of minor importance. The results of this study demonstrate that sorption of DOM has a strong potential to mobilize arsenic from soils and sediments.
Covering only 3% of the land area, northern peatlands store about 30% of the global soil carbon and account for 5 to 10% of the global methane burden to the atmosphere. A review of the literature on ...net ecosystem exchange, net primary productivity, carbon mineralization, methane emissions, and dissolved organic carbon dynamics indicates that peatlands can be both C sources and sinks. The temporal and spatial variability of fluxes is large, but a substantial portion of this variation can be explained by environmental and ecological variables. Uncertainty in predictions about carbon dynamics under changing environmental conditions arises from a number of knowledge gaps: (i) the understanding of how organic matter is mineralized and partitioned into carbon dioxide, methane, and dissolved organic carbon is insufficient; (ii) little is known about the consequences of longterm and short-term disturbances, such as elevated carbon dioxide concentrations, nitrogen and sulfur deposition, fire, and droughts, on the individual components of the carbon cycle; (iii) models that capture the dynamic interaction of the processes and their controls have not been developed yet, with the notable exception of methane dynamics.
Elevated nitrogen (N) deposition changes the retention, transformation, and fluxes of N in ombrotrophic peatlands. To evaluate such effects we applied a .sup.15 N tracer (NH.sub.4 â¯.sup.15 ...NO.sub.3) at a rate of 2.3â¯gâ¯Nâ¯m.sup.-2 â¯yr.sup.-1 to mesocosms of five European peatlands with differing long-term N deposition rates for a period of 76 days of dry and 90 days of wet conditions. We determined background N content and moss length growth, and recovered the .sup.15 N tracer from the mosses, graminoids, shrubs, the peat, and dissolved N. Background N contents in Sphagnum mosses increased from 5.5 (Degerö Stormyr, deposition < 0.2â¯gâ¯Nâ¯m.sup.-2 â¯yr.sup.-1) up to 12.2â¯mgâ¯g.sup.-1 (Frölichshaier Sattelmoor, 4.7-6.0â¯gâ¯Nâ¯m.sup.-2 â¯yr.sup.-1). In peat from Degerö, nitrate and ammonium concentrations were below 3â¯mgâ¯L.sup.-1, whereas up to 30 (nitrate) and 11â¯mgâ¯L.sup.-1 (ammonium) was found in peat from Frölichshaier Sattelmoor. Sphagnum mosses (down to 5â¯cm below surface) generally intercepted large amounts of .sup.15 N (0.2-0.35 mgâ¯g.sup.-1) and retained the tracer most effectively relative to their biomass. Similar quantities of the .sup.15 N were recovered from the peat, followed by shrubs, graminoids, and the dissolved pool. At the most polluted sites we recovered more .sup.15 N from shrubs (up to 12.4â¯%) and from nitrate and ammonium (up to 0.7â¯%). However, no impact of N deposition on .sup.15 N retention by Sphagnum could be identified and their length growth was highest under high N background deposition. Our experiment suggests that the decline in N retention at levels above ca. 1.5â¯gâ¯m.sup.-2 â¯yr.sup.-1, as expressed by elevated near-surface peat N content and increased dissolved N concentrations, is likely more modest than previously thought. This conclusion is related to the finding that Sphagnum species can apparently thrive at elevated long-term N deposition rates in European peatlands.
Ponds smaller than 10 000 m2 likely account for about one-third of the global lake perimeter. The release of methane (CH4) and carbon dioxide (CO2) from these ponds is often high and significant on ...the landscape scale. We measured CO2 and CH4 fluxes in a temperate peatland in southern Ontario, Canada, in summer 2014 along a transect from the open water of a small pond (847 m2) towards the surrounding floating mat (5993 m2) and in a peatland reference area. We used a high-frequency closed chamber technique and distinguished between diffusive and ebullitive CH4 fluxes. CH4 fluxes and CH4 bubble frequency increased from a median of 0.14 (0.00 to 0.43) mmol m−2 h−1 and 4 events m−2 h−1 on the open water to a median of 0.80 (0.20 to 14.97) mmol m−2 h−1 and 168 events m−2 h−1 on the floating mat. The mat was a summer hot spot of CH4 emissions. Fluxes were 1 order of magnitude higher than at an adjacent peatland site. During daytime the pond was a net source of CO2 equivalents to the atmosphere amounting to 0.13 (−0.02 to 1.06) g CO2 equivalents m−2 h−1, whereas the adjacent peatland site acted as a sink of −0.78 (−1.54 to 0.29) g CO2 equivalents m−2 h−1. The photosynthetic CO2 uptake on the floating mat did not counterbalance the high CH4 emissions, which turned the floating mat into a strong net source of 0.21 (−0.11 to 2.12) g CO2 equivalents m−2 h−1. This study highlights the large small-scale variability of CH4 fluxes and CH4 bubble frequency at the peatland–pond interface and the importance of the often large ecotone areas surrounding small ponds as a source of greenhouse gases to the atmosphere.
Peatlands are significant global methane (CH4) sources, but processes governing CH4 dynamics have been predominantly studied in the Northern Hemisphere. Southern hemispheric and tropical bogs can be ...dominated by cushion-forming vascular plants (e.g. Astelia pumila, Donatia fascicularis). These cushion bogs are found in many (mostly southern) parts of the world but could also serve as extreme examples for densely rooted northern hemispheric bogs dominated by rushes and sedges. We report highly variable summer CH4 emissions from different microforms in a Patagonian cushion bog as determined by chamber measurements. Driving biogeochemical processes were identified from pore water profiles and carbon isotopic signatures. Intensive root activity throughout a rhizosphere stretching over 2 m in depth accompanied by molecular oxygen release created aerobic microsites in water-saturated peat, leading to a thorough CH4 oxidation (< 0.003 mmol L−1 pore water CH4, enriched in δ13C-CH4 by up to 10 ‰) and negligible emissions (0.09±0.16 mmol CH4 m−2 d−1) from Astelia lawns. In sparsely or even non-rooted peat below adjacent pools pore water profile patterns similar to those obtained under Astelia lawns, which emitted very small amounts of CH4 (0.23±0.25 mmol m−2 d−1), were found. Below the A. pumila rhizosphere pore water concentrations increased sharply to 0.40±0.25 mmol CH4 L−1 and CH4 was predominantly produced by hydrogenotrophic methanogenesis. A few Sphagnum lawns and – surprisingly – one lawn dominated by cushion-forming D. fascicularis were found to be local CH4 emission hotspots with up to 1.52±1.10 mmol CH4 m−2 d−1 presumably as root density and molecular oxygen release dropped below a certain threshold. The spatial distribution of root characteristics supposedly causing such a pronounced CH4 emission pattern was evaluated on a conceptual level aiming to exemplify scenarios in densely rooted bogs. We conclude that presence of cushion vegetation as a proxy for negligible CH4 emissions from cushion bogs needs to be interpreted with caution. Nevertheless, overall ecosystem CH4 emissions at our study site were probably minute compared to bog ecosystems worldwide and widely decoupled from environmental controls due to intensive root activity of A. pumila, for example.
A combination of rate measurements of iron(III)oxide and sulfate reduction, thermodynamic data, and pore-water and solid phase analyses was used to evaluate the relative significance of iron and ...sulfate reduction in the sediments of an acidic strip mining lake (Lake 116, Brandenburg, Germany). The rate of sulfate reduction was determined using a 35S-radiotracer method. Rates of iron turnover were quantified by mass balances based on pore-water concentration profiles. The differences in Gibbs free energy yield from reduction of iron and sulfate and from methanogenesis were calculated from individual redox couples and concentrations of reactants to account for the influence of high Fe2+ concentrations and differing mineral phases. Integrated (O-20 cm) mean rates of sulfate reduction were 1.2 (pelagial), respectively 5.2 (littoral) mmol (m2d)-1. Based on electron equivalents, the estimated iron reduction rates reached between about 50 % (pelagial) and 75 % (littoral) of the sulfate reduction rates. Compared to conditions usually assumed in the literature, in the sediments Gibbs free energy advantage of iron reduction over sulfate reduction was reduced frmm +11 KJeq-1 to a range of about +7 KJeq-1 (ferrihydrite, "reactive iron") to -6 KJeq-1 (goethite). This indicates that iron reduction was thermodynamically favored to sulfate reduction only if amorphous iron(III)oxides were available and is in accordance to the high competitiveness of sulfate reducers in the sediment. While total iron concentration in the sediments was high (up to 80% of the dryweight), reactive iron only accounted for 11-38% and was absolutely and relatively diminished in the zone of iron reduction. Pore-water concentration gradients and 137CS profiles indicated that little or no bioturbation occurred in the sediments, probably inhibiting the renewal of reactive iron. We further hypothesize that the reactivity of the iron oxide surfaces was reduced due to adsorption of DOM, suggested by IR spectra of the DOM and by a surface coverage estimate using literature data. Pelagial and littoral sediments displayed different dynamics. At the littoral relative iron reduction rate estimates were higher, iron sulfides were not accumulated and residence times of iron oxides were short compared to the pelagial. At the littoral site reoxidation of iron sulfides probably resulted in the renewal of reactive iron(III)oxides, possibly allowing for higher relative rates of iron reduction.PUBLICATION ABSTRACT
Changes of water table position influence carbon cycling in peatlands, but effects on the sources and sinks of carbon are difficult to isolate and quantify in field investigations due to seasonal ...dynamics and covariance of variables. We thus investigated carbon fluxes and dissolved carbon production in peatland mesocosms from two acidic and oligotrophic peatlands under steady state conditions at two different water table positions. Exchange rates and CO2, CH4 and DOC production rates were simultaneously determined in the peat from diffusive-advective mass-balances of dissolved CO2, CH4 and DOC in the pore water. Incubation experiments were used to quantify potential CO2, CH4, and DOC production rates. The carbon turnover in the saturated peat was dominated by the production of DOC (10-15 mmol m-2 d-1) with lower rates of DIC (6.1-8.5 mmol m-2 d-1) and CH4 (2.2-4.2 mmol m-2 d-1) production. All production rates strongly decreased with depth indicating the importance of fresh plant tissue for dissolved C release. A lower water table decreased area based rates of photosynthesis (24-42%), CH4 production (factor 2.5-3.5) and emission, increased rates of soil respiration and microbial biomass C, and did not change DOC release. Due to the changes in process rates the C net balance of the mesocosms shifted by 36 mmol m-2 d-1. According to our estimates the change in C mineralization contributed most to this change. Anaerobic rates of CO2 production rates deeper in the peat increased significantly by a factor of 2-3.5 (DOC), 2.9-3.9 (CO2), and 3-14 (CH4) when the water table was lowered by 30 cm. This phenomenon might have been caused by easing an inhibiting effect by the accumulation of CO2 and CH4 when the water table was at the moss surface.
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