ALBERT

Description: Permafrost-affected tundra soils are significant sources of the climate-relevant trace gas methane (CH4). The observed accelerated warming of the Arctic will cause a deeper permafrost thawing followed by increased carbon mineralization and CH4 formation in water saturated tundra soils which might cause a positive feedback to climate change. Aerobic CH4 oxidation is regarded as the key process reducing CH4 emissions from wetlands, but quantification of turnover rates has remained difficult so far. The application of carbon stable isotope fractionation enables the in situ quantification of CH4 oxidation efficiency in arctic wetland soils. The aim of the current study is to quantify CH4 oxidation efficiency in permafrost-affected tundra soils in Russia's Lena River Delta based on stable isotope signatures of CH4. Therefore, depth profiles of CH4 concentrations and δ13CH4-signatures were measured and the fractionation factors for the processes of oxidation (αox) and diffusion (αdiff) were determined. Most previous studies employing stable isotope fractionation for the quantification of CH4 oxidation in soils of other habitats (e.g. landfill cover soils) have assumed a gas transport dominated by advection (αtrans = 1). In tundra soils, however, diffusion is the main gas transport mechanism, aside from ebullition. Hence, diffusive stable isotope fractionation has to be considered. For the first time, the stable isotope fractionation of CH4 diffusion through water-saturated soils was determined with an αdiff = 1.001 ± 0.000 (n = 3). CH4 stable isotope fractionation during diffusion through air-filled pores of the investigated polygonal tundra soils was αdiff = 1.013 ± 0.003 (n = 18). Furthermore, it was found that αox differs widely between sites and horizons (mean αox, = 1.017 ± 0.009) and needs to be determined individually. The impact of both fractionation factors on the quantification of CH4 oxidation was analyzed by considering both the potential diffusion rate under saturated and unsaturated conditions and potential oxidation rates. For a submerged organic rich soil, the data indicate a CH4 oxidation efficiency of 50% at the anaerobic-aerobic interface in the upper horizon. The improved in situ quantification of CH4 oxidation in wetlands enables a better assessment of current and potential CH4 sources and sinks in permafrost affected ecosystems and their potential strengths in response to global warming.

Description: Natural peatlands are important carbon sinks and sources of methane (CH4). In contrast, drained peatlands turn from a carbon sink to a carbon source and potentially emit nitrous oxide (N2O). Rewetting of peatlands thus implies climate change mitigation. However, data about the time span that is needed for the re-establishment of the carbon sink function by restoration is scarce. We therefore investigated the annual greenhouse gas (GHG) balances of three differently vegetated bog sites 30 years after rewetting. All three vegetation communities turned out to be sources of carbon dioxide (CO2) ranging between 0.6 ± 1.43 t CO2 ha-2 yr-1 (Sphagnum-dominated vegetation) and 3.09 ± 3.86 t CO2 ha-2 yr-1 (vegetation dominated by heath). While accounting for the different global warming potential (GWP) of the three greenhouse gases, the annual GHG balance was calculated. Emissions ranged between 25 and 53 t CO2-eq ha-1 yr-1 and were dominated by large emissions of CH4 (22 up to 51 t CO2-eq ha-1 yr-1), while highest rates were found at purple moor grass (Molinia caerulea) stands. These are to our knowledge the highest CH4 emissions so far reported for bog ecosystems in temperate Europe. As the restored area was subject to large fluctuations in water table, we conclude that the high CH4 emission rates were caused by a combination of both the temporal inundation of the easily decomposable plant litter of this grass species and the plant-mediated transport through its tissues. In addition, as a result of the land use history, the mixed soil material can serve as an explanation. With regards to the long time span passed since rewetting, we note that the initial increase in CH4 emissions due to rewetting as described in the literature is not limited to a short-term period.

Description: The Lena River Delta, which is the largest delta in the Arctic, extends over an area of 32 000 km2 and likely holds more than half of the entire soil organic carbon (SOC) mass stored in the seven major deltas in the northern permafrost regions. The geomorphic units of the Lena River Delta which were formed by true deltaic sedimentation processes are a Holocene river terrace and the active floodplains. Their mean SOC stocks for the upper 1 m of soils were estimated at 29 kg m−2 ± 10 kg m−2 and at 14 kg m−2 ± 7 kg m−2, respectively. For the depth of 1 m, the total SOC pool of the Holocene river terrace was estimated at 121 Tg ± 43 Tg, and the SOC pool of the active floodplains was estimated at 120 Tg ± 66 Tg. The mass of SOC stored within the observed seasonally thawed active layer was estimated at about 127 Tg assuming an average maximum active layer depth of 50 cm. The SOC mass which is stored in the perennially frozen ground at the increment 50–100 cm soil depth, which is currently excluded from intense biogeochemical exchange with the atmosphere, was estimated at 113 Tg. The mean nitrogen (N) stocks for the upper 1 m of soils were estimated at 1.2 kg m−2 ± 0.4 kg m−2 for the Holocene river terrace and at 0.9 kg m−2 ± 0.4 kg m−2 for the active floodplain levels, respectively. For the depth of 1 m, the total N pool of the river terrace was estimated at 4.8 Tg ± 1.5 Tg, and the total N pool of the floodplains was estimated at 7.7 Tg ± 3.6 Tg. Considering the projections for deepening of the seasonally thawed active layer up to 120 cm in the Lena River Delta region within the 21st century, these large carbon and nitrogen stocks could become increasingly available for decomposition and mineralization processes.

Description: Soils are an important compartment of ecosystems and have the ability to buffer and immobilize substances of natural and anthropogenic origin to prevent their movement to other environment compartments. Predicted climatic changes together with other anthropogenic influences on Arctic terrestrial environments may affect biogeochemical processes enhancing leaching and migration of trace elements in permafrost-affected soils. This is especially important since Arctic ecosystems are considered to be highly sensitive to climatic changes as well as to chemical contamination. This study characterises background levels of trace metals in permafrost-affected soils of the Lena River delta and its hinterland in northern Siberia (73.5–69.5° N), representing a remote region far from evident anthropogenic trace metal sources. Investigations on the element content of iron (Fe), arsenic (As), manganese (Mn), zinc (Zn), nickel (Ni), copper (Cu), lead (Pb), cadmium (Cd), cobalt (Co), and mercury (Hg) in different soil types developed in different geological parent materials have been carried out. The highest median concentrations of Fe and Mn were observed in soils belonging to ice-rich permafrost sediments formed during the Pleistocene (ice-complex) while the highest median values of Ni, Pb and Zn were found in soils of both the ice-complex and the Holocene estuarine terrace of the Lena River delta region, as well as in the southernmost study unit of the hinterland area. Detailed observations of trace metal distribution on the micro scale showed that organic matter content, soil texture and iron-oxide contents influenced by cryogenic processes, temperature, and hydrological regimes are the most important factors determining the metal abundance in permafrost-affected soils. The observed range of trace element background concentrations was similar to trace metal levels reported for other pristine northern areas.

Description: This paper evaluates the relative contribution of light and temperature on net ecosystem CO2 uptake during the 2006 growing season in a polygonal tundra ecosystem in the Lena River Delta in Northern Siberia (72°22´ N, 126°30´ E). The occurrence and frequency of warm periods may be an important determinant of the magnitude of the ecosystem's carbon sink function, as they drive temperature-induced changes in respiration. Hot spells during the early portion of the growing season, when the photosynthetic apparatus of vascular plants is not fully developed, are shown to be more influential in creating positive mid-day surface-to-atmosphere net ecosystem CO2 exchange fluxes than those occurring later in the season. In this work we also develop and present a multi-step bulk flux partition model to better account for tundra plant physiology and the specific light conditions of the arctic region. These conditions preclude the successful use of traditional partition methods that derive a respiration–temperature relationship from all nighttime data or from other bulk approaches that are insensitive to temperature or light stress. Nighttime growing season measurements are rare during the arctic summer, however, so the new method allows for temporal variation in the parameters describing both ecosystem respiration and gross uptake by fitting both processes at the same time. Much of the apparent temperature sensitivity of respiration seen in the traditional partition method is revealed in the new method to reflect seasonal changes in basal respiration rates. Understanding and quantifying the flux partition is an essential precursor to describing links between assimilation and respiration at different timescales, as it allows a more confident evaluation of measured net exchange over a broader range of environmental conditions. The growing season CO2 sink estimated by this study is similar to those reported previously for this site, and is substantial enough to withstand the long, low-level respiratory CO2 release during the rest of the year to maintain the site's CO2 sink function on an annual basis.

Description: Natural peatlands are important carbon sinks and sources of methane (CH4). In contrast, drained peatlands turn from a carbon sink to a carbon source and potentially emit nitrous oxide (N2O). Rewetting of peatlands thus potentially implies climate change mitigation. However, data about the time span that is needed for the re-establishment of the carbon sink function by restoration are scarce. We therefore investigated the annual greenhouse gas (GHG) balances of three differently vegetated sites of a bog ecosystem 30 years after rewetting. All three vegetation communities turned out to be sources of carbon dioxide (CO2) ranging between 0.6 ± 1.43 t CO2 ha−2 yr−1 (Sphagnum-dominated vegetation) and 3.09 ± 3.86 t CO2 ha−2 yr−1 (vegetation dominated by heath). While accounting for the different global warming potential (GWP) of CO2, CH4 and N2O, the annual GHG balance was calculated. Emissions ranged between 25 and 53 t CO2-eq ha−1 yr−1 and were dominated by large emissions of CH4 (22–51 t CO2-eq ha−1 yr−1), with highest rates found at purple moor grass (Molinia caerulea) stands. These are to our knowledge the highest CH4 emissions so far reported for bog ecosystems in temperate Europe. As the restored area was subject to large fluctuations in the water table, we assume that the high CH4 emission rates were caused by a combination of both the temporal inundation of the easily decomposable plant litter of purple moor grass and the plant-mediated transport through its tissues. In addition, as a result of the land use history, mixed soil material due to peat extraction and refilling can serve as an explanation. With regards to the long time span passed since rewetting, we note that the initial increase in CH4 emissions due to rewetting as described in the literature is not inevitably limited to a short-term period.

Description: Permafrost-affected tundra soils are significant sources of the climate-relevant trace gas methane (CH4). The observed accelerated warming of the arctic will cause deeper permafrost thawing, followed by increased carbon mineralization and CH4 formation in water-saturated tundra soils, thus creating a positive feedback to climate change. Aerobic CH4 oxidation is regarded as the key process reducing CH4 emissions from wetlands, but quantification of turnover rates has remained difficult so far. The application of carbon stable isotope fractionation enables the in situ quantification of CH4 oxidation efficiency in arctic wetland soils. The aim of the current study is to quantify CH4 oxidation efficiency in permafrost-affected tundra soils in Russia's Lena River delta based on stable isotope signatures of CH4. Therefore, depth profiles of CH4 concentrations and δ13CH4 signatures were measured and the fractionation factors for the processes of oxidation (αox) and diffusion (αdiff) were determined. Most previous studies employing stable isotope fractionation for the quantification of CH4 oxidation in soils of other habitats (such as landfill cover soils) have assumed a gas transport dominated by advection (αtrans = 1). In tundra soils, however, diffusion is the main gas transport mechanism and diffusive stable isotope fractionation should be considered alongside oxidative fractionation. For the first time, the stable isotope fractionation of CH4 diffusion through water-saturated soils was determined with an αdiff = 1.001 ± 0.000 (n = 3). CH4 stable isotope fractionation during diffusion through air-filled pores of the investigated polygonal tundra soils was αdiff = 1.013 ± 0.003 (n = 18). Furthermore, it was found that αox differs widely between sites and horizons (mean αox = 1.017 ± 0.009) and needs to be determined on a case by case basis. The impact of both fractionation factors on the quantification of CH4 oxidation was analyzed by considering both the potential diffusion rate under saturated and unsaturated conditions and potential oxidation rates. For a submerged, organic-rich soil, the data indicate a CH4 oxidation efficiency of 50% at the anaerobic–aerobic interface in the upper horizon. The improved in situ quantification of CH4 oxidation in wetlands enables a better assessment of current and potential CH4 sources and sinks in permafrost-affected ecosystems and their potential strengths in response to global warming.

Description: The exchange fluxes of carbon dioxide between wet arctic polygonal tundra and the atmosphere were investigated by the micrometeorological eddy covariance method. The investigation site was situated in the centre of the Lena River Delta in Northern Siberia (72°22' N, 126°30' E). The study region is characterized by a polar and distinctly continental climate, very cold and ice-rich permafrost and its position at the interface between the Eurasian continent and the Arctic Ocean. The soils at the site are characterized by high organic matter content, low nutrient availability and pronounced water logging. The vegetation is dominated by sedges and mosses. The micrometeorological campaigns were performed during the periods July–October 2003 and May–July 2004 which included the period of snow and soil thaw as well as the beginning of soil refreeze. The main CO2 exchange processes, the gross photosynthesis and the ecosystem respiration, were found to be of a generally low intensity. The gross photosynthesis accumulated to –432 g m−2 over the photosynthetically active period (June–September). The contribution of mosses to the gross photosynthesis was estimated to be about 40%. The diurnal trend of the gross photosynthesis was mainly controlled by the incoming photosynthetically active radiation. During midday the photosynthetic apparatus of the canopy was frequently near saturation and represented the limiting factor on gross photosynthesis. The synoptic weather conditions strongly affected the exchange fluxes of CO2 by changes in cloudiness, precipitation and pronounced changes of air temperature. The ecosystem respiration accumulated to +327 g m−2 over the photosynthetically active period, which corresponds to 76% of the CO2 uptake by photosynthesis. However, the ecosystem respiration continued at substantial rates during autumn when photosynthesis had ceased and the soils were still largely unfrozen. The temporal variability of the ecosystem respiration during summer was best explained by an exponential function with surface temperature, and not soil temperature, as the independent variable. This was explained by the major role of the plant respiration within the CO2 balance of the tundra ecosystem. The wet polygonal tundra of the Lena River Delta was observed to be a substantial CO2 sink with an accumulated net ecosystem CO2 exchange of –119 g m−2 over the summer and an estimated annual net ecosystem CO2 exchange of –71 g m−2.