Climate Change and...
- Climate Variability
- Climate Models
Effects of Climate Change
ABSTRACT: High-latitude peatlands are important soil carbon sinks. In these ecosystems, the mineralization of carbon and nitrogen are constrained by low temperatures and low nutrient concentrations in plant litter and soil organic matter. Global warming is predicted to increase soil N availability for plants at high-latitude sites. We applied N fertilizer as an experimental analogue for this increase. In a three-year field experiment we studied N fertilization effects on leaf litter decomposition and N dynamics of the four dominant plant species (comprising >75% of total aboveground biomass) in a sub-arctic bog in northern Sweden. The species wereEmpetrum nigrum (evergreen shrub),Eriophorum vaginatum (graminoid),Betula nana (deciduous shrub) andRubus chamaemorus (perennial forb). In the controls, litter mass loss rates increased in the order:Empetrum <Eriophorum <Betula <Rubus . Increased N availability had variable, species-specific effects: litter mass loss rates (expressed per unit litter mass) increased inEmpetrum , did not change inEriophorum andBetula and decreased inRubus . In the leaf litter from the controls, we measured no or only slight net N mineralization even after three years. In the N-fertilized treatments we found strong net N immobilization, especially inEriophorum andBetula . This suggests that, probably owing to substantial chemical and/or microbial immobilization, additional N supply does not increase the rate of N cycling for at least the first three years.
ABSTRACT: Large amounts of soil carbon deposited in permafrost may be released due to deeper seasonal thawing under the climatic conditions projected for the future. An increase in the volume of the available organic material together with the higher ground temperatures may lead to enhanced emission of greenhouse gasses. Particular concerns are associated with methane, which has a much stronger greenhouse effect than an equal amount of CO2 . Production of methane is favored in the wetlands, which occupy up to 0.7 million km2 in Russian permafrost regions and have accumulated about 50 Gt of carbon (Gt C). We used the permafrost model and several climatic scenarios to construct projections of the soil temperature and the depth of seasonal thawing. To evaluate the effect of such changes on the volume of the seasonally thawing organic material, we overlaid the permafrost projections on the digitized geographically referenced contours of 59 846 wetlands in the Russian Arctic. Results for the mid-21st century climate indicated up to 50% increase in the volume of organic substrate in the northernmost locations along the Arctic coast and in East Siberia, where wetlands are sparse, and a relatively small increase by 10%–15% in West Siberia, where wetlands occupy 50%–80% of the land. We developed a soil carbon model and used it to estimate the changes in the methane fluxes due to higher soil temperature and increased substrate availability. According to our results, by mid-21st century the annual net flux of methane from Russian permafrost regions may increase by 6–8 Mt, depending on climatic scenario. If other sinks and sources of methane remain unchanged, this may increase the overall content of methane in the atmosphere by approximately 100 Mt, or 0.04 ppm, and lead to 0.012 °C global temperature rise.
Callaghan, T. V., Bjorn, L. O., Chernov, Y., Chapin, T., Christensen, T. R., Huntley, B., Ims, R. A., Johansson, M., Jolly, D., Jonasson, S., Matveyeva, N., Panikov, N., Oechel, W., Shaver, G. R. (2004). Effects on the function of Arctic ecosystems in the short- and long-term perspectives. Ambio 33 (7): 448-458
ABSTRACT: Historically, the function of Arctic ecosystems in terms of cycles of nutrients and carbon has led to low levels of primary production and exchanges of energy, water and greenhouse gases have led to low local and regional cooling. Sequestration of carbon from atmospheric CO2 , in extensive, cold organic soils and the high albedo from low, snow-covered vegetation have had impacts on regional climate. However, many aspects of the functioning of Arctic ecosystems are sensitive to changes in climate and its impacts on biodiversity. The current Arctic climate results in slow rates of organic matter decomposition. Arctic ecosystems therefore tend to accumulate organic matter and elements despite low inputs. As a result, soil-available elements like nitrogen and phosphorus are key limitations to increases in carbon fixation and further biomass and organic matter accumulation. Climate warming is expected to increase carbon and element turnover, particularly in soils, which may lead to initial losses of elements but eventual, slow recovery. Individual species and species diversity have clear impacts on element inputs and retention in Arctic ecosystems. Effects of increased CO2 and UV-B on whole ecosystems, on the other hand, are likely to be small although effects on plant tissue chemisty, decomposition and nitrogen fixation may become important in the long-term. Cycling of carbon in trace gas form is mainly as CO2 and CH4 . Most carbon loss is in the form of CO2 , produced by both plants and soil biota. Carbon emissions as methane from wet and moist tundra ecosystems are about 5% of emissions as CO2 and are responsive to warming in the absence of any other changes. Winter processes and vegetation type also affect CH4 emissions as well as exchanges of energy between biosphere and atmosphere. Arctic ecosystems exhibit the largest seasonal changes in energy exchange of any terrestrial ecosystem because of the large changes in albedo from late winter, when snow reflects most incoming radiation, to summer when the ecosystem absorbs most incoming radiation. Vegetation profoundly influences the water and energy exchange of Arctic ecosystems. Albedo during the period of snow cover declines from tundra to forest tundra to deciduous forest to evergreen forest. Shrubs and trees increase snow depth which in turn increases winter soil temperatures. Future changes in vegetation driven by climate change are therefore, very likely to profoundly alter regional climate.
ABSTRACT: Permafrost soils are an important reservoir of carbon (C) in boreal and arctic ecosystems. Rising global temperature is expected to enhance decomposition of organic matter frozen in permafrost, and may cause positive feedback to warming as CO2 is released to the atmosphere. Significant amounts of organic matter remain frozen in thick mineral soil (loess) deposits in northeastern Siberia, but the quantity and lability of this deep organic C is poorly known. Soils from four tundra and boreal forest locations in northeastern Siberia that have been continuously frozen since the Pleistocene were incubated at controlled temperatures (5, 10 and 15°C) to determine their potential to release C to the atmosphere when thawed. Across all sites, CO2 with radiocarbon (14 C) ages ranging between ~21 and 24 ka bp was respired when these permafrost soils were thawed. The amount of C released in the first several months was strongly correlated to C concentration in the bulk soil in the different sites, and this correlation remained the same for fluxes up to 1 year later. Fluxes were initially strongly related to temperature with a mean Q10 value of 1.9±0.3 across all sites, and later were unrelated to temperature but still correlated with bulk soil C concentration. Modeled inversions ofΔ14 CO2 values in respiration CO2 and soil C components revealed mean contribution of 70% and 26% from dissolved organic C to respiration CO2 in case of two permafrost soils, while organic matter fragments dominated respiration (mean 68%) from a surface mineral soil that served as modern reference sample. Our results suggest that if 10% of the total Siberian permafrost C pool was thawed to a temperature of 5°C, about 1 Pg C will be initially released from labile C pools, followed by respiration of ~40 Pg C to the atmosphere over a period of four decades.
ABSTRACT: Moisture is an important control on atmospheric CH4 consumption and CO2 production in soil. Wet conditions limit these microbial activities by restricting CH4 and O2 diffusion and dry conditions limit microbial activity due to physiological water stress. We examined the relationship between soil moisture and these biogeochemical activities in five Alaskan soils with varying physical properties. Three expressions of soil moisture, absolute water content (g H2O g−1 dry soil), water potential and percent of water-holding capacity (%WHC), were compared for their abilities to predict microbial activity in the different soils. We also examined the physiological responses of CH4 oxidizers and the general microbial community to changes in water potential. The quantitative relationship between absolute water content and microbial activity varied widely among soils with different textures. The relationship between microbial activity and water potential was asymmetrical and differed between upland and wetland soils. In contrast, the parabolic relationship between %WHC and CH4 consumption was symmetrical and similar among the five soils. CO2 production also related to %WHC similarly across soils. Maximum atmospheric CH4 consumption occurred between 20-40% WHC in all soils with a mean optimum of 34% WHC, whereas CO2 production was maximal above 50% WHC. For CH4 oxidation, optimum water potential was −0.3 to −0.2 MPa in upland soils, and about −0.02 MPa in a wetland soil. Our results demonstrate that %WHC is a powerful expression for quantitatively relating microbial activity responses to moisture across physically diverse soils and may be useful for modeling the response of biogeochemical processes, especially atmospheric CH4 consumption, to climate change. Our data also suggest that CH4 oxidizers in upland soils are adapted to growth on atmospheric CH4 and that CH4 consumption in upland taiga soils may be decreased by altered soil moisture, regardless of whether conditions become wetter or drier.
ABSTRACT: CO2 and CH4 fluxes were monitored over 4 years in a range of taiga forests along the Tanana River in interior Alaska. Floodplain alder and white spruce sites and upland birch/aspen and white spruce sites were examined. Each site had control, fertilized, and sawdust amended plots; flux measurements began during the second treatment year. CO2 emissions decreased with successional age across the sites (alder, birch/aspen, and white spruce, in order of succession) regardless of landscape position. Although CO2 fluxes showed an exponential relationship with soil temperature, the response of CO2 production to moisture fit an asymptotic model. Of the manipulations, only N fertilization had an effect on CO2 flux, decreasing flux in the floodplain sites but increasing it in the birch/aspen site. Landscape position was the best predictor of CH4 flux. The two upland sites consumed CH4 at similar rates (approximately 0.5 mg C m−2 d−1 ), whereas the floodplain sites had lower consumption rates (0–0.3 mg C m−2 d−1 ). N fertilization and sawdust both inhibited CH4 consumption in the upland birch/aspen and floodplain spruce sites but not in the upland spruce site. The biological processes driving CO2 fluxes were sensitive to temperature, moisture, and vegetation, whereas CH4 fluxes were sensitive primarily to landscape position and biogeochemical disturbances. Hence, climate change effects on C-gas flux in taiga forest soils will depend on the relationship between soil temperature and moisture and the concomitant changes in soil nutrient pools and cycles.
Hossain, M.F., Zhang, Y., Chen, W., Wang, J., Pavlic, G. (2007). Soil organic carbon content in northern Canada: A database of field measurements and its analysis. Canadian Journal of Soil Science 87 (3): 259-268
ABSTRACT: Arctic and sub-arctic soils contain a large amount of organic carbon in their topsoil horizons and in the upper layers of permafrost. There is concern that climate warming could release this soil organic carbon (SOC) to the atmosphere as greenhouse gases. However, information about the profile features and spatial distribution of SOC in northern ecosystems are far less than for other regions. In this study, we compiled all the available field measurements of SOC in northern Canada and developed a database. Including our recent measurements, the database contains 438 profiles with 1473 soil horizons. We analyzed the profile features and the regional patterns of SOC in northern Canada based on this database. The results show that the SOC content of subsurface soils is relatively high in northern regions because of the alternate freeze-thaw actions. In the top 100 cm of soils, 40% of the SOC is located in the 50- to 100-cm layer. The SOC content is lower in northern Arctic and in mountainous regions. The average upland SOC content in northern Canada is higher than in other world biomes (i.e., croplands, temperate forest, tropical savannas, and tropical forest) except temperate grasslands and boreal forest. Key words: Soil organic carbon, northern Canada, database, arctic and sub-arctic.
Ilvesniemi, H., Kahkonen, M. A., Pumpanen, J., Rannik, U., Wittmann, C., Peramaki, M., Keronen, P., Hari, P., Vesala, T., Salkinoja-Salonen, M. (2005). Wintertime CO2 evolution from a boreal forest ecosystem. Boreal Environment Research 10 (5): 401-408
ABSTRACT: We investigated wintertime ecosystem activity and CO2 efflux over three winters (1 November–28 February 1997–2000) in a boreal Scots pine stand in Finland. During the three winters the cumulative wintertime CO2 efflux measured with continuously operating soil chambers directly from the soil surface was between 103 and 144 g m–2 , and between 240 and 330 g m–2 when measured by an eddy covariance method or estimated from the soil sample endogenous CO2 production. The flux measured directly from the soil surface is probably an underestimation due to the ice formation within the chamber. Photosynthesis was found to be active also during winter and metabolic activity was found to show extrapolated zero at –5 °C to –10 °C.
Khvorostyanov, D. V., Ciais, P., Krinner, G., Zimov, S. A., Corradi, Ch., Guggenberger, G. (2008). Vulnerability of permafrost carbon to global warming. Part II: sensitivity of permafrost carbon stock to global warming. Tellus B 60 (2): 265-275
ABSTRACT: In the companion paper (Part I), we presented a model of permafrost carbon cycle to study the sensitivity of frozen carbon stocks to future climate warming. The mobilization of deep carbon stock of the frozen Pleistocene soil in the case of rapid stepwise increase of atmospheric temperature was considered. In this work, we adapted the model to be used also for floodplain tundra sites and to account for the processes in the soil active layer. The new processes taken into account are litter input and decomposition, plant-mediated transport of methane, and leaching of exudates from plant roots. The SRES-A2 transient climate warming scenario of the IPSL CM4 climate model is used to study the carbon fluxes from the carbon-rich Pleistocene soil with seasonal active-layer carbon cycling on top of it. For a point to the southwest from the western branch of Yedoma Ice Complex, where the climate warming is strong enough to trigger self-sustainable decomposition processes, about 256 kg C m−2 , or 70% of the initial soil carbon stock under present-day climate conditions, are emitted to the atmosphere in about 120 yr, including 20 kgC m−2 released as methane. The total average flux of CO2 and methane emissions to the atmosphere during this time is of 2.1 kg C m−2 yr−1 . Within the Yedoma, whose most part of the territory remains relatively cold, the emissions are much smaller: 0.2 kg C m−2 yr−1 between 2050 and 2100 for Yakutsk area. In a test case with saturated upper-soil meter, when the runoff is insufficient to evacuate the meltwater, 0.05 kg CH4 m−2 yr−1 on average are emitted as methane during 250 yr starting from 2050. The latter can translate to the upper bound of 1 Gt C yr−1 in CO2 equivalent from the 1 million km2 area of the Yedoma.
Kutzbach, L., Wille, C., Pfeiffer, E.-M. (2007). The exchange of carbon dioxide between wet arctic tundra and the atmosphere at the Lena River Delta, Northern Siberia. Biogeosciences Discussions 4 (3): 1953-2005
ABSTRACT: 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 .
ABSTRACT: Tundra-atmosphere exchanges of carbon dioxide (CO2 ) and water vapour were measured near Daring Lake, Northwest Territories in the Canadian Low Arctic for 3 years, 2004–2006. The measurement period spanned late-winter until the end of the growing period. Mean temperatures during the measurement period varied from about 2 °C less than historical average in 2004 and 2005 to 2 °C greater in 2006. Much of the added warmth in 2006 occurred at the beginning of the study, when snow melt occurred 3 weeks earlier than in the other years. Total precipitation in 2006 (163 mm) was more than double that of the driest year, 2004 (71 mm). The tundra was a net sink for CO2 carbon in all years. Mid-summer net ecosystem exchange of CO2 (NEE) achieved maximum values of −1.3 g C m−2 day−1 (2004) to −1.8 g C m−2 day−1 (2006). Accumulated NEE values over the 109-day period were −32,−51 and −61 g C m−2 in 2004, 2005 and 2006, respectively. The larger CO2 uptake in 2006 was attributed to the early spring coupled with warmer air and soil conditions. In 2004, CO2 uptake was limited by the shorter growing season and mid-summer dryness, which likely reduced ecosystem productivity. Seasonal total evapotranspiration (ET) ranged from 130 mm (2004) to 181 mm (2006) and varied in accordance with the precipitation received and with the timing of snow melt. Maximum daily ET rates ranged from 2.3 to 2.7 mm day−1 , occurring in mid July. Ecosystem water use efficiency (WUEeco) varied slightly between years, ranging from 2.2 in the driest year to 2.5 in the year with intermediate rainfall amounts. In the wettest year, increased soil evaporation may have contributed to a lower WUEeco (2.3). We speculate that most, if not all, of the modest growing season CO2 sink measured at this site could be lost due to fall and winter respiration leading to the tundra being a net CO2 source or CO2 neutral on an annual basis. However, this hypothesis is untested as yet.
Loya, W. M., Johnson, L. C., Kling, G. W., King, J. Y., Reeburgh, W. S., Nadelhoffer, K. J. (2002). Pulse-labeling studies of carbon cycling in arctic tundra ecosystems: Contribution of photosynthates to soil organic matter. Global Biogeochemical Cycles 16 (4): 1101
ABSTRACT: To increase our understanding of carbon (C) cycling and storage in soils, we used14 C to trace C from roots into four soil organic matter (SOM) fractions and the movement of soil microbes in arctic wet sedge and tussock tundra. For both tundra types, the proportion of14 C activity in the soil was 6% of the total14 C-CO2 taken up by plants at each of the four harvests conducted 1, 7, 21, and 68 days after labeling. In tussock tundra, we observed rapid microbial transformation of labile C from root exudates into more stable SOM. In wet sedge tundra, there appears to be delayed or indirect microbial use of root exudates. The net amount of14 C label transferred to SOM by the end of the season in both tundra types was approximately equal to the amount transferred to soils 1 day after labeling, suggesting that transfer of14 C tracer from roots to soils continued through the growing season. Overall, C inputs from living roots contributes 24 g C m−2 yr−1 in tussock tundra and 8.8 g C m−2 yr−1 in wet sedge tundra. These results suggest rapid belowground allocation of C by plants and subsequent incorporation of much of this C into storage in the SOM.
Meyer, H., Kaiser, C., Biasi, C., Haemmerle, R., Rusalimova, O., Lashchinsky, N., Baranyi, C., Daims, H., Barsukov, P., Richter, A. (2006). Soil carbon and nitrogen dynamics along a latitudinal transect in western Siberia, Russia. Biogeochemistry 81 (2): 239-252
ABSTRACT: An 1800-km South to North transect (N 53°43′ to 69°43′) through Western Siberia was established to study the interaction of nitrogen and carbon cycles. The transect comprised all major vegetation zones from steppe, through taiga to tundra and corresponded to a natural temperature gradient of 9.5°C mean annual temperature (MAT). In order to elucidate changes in the control of C and N cycling along this transect, we analyzed physical and chemical properties of soils and microbial structure and activity in the organic and in the mineral horizons, respectively. The impact of vegetation and climate exerted major controls on soil C and N pools (e.g., soil organic matter, total C and dissolved inorganic nitrogen) and process rates (gross N mineralization and heterotrophic respiration) in the organic horizons. In the mineral horizons, however, the impact of climate and vegetation was less pronounced. Gross N mineralization rates decreased in the organic horizons from south to north, while remaining nearly constant in the mineral horizons. Especially, in the northern taiga and southern tundra gross nitrogen mineralization rates were higher in the mineral compared to organic horizons, pointing to strong N limitation in these biomes. Heterotrophic respiration rates did not exhibit a clear trend along the transect, but were generally higher in the organic horizon compared to mineral horizons. Therefore, C and N mineralization were spatially decoupled at the northern taiga and tundra. The climate change implications of these findings (specifically for the Arctic) are discussed.
Muraoka, H., Noda, H., Uchida, M., Ohtsuka, T., Koizumi, H., Nakatsubo, T. (2008). Photosynthetic characteristics and biomass distribution of the dominant vascular plant species in a high Arctic tundra ecosystem, Ny-Ålesund, Svalbard: implications for their role in ecosystem carbon gain. Journal of Plant Research 121 (2): 137-145
ABSTRACT: Studies on terrestrial ecosystems in the high Arctic region have focused on the response of these ecosystems to global environmental change and their carbon sequestration capacity in relation to ecosystem function. We report here our study of the photosynthetic characteristics and biomass distribution of the dominant vascular plant species,Salix polaris ,Dryas octopetala andSaxifraga oppositifolia , in the high Arctic tundra ecosystem at Ny-Ålesund, Svalbard (78.5°N, 11.5°E). We also estimated net primary production (NPP) along both the successional gradient created by the proglacial chronosequence and the topographical gradient. The light-saturated photosynthesis rate (A max ) differed among the species, with approximately 124.1 nmol CO2 g−1 leaf s−1 forSal. polaris , 57.8 forD. octopetala and 24.4 forSax. oppositifolia , and was highly correlated with the leaf nitrogen (N) content for all three species. The photosynthetic N use efficiency was the highest inSal. polaris and lowest inSax. oppositifolia . Distributions ofSal. polaris andD. octopetala were restricted to the area where soil nutrient availability was high, whileSax. oppositifolia was able to establish at the front of a glacier, where nutrient availability is low, but tended to be dominated by other vascular plants in high nutrient areas. The NPP reflected the photosynthetic capacity and biomass distribution in that it increased with the successional status; the contribution of Sal. polaris reached as high as 12-fold that ofSax. oppositifolia .
Nakatsubo, T., Bekku, Y. S., Uchida, M., Muraoka, H., Kume, A., Ohtsuka, T., Masuzawa, T., Kanda, H., Koizumi, H. (2005). Ecosystem development and carbon cycle on a glacier foreland in the high Arctic, Ny-Alesund, Svalbard. Journal of Plant Research 118 (3): 173-179
ABSTRACT: The Arctic terrestrial ecosystem is thought to be extremely susceptible to climate change. However, because of the diverse responses of ecosystem components to change, an overall response of the ecosystem carbon cycle to climate change is still hard to predict. In this review, we focus on several recent studies conducted to clarify the pattern of the carbon cycle on the deglaciated area of Ny-Ålesund, Svalbard in the high Arctic. Vegetation cover and soil carbon pools tended to increase with the progress of succession. However, even in the latter stages of succession, the size of the soil carbon pool was much smaller than those reported for the low Arctic tundra. Cryptogams contributed the major proportion of phytomass in the later stages. However, because of water limitation, their net primary production was smaller than that of the vascular plants. The compartment model that incorporated major carbon pools and flows suggested that the ecosystem of the later stages is likely to be a net sink of carbon at least for the summer season. Based on the eco-physiological characteristics of the major ecosystem components, we suggest several possible scenarios of future changes in the ecosystem carbon cycle.
ABSTRACT: A large portion of carbon (C) is stored in the world’s soils, including those of peatlands, wetlands and permafrost. However, there is disagreement regarding the effects of climate change on the rate of organic matter decomposition in permafrost soils of the arctic. In this study it was hypothesized that soil exposed to a higher ambient temperature would have a greater flux of CO2 as well as a change in the metabolic diversity of culturable soil microorganisms. To evaluate this hypothesis we determined soil C dynamics, soil microbial respiration and activity, and13 C and15 N fractionation in laboratory incubations (at 14 and 21°C) for an organic-rich soil (Mesic Organic Cryosol) and a mineral soil (Turbic Cryosol) collected at the Daring Lake Research Station in Canada’s Northwest Territories. Soil organic C (SOC) and nitrogen (N) stocks (g m-2) and concentration (%) were significantly different (P < 0.05) between soil horizons for both soil types. Stable isotope analysis showed a significant enrichment ind13 C andd15 N with depth and a depletion ind13 C andd15 N with increasing SOC and N concentration. In laboratory incubations, microbial respiration showed three distinct phases of decomposition: a phase with a rapidly increasing rate of respiration (phase 1), a phase in which respiration reached a peak midway through the incubation (phase 2), and a phase in the latter part of the incubation in which respiration stabilized at a lower flux than that of the first phase (phase 3). Fluxes of CO2 were significantly greater at 21°C than at 14°C. Thed13C of the evolved CO2 became significantly enriched with time with the greatest enrichment occurring in phase 2 of the incubation. Soil microbial activity, as measured using Biolog EcoplatesTM, showed a significantly greater average well color development, richness, and Shannon index at 21°C; again the greatest change occurred in phase 2 of the incubation. Principal component analysis (PCA) of the Biolog data also showed a change in the distinct clustering of the soil microbial activity, showing that C sources from the soil were metabolized differently with time at 21 than at 14°C, and between soil horizons. Our results show that Canadian arctic soils contain large stores of C, which readily decompose, and that substantial increases in CO2 emissions and changes in the metabolic diversity of culturable soil microorganisms may occur when ambient temperatures increase from 14 to 21°C.
ABSTRACT: Carbon balance of intact arctic tundra microcosms (soil cores with vegetation) has been shown to be sensitive to small changes in water table. The persistence of the effect of water table on CO2 flux in darkness, and the large reduction of such an effect in sand-culture microcosms without peat-degrading microorganisms have led to the conclusion that rates of microbial degradation of peat are most likely responsible for the bulk of previously observed effects of water table on net ecosystem CO2 flux of arctic tundra microcosms. In our experiment using sand cultures, we could not detect any significant effects of changes in water table or of increasing the atmospheric CO2 concentration on the growth of plants of two dominant graminoid species (Dupontia fisheri andCarex aquatilis ).
ABSTRACT: A dynamic vegetation model has been used to predict patterns of recent past and potential future change in taiga forest ecosystems of interior Alaska. The model, called CASA (Carnegie Ames Stanford Approach), is a process-based ecosystem depiction of plant and soil processes, including all major cycles of water and carbon. CASA’s dynamic vegetation component is intended to facilitate coupling to general circulation models of the atmosphere, which require mechanistic fluxes and feedbacks from terrestrial vegetation. Simulation results for selected Alaska sites of Denali National Park suggest that the past 50-year climate trends of warming temperatures may shift the taiga ecosystem from dominance by evergreen needleleaf trees to a more mixed broadleaf–needleleaf tree composition. For other (higher elevation) areas of Denali, our model predicts that a difference of only about 3 °C in mean annual air temperatures appears to differentiate the permanent presence of tundra vegetation forms over taiga forest. The model predicts that over the 1950–1999 climate record at Denali station, the changing taiga ecosystems were net sinks for atmospheric CO2 of about 1.3 kg C m−2 . During the warm 1990s, these forests were predicted to be net carbon sinks of more than 15 g C m−2 per year in 8 out of 10 years. Predicted NPP for the forest continues to increase with a projected warming trend for the next 25 years at a mean rate of about +1.2 g C m−2 per year. On the basis of these model results, a series of crucial field site measurements can be identified for inclusion in subsequent long-term ecological studies of the changing taiga forest.
Prokushkin, A. S., Kajimoto, T., Prokushkin, S. G., Mcdowell, H., Abaimov, A. P., Matsuura, Y. (2005). Climatic factors influencing fluxes of dissolved organic carbon from the forest floor in a continuous-permafrost Siberian watershed. Canadian Journal of Forest Research 35 (9): 2130-2140
ABSTRACT: Fluxes of dissolved organic carbon (DOC) in forested watersheds underlain by permafrost are likely to vary with changes in climatic regime that increase soil moisture and temperature. We examined the effects of temporal and spatial variations in soil temperature and moisture on DOC fluxes from the forest floor of contrasting north- and south-facing slopes in central Siberia. DOC fluxes increased throughout the growing season (June–September) on both slopes in 2002 and 2003. The most favorable combination of moisture content and temperature (deepest active soil layer) occurred in September, and we believe this was the primary driver of increased DOC concentrations and flux in autumn. Total DOC flux for June–September was 12.6–17.6 g C·m–2 on the south-facing slope and 4.6–8.9 g C·m–2 on the north-facing slope. DOC concentrations in forest floor leachates increased with increasing temperature on the north-facing slope, but were almost unaffected by temperature on the south-facing slope. Our results suggest that water input in midseason from melting of ice or precipitation events is the primary factor limiting DOC production. Significant positive correlations between amounts of precipitation and DOC flux were found on both slopes. Dilution of DOC concentrations by high precipitation volumes was observed only for the forest floor leachates collected from the north-facing slope. Our results suggest that global warming will result in increased DOC production in forest floors of permafrost regions, and that precipitation patterns will play an important role in determining the magnitude of these changes in DOC flux as well as its interannual variability. However, the longer-term response of soils and DOC flux to a warming climate will be driven by changes in vegetation and microbial communities as well as by the direct results of temperature and moisture conditions.
Rodionow, A., Flessa, H., Kazansky, O., Guggenberger, G. (2006). Organic matter composition and potential trace gas production of permafrost soils in the forest tundra in northern Siberia. Geoderma 135: 49-62
ABSTRACT: The amount, quality and bioavailability of organic matter stored in permafrost soils are important factors determining the response of high-latitude soils to climate warming. In this study, we investigated the storage and composition (isotopic composition, lignin, pyrogenic carbon) of organic matter in mineral soils which are differently affected by permafrost, and we determined the potential CO2 emission, CH4 exchange and N2 O emission of these soils at different temperature (5 °C and 15 °C) and moisture (60% of the maximum water-holding capacity [WHC] and completely water saturated) in a laboratory incubation experiment. Soil samples were collected in the summer of 2002 and 2003 from the mineral soils of the Grawijka Creek catchment in the Siberian forest tundra and for comparison, from a fertilized grassland in Germany.
The depth of the seasonal thaw layer of the Siberian soils ranged from 15 cm to > 90 cm and was greater in soils located on slopes than in soils of plane areas where drainage was poor and soils showed gleyic properties.
The soil organic carbon (SOC) concentration ranged from 14 to 74 g kg− 1 in the upper 20 cm and from 5 to 128 g kg− 1 in the subsoil. The total SOC and N accumulation in the upper 30–40 cm were larger in soils with a seasonal thaw layer < 40 cm (up to 23 kg C m−2 and 1.3 kg N m−2 ) than in soils without permafrost in the upper 90 cm (approximately 8 kg C m−2 and 0.6 kg N m−2 ). The concentration of lignin-derived CuO oxidation products in soil OC were larger in a soil without permafrost than in the permafrost soils in which lignin oxidation appears to be more advanced. All soil samples from the forest tundra contained considerable amounts of black carbon (up to 57 g C kg-1 SOC), which indicates the importance of fire in this ecosystem. Water logging in the permafrost soils seems to restrain the decomposition of black carbon. Soil organic carbon mineralization in the gleyic permafrost soils increased by a factor of approximately 4 if soil temperature was raised from 5 to 15 °C and soil moisture reduced from complete water saturation to 60% WHC. Emission of N2O was negligible from all Siberian soils but very high from the fertilized grassland soil at complete water saturation. At 60% WHC, all forest tundra soils were a net-sink for atmospheric methane with significantly larger CH4 uptake in the A horizon of the soil without permafrost (~0.1 ng CH4–C h-1 g-1 ) than in the A horizon of the permafrost soils (< 0.02 ng CH4 –C h-1 g-1 ). The results show that permafrost distribution is an important factor determining storage and composition of SOC in the Grawijka Creek area and that permafrost distribution may considerably affect current and future net fluxes of the greenhouse gases CO2 and CH4 in this region.
ABSTRACT: Four tundra and taiga soils were experimentally subjected to three freeze-thaw cycles (5 days each at −5°C and +5°C). After each thaw, there was an initial pulse (<24 h) in microbial respiration. The total amount of C respired in each thaw period was largest during the first cycle and decreased in successive cycles. Three cycles caused a net increase in total respired C relative to the +5°C control in wet meadow tundra soil, a reduction in birch soil, and had no net effect in either alder-poplar or tussock tundra soil. These different patterns apparently resulted from differences in the quality of the soil organic matter and the relative activity of the microbial biomass. Net N mineralization was generally enhanced relative to the +5°C controls in the first cycle, but was inhibited in the third cycle, similar to what was observed with C. Over multiple freeze-thaw cycles, the initial response of C and N mineralization appear to be driven by release from the microbial biomass, while over the longer-term, the response is driven by the reduction in attack on soil organic matter resulting from a reduced microbial population.
Schimel, J. P., Gulledge, J. M., Clein-Curley, J. S., Lindstrom, J. E., Braddock, J. F. (1999). Moisture effects on microbial activity and community structure in decomposing birch litter in the Alaskan taiga. Soil Biology and Biochemistry 31 (6): 831-838
ABSTRACT: We carried out a field experiment to evaluate the effect of moisture regime on microbial biomass and activity in birch litter in the Alaskan taiga. Litter bags were placed in one of three treatments: continuously moist (0.5 cm water d−1 ), cycling (0.5 cm water weekly), and ‘natural', which experienced two natural dry–wet cycles of 2 weeks dry followed by rain. The experiment lasted for 1 month. Each week we collected litter bags and analyzed microbial respiration and biomass C and N. In the last two cycles we analyzed bacterial substrate use on Biolog GN plates. There were strong overall correlations between biomass, respiration and litter moisture content. However, the different treatments had significantly different rates of respiration, biomass and respiratory quotient (qCO2 ) that could not be explained by moisture content directly. The natural treatment had lower respiration rates and biomass than the wet or cycling samples, indicating that the 2-week droughts in the natural treatment reduced microbial populations and activity to a greater degree than did shorter droughts. Episodic drying and rewetting considerably decreased the number of Biolog substrates used. This experiment showed that the size and functioning of the litter microbial community was strongly affected by its stress history.
Sitch, S., Mcguire, A. D., Kimball, J., Gedney, N., Gamon, J., Engstrom, R., Wolf, A., Zhuang, Q., Clein, J., McDonald, K.C. (2007). Assessing the carbon balance of circumpolar Arctic tundra using remote sensing and process modeling. Ecological Applications 17 (1): 213-234
ABSTRACT: This paper reviews the current status of using remote sensing and process-based modeling approaches to assess the contemporary and future circumpolar carbon balance of Arctic tundra, including the exchange of both carbon dioxide and methane with the atmosphere. Analyses based on remote sensing approaches that use a 20-year data record of satellite data indicate that tundra is greening in the Arctic, suggesting an increase in photosynthetic activity and net primary production. Modeling studies generally simulate a small net carbon sink for the distribution of Arctic tundra, a result that is within the uncertainty range of field-based estimates of net carbon exchange. Applications of process-based approaches for scenarios of future climate change generally indicate net carbon sequestration in Arctic tundra as enhanced vegetation production exceeds simulated increases in decomposition. However, methane emissions are likely to increase dramatically, in response to rising soil temperatures, over the next century. Key uncertainties in the response of Arctic ecosystems to climate change include uncertainties in future fire regimes and uncertainties relating to changes in the soil environment. These include the response of soil decomposition and respiration to warming and deepening of the soil active layer, uncertainties in precipitation and potential soil drying, and distribution of wetlands. While there are numerous uncertainties in the projections of process-based models, they generally indicate that Arctic tundra will be a small sink for carbon over the next century and that methane emissions will increase considerably, which implies that exchange of greenhouse gases between the atmosphere and Arctic tundra ecosystems is likely to contribute to climate warming.
ABSTRACT: We evaluated the balance of production and decomposition in natural ecosystems ofPinus sylvestris ,Larix sibirica andBetula pendulain the southern boreal forests of central Siberia, using the Yenisei transect. We also investigated whether anthropogenic disturbances (logging, fire and recreation pressure) influence the carbon budget.Pinus andLarix stands up to age class VI act as a net sink for atmospheric carbon. Mineralization rates in youngBetula forests exceed rates of uptake via photosynthesis assimilation. Old-growth stands of all three forest types are CO2 sources to the atmosphere. The prevalence of old-growthLarix in the southern taiga suggests thatLarix stands are a net source of CO2 . The CO2 flux to the atmosphere exceeds the uptake of atmospheric carbon via photosynthesis by 0.23 t C.ha−1 .yr−1 (47%).Betula andPinus forests are net sinks, as photosynthesis exceeds respiration by 13% and 16% respectively. The total carbon flux fromPinus ,Larix andBetula ecosystems to the atmosphere is 10 387 thousand tons C.yr−1 . Net Primary Production (0.935 t-C.ha−1 ) exceeds carbon release from decomposition of labile and mobile soil organic matter (Rh) by 767 thousand tons C (0.064 t-C.ha−1 ), so that these forests are net C-sinks. The emissions due to decomposition of slash (101 thousand tons C; 1.0%) and from fires (0.21%) are very small. The carbon balance of human-disturbed forests is significantly different. A sharp decrease in biomass stored inPinus andBetula ecosystems leads to decreased production. As a result, the labile organic matter pool decreased by 6–8 times; course plant residues with a low decomposition rate thus dominate this pool. Annual carbon emissions to the atmosphere from these ecosystems are determined primarily by decomposing fresh litterfall. This source comprises 40–79% of the emissions from disturbed forests compared to only 13–28% in undisturbed forests. The ratio of emissions to production (NPP) is 20–30% in disturbed and 52–76% in undisturbed forests.
ABSTRACT: We conducted plant species removals, air temperature manipulations, and vegetation and soil transplants in Alaskan wet-meadow and tussock tundra communities to determine the relative importance of vegetation type and environmental variables in controlling ecosystem methane (CH4 ) and carbon dioxide (CO2 ) flux. Plastic greenhouses placed over wet-meadow tundra increased air temperature, soil temperature, and soil moisture, but did not affect CH4 or CO2 flux (measured in the dark). By contrast, removal of sedges in the wet meadow significantly decreased flux of CH4 , while moss removal tended to increaseCH4 emissions. At 15 cm depth, pore-water CH4 concentrations were higher in sedge-removal than in control plots, suggesting that sedges contribute to CH4 emissions by transporting CH4 from anaerobic soil to the atmosphere, rather than by promoting methanogenesis. In reciprocal-ecosystem transplants between the wet-meadow and tussock tundra communities, CH4 and CO2 emissions were higher overall in thewet-meadow site, but were unrelated to transplant origin. Methane flux was correlated with local variation in soil temperature, thaw depth, and water-table depth, but the relative importance ofthese factors varied through the season. Our result ssuggest that future changes in CH4 and CO2 flux in response to climatic change will be more strongly mediated by large-scale changes in vegetation and soil parameters than by direct temperature effects.
Vourlitis,G. C., Boynton,B., Verfaillie J.,Jr, Zulueta,R., Hastings,S. J., Oechel,W. C., Hope,A., Stow,D. (2000). Physiological models for scaling plot measurements of CO2 flux across an arctic tundra landscape. Ecologcial Applications 10 (1): 60-72
ABSTRACT: Regional estimates of arctic ecosystem CO2 exchange are required because of the large soil carbon stocks located in arctic regions, the potentially large global-scale feedbacks associated with climate-change-induced alterations in arctic ecosystem C sequestration, and the substantial small-scale (1–10 m2 ) heterogeneity of arctic vegetation and hydrology. Because the majority of CO2 flux data for arctic ecosystems are derived from plot-scale studies, a scaling routine that can provide reliable estimates of regional CO2 flux is required. This study combined data collected from chamber measurements of CO2 exchange, meteorology, hydrology, and surface reflectance with simple physiological models to quantify the diurnal and seasonal dynamics of whole-ecosystem respiration (R), gross primary production (GPP), and net CO2 exchange (F) of wet- and moist-sedge tundra ecosystems of arctic Alaska. Diurnal fluctuations in R were expressed as exponential functions of air temperature, whereas diurnal fluctuations in GPP were described as hyperbolic functions of diurnal photosynthetic photon flux density (PPFD). Daily integrated rates of R were expressed as an exponential function of average daily water table depth and temperature, whereas daily fluctuations in GPP were described as a hyperbolic function of average daily PPFD and a sigmoidal function of the normalized difference vegetation index (NDVI) calculated from satellite imagery. These models described, on average, 75–97% of the variance in diurnal R and GPP, and 78–95% of the variance in total daily R and GPP. Model results suggest that diurnal F can be reliably predicted from meteorology (radiation and temperature), but over seasonal time scales, information on hydrology and phenology is required to constrain the response of GPP and R to variations in temperature and radiation.
Using these physiological relationships and information about the spatial variance in surface features across the landscape, measurements of CO2 exchange in 0.5-m2 plots were extrapolated to the hectare scale. Compared to direct measurements of hectare-scale F made using eddy covariance, the scaled estimate of seasonally integrated F was within 20% of the observed value. With a minimum of input data, these models allowed plot measurements of arctic ecosystem CO2 exchange to be confidently scaled in space and time.
ABSTRACT: Climate warming will thaw permafrost, releasing trapped carbon from this high-latitude reservoir and further exacerbating global warming.
W.R. Rouse, M.S.V. Douglas, R.E. Hecky, A.E. Hershey, G.W. Kling, L.E> Lesack, P. Marsh, M. McDonald, B.J. Nicholson, N.T. Roulet, J.P. Smol (1997). Effects of climate change on the freshwaters of Arctic and subarctic North America. Hydrological Processes 11 (8): 873-902
ABSTRACT: Region 2 comprises arctic and subarctic North America and is underlain by continuous or discontinuous permafrost. Its freshwater systems are dominated by a low energy environment and cold region processes. Central northern areas are almost totally influenced by arctic air masses while Pacific air becomes more prominent in the west, Atlantic air in the east and southern air masses at the lower latitudes. Air mass changes will play an important role in precipitation changes associated with climate warming. The snow season in the region is prolonged resulting in long-term storage of water so that the spring flood is often the major hydrological event of the year, even though, annual rainfall usually exceeds annual snowfall. The unique character of ponds and lakes is a result of the long frozen period, which affects nutrient status and gas exchange during the cold season and during thaw. GCM models are in close agreement for this region and predict temperature increases as large as 4°C in summer and 9°C in winter for a 2 × CO2 scenario. Palaeoclimate indicators support the probability that substantial temperature increases have occurred previously during the Holocene. The historical record indicates a temperature increase of > 1°C in parts of the region during the last century. GCM predictions of precipitation change indicate an increase, but there is little agreement amongst the various models on regional disposition or magnitude. Precipitation change is as important as temperature change in determining the water balance. The water balance is critical to every aspect of hydrology and limnology in the far north. Permafrost close to the surface plays a major role in freshwater systems because it often maintains lakes and wetlands above an impermeable frost table, which limits the water storage capabilities of the subsurface. Thawing associated with climate change would, particularly in areas of massive ice, stimulate landscape changes, which can affect every aspect of the environment. The normal spring flooding of ice-jammed north-flowing rivers, such as the Mackenzie, is a major event, which renews the water supply of lakes in delta regions and which determines the availability of habitat for aquatic organisms. Climate warming or river damming and diversion would probably lead to the complete drying of many delta lakes. Climate warming would also change the characteristics of ponds that presently freeze to the bottom and result in fundamental changes in their limnological characteristics. At present, the food chain is rather simple usually culminating in lake trout or arctic char. A lengthening of the growing season and warmer water temperature would affect the chemical, mineral and nutrient status of lakes and most likely have deleterious effects on the food chain. Peatlands are extensive in region 2. They would move northwards at their southern boundaries, and, with sustained drying, many would change form or become inactive. Extensive wetlands and peatlands are an important component of the global carbon budget, and warmer and drier conditions would most likely change them from a sink to a source for atmospheric carbon. There is some evidence that this may be occurring already. Region 2 is very vulnerable to global warming. Its freshwater systems are probably the least studied and most poorly understood in North America. There are clear needs to improve our current knowledge of temperature and precipitation patterns; to model the thermal behaviour of wetlands, lakes and rivers; to understand better the interrelationships of cold region rivers with their basins; to begin studies on the very large lakes in the region; to obtain a firm grasp of the role of northern peatlands in the global carbon cycle; and to link the terrestrial water balance to the thermal and hydrological regime of the polar sea. Overall, there is a strong need for basic research and long-term monitoring.
Khvorostyanov, D. V., G. Krinner, P. Ciais, P. Heimann, S. A. Zimov (2008). Vulnerability of permafrost carbon to global warming. Part I: model description and role of heat generated by organic matter decomposition. Tellus: Series B 60 (2): 250-264
ABSTRACT: We constructed a new model to study the sensitivity of permafrost carbon stocks to future climate warming. The one-dimensional model solves an equation for diffusion of heat penetrating from the overlying atmosphere and takes into account additional in situ heat production by active soil microorganisms. Decomposition of frozen soil organic matter and produced CO2 and methane fluxes result from an interplay of soil heat conduction and phase transitions, respiration, methanogenesis and methanotrophy processes. Respiration and methanotrophy consume soil oxygen and thus can only develop in an aerated top-soil column. In contrast, methanogenesis is not limited by oxygen and can be sustained within the deep soil, releasing sufficient heat to further thaw in depth the frozen carbon-rich soil organic matter. Heat production that accompanies decomposition and methanotrophy can be an essential process providing positive feedback to atmospheric warming through self-sustaining transformation of initially frozen soil carbon into CO2 and CH4 . This supplementary heat becomes crucial, however, only under certain climate conditions. Oxygen limitation to soil respiration slows down the process, so that the mean flux of carbon released during the phase of intense decomposition is more than two times less than without oxygen limitation. Taking into account methanogenesis increases the mean carbon flux by 20%. Part II of this study deals with mobilization of frozen carbon stock in transient climate change scenarios with more elaborated methane module, which makes it possible to consider more general cases with various site configurations. Part I (this manuscript) studies mobilization of 400 Gt C carbon stock of the Yedoma in response to a stepwise rapid warming focusing on the role of supplementary heat that is released to the soil during decomposition of organic matter.