Jukka Alm

CARBON BALANCE OF A BOREAL BOG DURING A YEAR WITH AN EXCEPTIONALLY DRY SUMMER

Northern peatlands are important terrestrial carbon stores, and they show large spatial and temporal variation in the atmospheric exchange of CO2 and CH4. Thus, annual carbon balance must be studied in detail in order to predict the climatic responses of these ecosystems. Closed-chamber methods were used to study CO2 and CH4 in hollow, Sphagnum angustifolium lawn, S. fuscum lawn, and hummock microsites within an om- brotrophic S. fuscum bog. Micrometeorological tower measurements were used as a ref- erence for the CH4 efflux from the bog. Low precipitation during May-August in 1994 (84 mm below the long-term average for the same period) and a warm July-August period caused the water table to drop by more than 15 cm below the peat surface in the hollows and to 48 cm below the surface in high hummocks. Increased annual total respiration exceeded gross production and resulted in a net C loss of 4-157 g/m2 in the different microsites. Drought probably caused irreversible desiccation in some lawns of S. angus- tifolium and S. balticum and in S. fuscum in the hummocks, while S. balticum growing in hollows retained its moisture and even increased its photosynthetic capacity during the July-August period. Seasonal (12 May-4 October) CH4 emissions ranged from 2 g CH4- C/M2 in drier S. fuscum hummocks and lawns to 7 and 14 g/m2 in wetter S. angustifolium- S. balticum lawns and hollows, respectively. Aerodynamic gradient measurements at the tower showed slightly higher CH4 flux rates than the average estimates for the whole bog obtained by closed-chamber methods. Winter C efflux comprised 30 g C02-C/m2 and 1 g CH4-C/m2 out of a total loss of 90 g C/M2 on average in the bog, and there was an estimated annual loss of 7 g C/M2 by leaching. This study shows how delicately the boreal bogs C balance in different microsites depends on climatic variations, especially the distribution of precipitation. It also confirms that severe C losses can occur in boreal bogs during extended summer droughts, even in years with annual temperatures close to the long-term average and with precipitation clearly greater than the long-term average.

Methane fluxes on boreal peatlands of different fertility and the effect of long‐term experimental lowering of the water table on flux rates

Methane (CH4) fluxes were measured at 17 peatland sites with different nutritional and hydrological characteristics in the southern and middle boreal zones in Finland by a static chamber technique. Many of the natural peatlands also had counterparts drained for forestry 30–50 years ago. The mean emissions from May to September were 8.0 g CH4 m−2 for natural ombrogenous bogs and 19.0 g CH4 m−2 for natural minerogenous fens thus being higher than the 2 g CH4 m−2 yr−1 estimated for the Canadian peatlands. Change in the mean water table level had greater effect on CH4 fluxes on natural fens than on natural bogs. The mean CH4 emissions on drained bogs and fens were 3.9 g CH4 m−2 and 0.3 g CH4m−2, respectively. Some drained fens even had CH4 uptake from the atmosphere. The change in the mean water table had the lowest effect on CH4 fluxes on drained peatlands. The CH4 fluxes on peatlands (natural fens and bogs and drained peatlands) are associated with peat aeration, nutrient level, vegetation cover, peat compaction, peat temperature, and finally with microbial processes responsible for the net release of CH4. We could explain 67% of the variation in mean CH4 fluxes on Finnish peatlands by measuring the mean water table, peat bulk density, and peat pH. The present results can be used to predict the possible changes in CH4 emissions on peatlands if the climate is drying in north. For example, lowering of the present water table by 10 cm would induce a 70% reduction in the CH4 emissions from fens and a 45% reduction from bogs.

Fluxes of methane, carbon dioxide and nitrous oxide in boreal lakes and potential anthropogenic effects on the aquatic greenhouse gas emissions.

We have examined how some major catchment disturbances may affect the aquatic greenhouse gas fluxes in the boreal zone, using gas flux data from studies made in 1994-1999 in the pelagic regions of seven lakes and two reservoirs in Finland. The highest pelagic seasonal average methane (CH(4)) emissions were up to 12 mmol x m(-2) x d(-1) from eutrophied lakes with agricultural catchments. Nutrient loading increases autochthonous primary production in lakes, promoting oxygen consumption and anaerobic decomposition in the sediments and this can lead to increased CH(4) release from lakes to the atmosphere. The carbon dioxide (CO(2)) fluxes were higher from reservoirs and lakes whose catchment areas were rich in peatlands or managed forests, and from eutrophied lakes in comparison to oligotrophic and mesotrophic sites. However, all these sites were net sources of CO(2) to the atmosphere. The pelagic CH(4) emissions were generally lower than those from the littoral zone. The fluxes of nitrous oxide (N(2)O) were negligible in the pelagic regions, apparently due to low nitrate inputs and/or low nitrification activity. However, the littoral zone, acting as a buffer for leached nitrogen, did release N(2)O. Anthropogenic disturbances of boreal lakes, such as increasing eutrophication, can change the aquatic greenhouse gas balance, but also the gas exchange in the littoral zone should be included in any assessment of the overall effect. It seems that autochthonous and allochthonous carbon sources, which contribute to the CH(4) and CO(2) production in lakes, also have importance in the greenhouse gas emissions from reservoirs.

Reconstruction of the carbon balance for microsites in a boreal oligotrophic pine fen, Finland

Abstract Carbon dioxide (CO2) exchange was studied at flark (minerotrophic hollow), lawn and hummock microsites in an oligotrophic boreal pine fen. Statistical response functions were constructed for the microsites in order to reconstruct the annual CO2 exchange balance from climate data. Carbon accumulation was estimated from the annual net CO2 exchange, methane (CH4) emissions and leaching of carbon. Due to high water tables in the year 1993, the average carbon accumulation at the flark, Eriophorum lawn, Carex lawn and hummock microsites was high, 2.91, 6.08, 2.83 and 2.66 mol C m–2, respectively, and for the whole peatland it was 5.66 mol m–2 year–1. During the maximum primary production period in midsummer, hummocks with low water tables emitted less methane than predicted from the average net ecosystem exchange (NEE), while the Carex lawns emitted slightly more. CH4 release during that period corresponded to 16% of the contemporary NEE. Annual C accumulation rate did not correlate with annual CH4 release in the microsites studied, but the total community CO2 release seemed to be related to CH4 emissions in the wet microsites, again excluding the hummocks. The dependence of CO2 exchange dynamics on weather events suggests that daily balances in C accumulation are labile and can change from net carbon uptake to net release, primarily in high hummocks on fens under warmer, drier climatic conditions.

Winter CO2, CH4 and N2O fluxes on some natural and drained boreal peatlands

CO2 and CH4 fluxes during the winter were measured at natural and drained bog and fen sites in eastern Finland using both the closed chamber method and calculations of gas diffusion along a concentration gradient through the snowpack. The snow diffusion results were compared with those obtained by chamber, but the winter flux estimates were derived from chamber data only. CH4 emissions from a poor bog were lower than those from an oligotrophic fen, while both CO2 and CH4 fluxes were higher in theCarex rostrata- occupied marginal (lagg) area of the fen than in the slightly less fertile centre. Average estimated winter CO2-C losses from virgin and drained forested peatlands were 41 and 68 g CO2-C m−2, respectively, accounting for 23 and 21% of the annual total CO2 release from the peat. The mean release of CH4-C was 1.0 g in natural bogs and 3.4 g m−2 in fens, giving rise to winter emissions averaging to 22% of the annual emission from the bogs and 10% of that from the fens. These wintertime carbon gas losses in Finnish natural peatlands were even greater than reported average long-term annual C accumulation values (less than 25g C m−2). The narrow range of 10–30% of the proportion of winter CO2 and CH4 emissions from annual emissions found in Finnish peatlands suggest that a wider generalization in the boreal zone is possible. Drained forested bogs emitted 0.3 g CH4-C m−2 on the average, while the effectively drained fens consumed an average of 0.01 g CH4-C m−2. Reason for the low CH4. efflux or net oxidation in drained peatlands probably lies in low substrate supply and thus low CH4 production in the anoxic deep peat layers. N2O release from a fertilized grassland site in November–May was 0.7 g N2O m−2, accounting for 38% of the total annual emission, while a forested bog released none and two efficiently drained forested fens 0.09 (28% of annual release) and 0.04 g N2O m−2 (27%) during the winter, respectively.

Change in fluxes of carbon dioxide, methane and nitrous oxide due to forest drainage of mire sites of different trophy

Northern peatlands accumulate atmospheric CO2 thus counteracting climate warming. However, CH4 which is more efficient as a greenhouse gas than CO2, is produced in the anaerobic decomposition processes in peat. When peatlands are taken for forestry their water table is lowered by ditching. We studied long-term effects of lowered water table on the development of vegetation and the annual emissions of CO2, CH4 and N2O in an ombrotrophic bog and in a minerotrophic fen in Finland. Reclamation of the peat sites for forestry had changed the composition and coverage of the field and ground layer species, and increased highly the growth of tree stand at the drained fen. In general, drainage increased the annual CO2 emissions but the emissions were also affected by the natural fluctuations of water table. In contrast to CO2, drainage had decreased the emissions of CH4, the drained fen even consumed atmospheric CH4. CO2 and CH4 emissions were higher in the virgin fen than in the virgin bog. There were no N2O emissions from neither type of virgin sites. Drainage had, however, highly increased the N2O emissions from the fen. The results suggest that post-drainage changes in gas fluxes depend on the trophy of the original mires.

Seasonal variation in CH4 emissions and production and oxidation potentials at microsites on an oligotrophic pine fen

Abstract Temporal and spatial variation in CH4 emissions was studied at hummock, Eriophorum lawn, flark and Carex lawn microsites in an oligotrophic pine fen over the growing season using a static chamber method, and CH4 production and oxidation potentials in peat profiles from hummock and flark were determined in laboratory incubation experiments. Emissions were lowest in the hummocks, and decreased with increasing hummock height, while in the lawns and flarks they increased with increasing sedge cover. Statistical response functions with water table and peat temperature as independent variables were calculated in order to reconstruct seasonal CH4 emissions by reference to the time series for peat temperature and water table specific to each microsite type. Mean CH4 emissions in the whole area in the snow-free period of 1993, weighted in terms of the proportions of the microsites, were 1.7 mol CH4 m–2. Potential CH4 production and oxidation rates were very low in the hummocks rising above the groundwater table, but were relatively similar when expressed per dry weight of peat both in the hummocks and flarks below the water table. The CH4 production potential increased in autumn at both microsites and CH4 oxidation potential seemed to decrease. The decrease in temperature in autumn certainly reduced in situ decomposition processes, possibly leaving unused substrates in the peat, which would explain the increase in CH4 production potential.

Methane production and oxidation potentials in relation to water table fluctuations in two boreal mires

We studied the response of methane production and oxidation potentials in a minerotrophic and an ombrotrophic mire to water table fluctuations. In profiles where water table had not varied, the water-saturated layers showed significant potentials while the unsaturated layers did not. The production potentials in the saturated layers below water level ranged from 0.1 to 2.4 m gC H4 h ˇ1 (g d.w.) ˇ 1 and oxidation potentials (first order reaction rate constants) betweenˇ0.010 andˇ0.120 h ˇ1 (g d.w.) ˇ 1 . In profiles with constant water level, the maximal production potential occurred 20 cm and maximal oxidation potential 10 cm below water level. When water table varied only a little, production potentials slightly increased towards the autumn. After a water level draw-down, in the profiles from the dry microsites, production and oxidation potentials were detected in layers that had been unsaturated up to 6 weeks. The maximal oxidation zone was shifted downwards during low water periods. In a wet microsite, a 2 week period of unsaturation eliminated the production potentials and decreased the oxidation potentials. After a rise in the water level, the potentials were reactivated more rapidly in the wet than in the dry microsites. # 1999 Elsevier Science Ltd. All rights reserved.

The contribution of plant roots to CO2 fluxes from organic soils

The CO2 released in soil respiration is formed from organic matter which differs in age and stability, ranging from soluble root exudates to more persistent plant remains. The contribution of roots, a relatively fast component of soil cycling, was studied in three experiments. (1) Willows were grown in a greenhouse and CO2 fluxes from the substrate soil (milled peat) and from control peat were measured. (2) CO2 fluxes from various peatland sites were measured at control points and points where the roots were severed from the plants. (3) CO2 fluxes in cultivated grassland established on peatland were measured in grassy subsites and in subsites where the growth of grass was prevented by regular tilling. The root-derived respiration followed the typical annual phenology of the vegetation, being at its maximum in the middle and late summer. All the experiments gave similar results, root-derived respiration accounting for 35–45% of total soil respiration in the middle and late summer at sites with an abundant vegetation. The root-derived respiration from the virgin peatland sites correlated well with the tree biomass, and also partly with the understorey vegetation, but in the drained sites the root effect was greater, even in the presence of less understorey vegetation than at virgin subsites.

High-resolution reconstruction of wetness dynamics in a southern boreal raised bog, Finland, during the late Holocene : a quantitative approach

A high-resolution plant macrofossil analysis was applied to investigate wetness dynamics in a southern Finnish boreal bog, Kontolanrahka, during the last 5000 years. The chronological control and the age—depth model were based on 40 AMS radiocarbon dates. Pollen analysis provided information on regional-scale vegetation changes. Macrofossil analysis revealed prominent changes in vegetation assemblages during the late Holocene, indicating fluctuations in water-table. The reconstruction suggests that at the coring point, which nowadays is a relatively wet lawn, habitat type has repeatedly varied between transient communities similar to those currently represented in dry hummocks, very wet lawns and even hollows. In order to quantify historical changes in water-table, Generalized Additive Models (GAM) were used to investigate the current relationships between surface plant species and water-table depth. Modern water-table measurements and a survey of associated plant communities along moisture gradients provided data for GAM-analyses. The plant species showed unimodal distributions with apparent optima and narrow tolerances along the water-table gradient. A transfer function for water-table reconstruction was created by calibrating plant macrofossil records against the modern vegetation/water-table relationship using the weighted averaging partial least squares (WA-PLS) regression method. The quantitative water-table reconstruction for the late Holocene showed that the water-table depth had varied between 38 and 2.5 cm, the root mean square error of prediction being 3 cm. The detected historical wet and dry shifts were compared with other similar data from Finland, Sweden and Estonia, and from Western Europe. Despite some similarities, especially during the last c. 1000 years, noticeable differences in timing and duration occur, suggesting they may not have been driven only by climate, but also by local factors, including surface fires.