Eeva-Stiina Tuittila

Annual cycle of methane emission from a boreal fen measured by the eddy covariance technique

The northern wetlands are one of the major sources of methane into the atmosphere. We measured annual methane emission from a boreal minerotrophic fen, Siikaneva, by the eddy covariance method. The average wintertime emissions were below 1 mg m-2 h-1, and the summertime emissions about 3.5 mg m-2 h-1. The water table depth did have any clear effect on methane emissions. During most of the year the emission depended on the temperature of peat below the water table. However, during the high and late summer the emission was independent on peat temperature as well. No diurnal cycle of methane flux was found. The total annual emission from the Siikaneva site was 12.6 g m-2. The emissions of the snow free period contributed 91% to the annual emission. The emission pulse during the snow melting period was clearly detectable but of minor importance adding only less than 3% to the annual emission. Over 20% of the carbon assimilated during the year as carbon dioxide was emitted as methane. Thus methane emission is an important component of the carbon balance of the Siikaneva fen. This indicates need of taking methane into account when studying carbon balances of northern fen ecosystems.

Restored cut-away peatland as a sink for atmospheric CO2

Abstract In a field study, we examined the relationship between vegetation, abiotic factors and the CO2 exchange dynamics of a cut-away peatland 20 years after production had ended. The main objective was to determine the effect of rewetting on the CO2 exchange dynamics, measured separately in Eriophorum vaginatum tussocks and intertussocks (almost non-vegetated surfaces) using closed-chamber techniques, one growing season before and three growing seasons after the rewetting treatment. Rewetting lowered total respiration (RTOT) and increased gross photosynthesis (PG), which resulted in a higher incorporation of CO2 into the system. The seasonal CO2 balance for the almost continuously submerged section of the rewetted site became positive 2 years after rewetting (9.1 g CO2-C m−2), and it was still higher in the 3rd year (64.5 g CO2-C m−2), i.e. the system accumulated carbon. In the driest section of the rewetted site the seasonal balance increased strongly, but the balance was still negative during the 3 years following rewetting with losses from the system of 44.1, 26.1, 38.3 g CO2-C m−2 in 1995, 1996 and 1997 respectively. At the control site seasonal balance during 1995–1997 varied between ecosystem C losses of 41.8 and 95.3 in an area with high Eriophorum cover and between 52.1 and 109.9 g CO2-C m−2 with lower cover. Simulation of a cut-away peatland with dense Eriophorum vegetation (Eriophorum cover 70%) showed that if the water level (WT) is low, the seasonal CO2 balance of the ecosystem can reach the compensation point of no net C change (PG = RTOT) only if weather conditions are favourable, but with a high WT the seasonal CO2 balance would be positive even under varying weather conditions. It can be concluded that with dense Eriophorum vegetation a restored cut-away peatland acts as a functional mire and becomes a sink for atmospheric CO2.

Status and restoration of peatlands in northern Europe

Environmental management of peatlands,landscape ecology and protection of keybiotopes have created needs and pressure torestore drained peatlands to natural mireecosystems. Here, we summarize differentapproaches and restoration techniquesdeveloped for peatland management inEstonia, Sweden, and Finland wherepeatlands are abundant. Without rewetting,plant colonisation on abandoned cut-awayareas is slow due to harsh hydrological andmicroclimatic conditions. However, after restoration, cut-away peatlands may returnto a functional state close to that ofpristine mires, and therefore restore a netcarbon sink function within a few years. Inaddition, restoration techniques can helpto create buffer zones between terrestrialand limnic ecosystems that reduces thenutrient loading imposed on watercourses byforestry operations. Restoration may alsobe important for peatland conservationprograms as drained peatlands are part ofpresent and future conservation areas.Finally, restoration actions in themselvescan have negative environmental impacts.For instance, inundation of peat surfacesresulting from the rewetting process oftenincreases phosphorus leaching. Efforts onpeatland restoration should focus onenvironmental monitoring, research onrestoration and its environmental impact aswell as public relations activities. Inthat respect, knowledge transfer betweenacademics and managers should generatesynergy benefits.

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.

CO2 exchange of a sedge fen in southern Finland—the impact of a drought period

Eddy covariance (EC) measurements of net ecosystem CO2 exchange (NEE) were conducted on a boreal sedge fen in southern Finland (61°50’N, 24.12’E) during a 1.5-yr period covering two summers in 2004.2005. The EC data were complemented by chamber measurements, which enabled the partition of the daytime NEE into respiration and photosynthesis. A special emphasis was put on the hydrometeorological responses of CO2 exchange during a drought period in July 2005. A mean CO2 efflux of 0.009 mg CO2 m-2 s-1 was observed during mid-winter (January.February), while the night-time respiration during the two Julys averaged 0.09 mgCO2 m-2 s-1. During both years the mean midday uptake in late July was about -0.16 mgCO2 m-2 s-1. An annual CO2 balance of -188 g CO2 m-2 was observed in 2005.Aslightly higher net sink of -219 gCO2 m-2 was estimated for 2004. The drought period experienced in July 2005 caused a clear depression in the daily NEE values. From the combined analysis of EC and chamber measurements it was concluded that this was mainly due to increased respiration, but evidence was also found of suppressed photosynthesis due to a high VPD.

Spatial variation in plant community functions regulates carbon gas dynamics in a boreal fen ecosystem

The aim of this study was to asses how the variability in carbon gas exchange at the plant community scale affected the C gas exchange estimates at the ecosystem scale in a fen that was homogeneous in a micrometeorological sense, that is, had an even surface topography and plant cover. CO2 and CH4 exchange was measured at the plant community scale with chambers and at the ecosystem scale with the eddy covariance (EC) technique. Community-scale measurements were upscaled to the ecosystem scale by weighting the community-specific estimates by the area of the community. All communities were net CO2 sinks and CH4 sources during the growing season, but net ecosystem production (NEP) and CH4 emissions ranged from 21 to 190 g CO2-C m-2 and from 4.3 to 13 g CH4-C m-2, respectively, between the communities. The seasonal estimates of NEP and CH4, upscaled to the 200 m radius from the EC tower, were 82 and 7.9 g CH4-C m-2, which agreed well with the EC measurements. As the communities differed markedly in their C gas dynamics, their proportions controlled the ecosystem scale estimates. Successful upscaling required detailed knowledge on the proportions and leaf area of the communities.

The role of Sphagnum mosses in the methane cycling of a boreal mire.

Peatlands are a major natural source of atmospheric methane (CH4). Emissions from Sphagnum-dominated mires are lower than those measured from other mire types. This observation may partly be due to methanotrophic (i.e., methane-consuming) bacteria associated with Sphagnum. Twenty-three of the 41 Sphagnum species in Finland can be found in the peatland at Lakkasuo. To better understand the Sphagnum-methanotroph system, we tested the following hypotheses: (1) all these Sphagnum species support methanotrophic bacteria; (2) water level is the key environmental determinant for differences in methanotrophy across habitats; (3) under dry conditions, Sphagnum species will not host methanotrophic bacteria; and (4) methanotrophs can move from one Sphagnum shoot to another in an aquatic environment. To address hypotheses 1 and 2, we measured the water table and CH4 oxidation for all Sphagnum species at Lakkasuo in 1-5 replicates for each species. Using this systematic approach, we included Sphagnum spp. with narrow and broad ecological tolerances. To estimate the potential contribution of CH4 to moss carbon, we measured the uptake of delta13C supplied as CH4 or as carbon dioxide dissolved in water. To test hypotheses 2-4, we transplanted inactive moss patches to active sites and measured their methanotroph communities before and after transplantation. All 23 Sphagnum species showed methanotrophic activity, confirming hypothesis 1. We found that water level was the key environmental factor regulating methanotrophy in Sphagnum (hypothesis 2). Mosses that previously exhibited no CH4 oxidation became active when transplanted to an environment in which the microbes in the control mosses were actively oxidizing CH4 (hypothesis 4). Newly active transplants possessed a Methylocystis signature also found in the control Sphagnum spp. Inactive transplants also supported a Methylocystis signature in common with active transplants and control mosses, which rejects hypothesis 3. Our results imply a loose symbiosis between Sphagnum spp. and methanotrophic bacteria that accounts for potentially 10-30% of Sphagnum carbon.

Methanotrophy induces nitrogen fixation during peatland development

Significance In peatlands, the external sources of nitrogen are mainly atmospheric, but the atmospheric nitrogen deposition alone cannot explain the long-term annual nitrogen accumulation rates to these ecosystems. Because of methodological problems, methane-induced fixation of atmospheric dinitrogen gas has been previously overlooked as an additional nitrogen input mechanism. We found that the activity of methane-oxidizing bacteria provides not only carbon but also nitrogen to peat mosses and, thus, contributes to carbon and nitrogen accumulation in peatlands, which store approximately one-third of the global soil carbon pool. Our results imply that nitrogen fixation in wetlands may be strongly underestimated when methods inhibiting methane oxidizers are used. Nitrogen (N) accumulation rates in peatland ecosystems indicate significant biological atmospheric N2 fixation associated with Sphagnum mosses. Here, we show that the linkage between methanotrophic carbon cycling and N2 fixation may constitute an important mechanism in the rapid accumulation of N during the primary succession of peatlands. In our experimental stable isotope enrichment study, previously overlooked methane-induced N2 fixation explained more than one-third of the new N input in the younger peatland stages, where the highest N2 fixation rates and highest methane oxidation activities co-occurred in the water-submerged moss vegetation.

Seasonality of rDNA- and rRNA-derived archaeal communities and methanogenic potential in a boreal mire

Methane (CH4) emissions from boreal wetlands show considerable seasonal variation, including small winter emissions. We addressed the seasonality of CH4-producing microbes by comparing archaeal communities and the rates and temperature response of CH4 production in a boreal fen at three key phases of growing season and in winter. Archaeal community analysis by terminal restriction fragment length polymorphism and cloning of 16S ribosomal DNA and reverse-transcribed RNA revealed slight community shifts with season. The main archaeal groups remained the same throughout the year and were Methanosarcinaceae, Rice cluster II and Methanomicrobiales-associated Fen cluster. These methanogens and the crenarchaeal groups 1.1c and 1.3 were detected from DNA and RNA, but the family Methanosaetaceae was detected only from RNA. Differences between DNA- and RNA-based results suggested higher stability of DNA-derived communities and better representation of the active CH4 producers in RNA. Methane production potential, measured as formation of CH4 in anoxic laboratory incubations, showed prominent seasonality. The potential was strikingly highest in winter, possibly due to accumulation of methanogenic substrates, and maximal CH4 production was observed at ca. 30 °C. Archaeal community size, determined by quantitative PCR, remained similar from winter to summer. Low production potential in late summer after a water level draw-down suggested diminished activity due to oxygen exposure. Our results indicated that archaeal community composition and size in the boreal fen varied only slightly despite the large fluctuations of methanogenic potential. Detection of mRNA of the methanogenic mcrA gene confirmed activity of methanogens in winter, accounting for previously reported winter CH4 emissions.

Impact of rewetting on the vegetation of a cut‐away peatland

We tested whether rewetting improved environ- mental conditions during peatland restoration and promoted colonization and development of mire vegetation. Vegetation change was monitored in a cut-away peatland one year before, and four years after, rewetting. Colonizers before rewetting were perennials, mostly typical of hummocks or bare peat surfaces. The main variation in vegetation was related to variation in the amounts of major nutrients and water table level. The wettest site with the highest nutrient level had the highest total vegetation cover and diversity, as well as some species typical of wet minerotrophic mires. Raising the water table level above, or close to, the soil surface promoted devel- opment of wet minerotrophic vegetation. Diversity initially decreased because of the disappearance of hummock vegeta- tion but started to recover in the third year. Eriophorum vaginatum and Carex rostrata were both favoured, and bryophytes typical of wet habitats colonized the site. Moder- ate rewetting promoted the development of Eriophorum vaginatum seedlings and an increase in the cover of tussocks. Bryophytes typical of disturbed peat surfaces spread. In the control site development continued slowly towards closed hummock vegetation. The study showed that raising the water level to, or above, soil surface promotes conditions wet enough for a rapid succession towards closed mire vegetation.