Sari Juutinen

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.

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.

Wetland chronosequence as a model of peatland development: Vegetation succession, peat and carbon accumulation

Model validation experiments are fundamental to ensure that the peat growth models correspond with the diversity in nature. We evaluated the Holocene Peatland Model (HPM) simulation against the field observations from a chronosequence of peatlands and peat core data. The ongoing primary peatland formation on the isostatically rising coast of Finland offered us an exceptional opportunity to study the peatland succession along a spatial continuum and to compare it with the past succession revealed by vertical peat sequences. The current vegetation assemblages, from the seashore to a 3000 year old bog, formed a continuum from minerotrophic to ombrotrophic plant communities. A similar sequence of plant communities was found in the palaeovegetation. The distribution of plant functional types was related to peat thickness and water-table depth (WTD) supporting the assumptions in HPM, though there were some differences between the field data and HPM. Palaeobotanical evidence from the oldest site showed a rapid fen–bog transition, indicated by a coincidental decrease in minerotrophic plant functional types and an increase in ombrotrophic plant functional types. The long-term mean rate of carbon (C) accumulation varied from 2 to 34 g C/m2 per yr, being highest in the intermediate age cohorts. Mean nitrogen (N) accumulation varied from 0.1 to 3.9 g N/m2 per yr being highest in the youngest sites. WTD was the deepest in the oldest sites and its variation there was temporally the least but spatially the highest. Evaluation of the HPM simulations against the field observations indicated that HPM reasonably well simulates peatland development, except for very young peatlands.

Vegetation feedbacks of nutrient addition lead to a weaker carbon sink in an ombrotrophic bog

To study vegetation feedbacks of nutrient addition on carbon sequestration capacity, we investigated vegetation and ecosystem CO2 exchange at Mer Bleue Bog, Canada in plots that had been fertilized with nitrogen (N) or with N plus phosphorus (P) and potassium (K) for 7-12 years. Gross photosynthesis, ecosystem respiration, and net CO2 exchange were measured weekly during May-September 2011 using climate-controlled chambers. A substrate-induced respiration technique was used to determine the functional ability of the microbial community. The highest N and NPK additions were associated with 40% less net CO2 uptake than the control. In the NPK additions, a diminished C sink potential was due to a 20-30% increase in ecosystem respiration, while gross photosynthesis rates did not change as greater vascular plant biomass compensated for the decrease in Sphagnum mosses. In the highest N-only treatment, small reductions in gross photosynthesis and no change in ecosystem respiration led to the reduced C sink. Substrate-induced microbial respiration was significantly higher in all levels of NPK additions compared with control. The temperature sensitivity of respiration in the plots was lower with increasing cumulative N load, suggesting more labile sources of respired CO2 . The weaker C sink potential could be explained by changes in nutrient availability, higher woody : foliar ratio, moss loss, and enhanced decomposition. Stronger responses to NPK fertilization than to N-only fertilization for both shrub biomass production and decomposition suggest that the bog ecosystem is N-P/K colimited rather than N-limited. Negative effects of further N-only deposition were indicated by delayed spring CO2 uptake. In contrast to forests, increased wood formation and surface litter accumulation in bogs seem to reduce the C sink potential owing to the loss of peat-forming Sphagnum.

Archaeal rRNA diversity and methane production in deep boreal peat.

Northern peatlands play a major role in the global carbon cycle as sinks for CO(2) and as sources of CH(4). These diverse ecosystems develop through accumulation of partially decomposed plant material as peat. With increasing depth, peat becomes more and more recalcitrant due to its longer exposure to decomposing processes. Compared with surface peat, deeper peat sediments remain microbiologically poorly described. We detected active archaeal communities even in the deep bottom layers (-220/-280 cm) of two Finnish fen-type peatlands by 16S rRNA-based terminal restriction fragment length polymorphism analysis. In the sediments of the northern study site, all detected archaea were methanogens with Rice Cluster II (RC-II) and Methanosaetaceae as major groups. In southern peatland, Crenarchaeota of a rare unidentified cluster were present together with mainly RC-II methanogens. RNA profiles showed a larger archaeal diversity than DNA-based community profiles, suggesting that small but active populations were better visualized with rRNA. In addition, potential methane production measurements indicated methanogenic activity throughout the vertical peat profiles.

Distribution of assimilated carbon in the system Phragmites australis-waterlogged peat soil after carbon-14 pulse labelling.

Abstract Short-term (3–6 days) and long-term (27 days) laboratory experiments were carried out to determine the distribution of assimilated C in the system Phragmites australis (common reed)-waterlogged fen soil after 14C pulse labelling. The investigated system of fen plants and anaerobic organic soil showed different patterns of assimilated 14C distribution when compared to systems with cultivated plants and aerobic mineral soil. Between 90% and 95% of the 14C in the system was found in the reed plants. A maximum of 2% of the assimilated plant 14C was released from the fen soil as CO2 and about 5–9% remained in the soil. The 14C remaining in the waterlogged fen soil of the reed plant had the same amount as that of a cultivated plant in mineral soil, despite lower 14C-release (i.e. rhizodeposition and root respiration) from reed roots. Assuming that root respiration of fen plants is low, this indicates that microbial C turnover in waterlogged fen soil is much slower than in mineral soil. The estimated quantity of the assimilated C remaining in the soil was of an ecologically relevant order of magnitude.

Development, carbon accumulation, and radiative forcing of a subarctic fen over the Holocene

Three-dimensional reconstructions of peatland development patterns, carbon (C) dynamics and the related radiative forcing (RF) were analyzed to improve understanding of peatland–climate feedback mechanisms. We investigated vertical and horizontal peat growth patterns of a subarctic fen (Lompolojänkkä) located in Finnish Lapland. We calculated C accumulation rates and, based on these and modern gas exchange measurements, developed different scenarios of Holocene carbon dioxide (CO2) and methane (CH4) fluxes and reconstructed the RF driven by these fluxes. Holocene C accumulation rates at Lompolojänkkä ranged between 2 and 30 g C/m2/yr. Plant macrofossil analysis suggests that a fen environment prevailed throughout the Holocene. However, net C accumulation rates showed an extended period of low C accumulation during the mid-Holocene. At the same time, the lateral expansion rate of the peat area also decreased. The C flux scenarios resulted in a positive RF effect (warming) on the atmosphere following peat initiation. The RF turned negative (cooling) several hundreds to 2000 years after peat initiation. Subsequently, the cooling effect increased steadily and was only temporarily interrupted when CH4 emissions were forced to increase in the model. Although climate had an important effect on peatland C dynamics, its influence on RF was buffered by the long-term history of C uptake.

The contribution of Phragmites australis litter to methane (CH4) emission in planted and non-planted fen microcosms

The contribution of Phragmites australis (Cav.) Trin. ex Steud. (common reed) litter as an origin of CH4-C was studied in a microcosm experiment. 14C-labelled, dried and ground P. australis root and shoot litter was buried in waterlogged, planted or non-planted fen microcosms. The evolution of 14CO2 and 14CH4 from the pots was monitored during the 35-day experiment. The 14C activity in the shoots and roots of the plants, soil, and inundation water was also analysed at the end of the experiment. Up to 40% of the released CH4-C originated from the added litter, whereas the rest originated from old soil organic matter. The comparison of planted and non-planted pots suggested that the contribution of recent plant-derived C (i.e. root exudates) to CH4 emission was negligible. The proportion of litter-derived CH4-C was significantly higher in the planted pots, suggesting that the presence of plants enhanced the formation of CH4 from litter. The major part of the initial 14C activity was recovered from the soil. About 3% was recovered from the inundation water, about 10% was emitted as CO2, and only <0.01% as CH4. However, these results demonstrated that plant litter and old soil organic matter are the major sources of CH4-C in fens during the early growth stage of P. australis.