Tuula Larmola

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.

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.

Peatland succession induces a shift in the community composition of Sphagnum-associated active methanotrophs

Sphagnum-associated methanotrophs (SAM) are an important sink for the methane (CH4) formed in boreal peatlands. We aimed to reveal how peatland succession, which entails a directional change in several environmental variables, affects SAM and their activity. Based on the pmoA microarray results, SAM community structure changes when a peatland develops from a minerotrophic fen to an ombrotrophic bog. Methanotroph subtypes Ia, Ib, and II showed slightly contrasting patterns during succession, suggesting differences in their ecological niche adaptation. Although the direct DNA-based analysis revealed a high diversity of type Ib and II methanotrophs throughout the studied peatland chronosequence, stable isotope probing (SIP) of the pmoA gene indicated they were active mainly during the later stages of succession. In contrast, type Ia methanotrophs showed active CH4 consumption in all analyzed samples. SIP-derived (13)C-labeled 16S rRNA gene clone libraries revealed a high diversity of SAM in every succession stage including some putative Methylocella/Methyloferula methanotrophs that are not detectable with the pmoA-based approach. In addition, a high diversity of 16S rRNA gene sequences likely representing cross-labeled nonmethanotrophs was discovered, including a significant proportion of Verrucomicrobia-related sequences. These results help to predict the effects of changing environmental conditions on SAM communities and activity.

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.

Terrestrial nitrogen cycling in Earth system models revisited

Understanding the degree to which nitrogen (N) availability limits land carbon (C) uptake under global environmental change represents an unresolved challenge. First-generation ‘C-only’ vegetation models, lacking explicit representations of N cycling, projected a substantial and increasing land C sink under rising atmospheric CO2 concentrations. This prediction was questioned for not taking into account the potentially limiting effect of N availability, which is necessary for plant growth (Hungate et al., 2003). More recent global models include coupled C and N cycles in land ecosystems (C–N models) and are widely assumed to be more realistic. However, inclusion of more processes has not consistently improved their performance in capturing observed responses of the global C cycle (e.g. Wenzel et al., 2014). With the advent of a new generation of global models, including coupled C, N, and phosphorus (P) cycling, model complexity is sure to increase; but model reliability may not, unless greater attention is paid to the correspondence of model process representations and empirical evidence. It was in this context that the ‘Nitrogen Cycle Workshop’ at Dartington Hall, Devon, UK was held on 1–5 February 2016. Organized by I. Colin Prentice and Benjamin D. Stocker (Imperial College London,UK), the workshopwas funded by theEuropeanResearchCouncil, project ‘Earth systemModelBias Reduction and assessing AbruptClimate change’ (EMBRACE).We gathered empirical ecologists and ecosystem modellers to identify key uncertainties in terrestrial C–N cycling, and to discuss processes that are missing or poorly represented in current models.

Long-term nutrient addition increased CH4 emission from a bog through direct and indirect effects

Peatlands are globally significant sources of atmospheric methane (CH4). While several studies have examined the effects of nutrient addition on CH4 dynamics, there are few long-term peatland fertilization experiments, which are needed to understand the aggregated effects of nutrient deposition on ecosystem functioning. We investigated responses of CH4 flux and production to long-term field treatments with three levels of N (1.6–6.4 g m−2 yr−1 as NH4NO3), potassium and phosphorus (PK, 5.0 g P and 6.3 g K m−2 yr−1 as KH2PO4), and NPK in a temperate bog. Methane fluxes were measured in the field from May to August in 2005 and 2015. In 2015 CH4 flux was higher in the NPK treatment with 16 years of 6.4 g N m−2 yr−1 than in the control (50.5 vs. 8.6 mg CH4 m−2 d−1). The increase in CH4 flux was associated with wetter conditions derived from peat subsidence. Incubation of peat samples, with and without short-term PK amendment, showed that potential CH4 production was enhanced in the PK treatments, both from field application and by amending the incubation. We suggest that changes in this bog ecosystem originate from long-term vegetation change, increased decomposition and direct nutrient effects on microbial dynamics.