cThe importance of anaerobic oxidation of methane (AOM) as a methane sink in freshwater systems is largely unexplored, particularly in peat ecosystems. Nitrite-dependent anaerobic methane oxidation (n-damo) was recently discovered and reported to be catalyzed by the bacterium "Candidatus Methylomirabilis oxyfera," which is affiliated with the NC10 phylum. So far, several "Ca. Methylomirabilis oxyfera" enrichment cultures have been obtained using a limited number of freshwater sediments or wastewater treatment sludge as the inoculum. In this study, using stable isotope measurements and porewater profiles, we investigated the potential of n-damo in a minerotrophic peatland in the south of the Netherlands that is infiltrated by nitrate-rich ground water. Methane and nitrate profiles suggested that all methane produced was oxidized before reaching the oxic layer, and NC10 bacteria could be active in the transition zone where countergradients of methane and nitrate occur. Quantitative PCR showed high NC10 bacterial cell numbers at this methane-nitrate transition zone. This soil section was used to enrich the prevalent NC10 bacteria in a continuous culture supplied with methane and nitrite at an in situ pH of 6.2. An enrichment of nitritereducing methanotrophic NC10 bacteria was successfully obtained. Phylogenetic analysis of retrieved 16S rRNA and pmoA genes showed that the enriched bacteria were very similar to the ones found in situ and constituted a new branch of NC10 bacteria with an identity of less than 96 and 90% to the 16S rRNA and pmoA genes of "Ca. Methylomirabilis oxyfera," respectively. The results of this study expand our knowledge of the diversity and distribution of NC10 bacteria in the environment and highlight their potential contribution to nitrogen and methane cycles.

Sphagnum-bog ecosystems have a limited capability to retain carbon and nutrients when subjected to increased nitrogen (N) deposition. Although it has been proposed that phosphorus (P) can dilute negative effects of nitrogen by increasing biomass production of Sphagnum mosses, it is still unclear whether P-addition can alleviate physiological N-stress in Sphagnum plants. A 3-year fertilisation experiment was conducted in lawns of a pristine Sphagnum magellanicum bog in Patagonia, where competing vascular plants were practically absent. Background wet deposition of nitrogen was low (≈ 0.1-0.2 g · N · m(-2) · year(-1)). Nitrogen (4 g · N · m(-2) · year(-1)) and phosphorus (1 g · P · m(-2) · year(-1)) were applied, separately and in combination, six times during the growing season. P-addition substantially increased biomass production of Sphagnum. Nitrogen and phosphorus changed the morphology of Sphagnum mosses by enhancing height increment, but lowering moss stem density. In contrast to expectations, phosphorus failed to alleviate physiological stress imposed by excess nitrogen (e.g. amino acid accumulation, N-saturation and decline in photosynthetic rates). We conclude that despite improving growth conditions by P-addition, Sphagnum-bog ecosystems remain highly susceptible to nitrogen additions. Increased susceptibility to desiccation by nutrients may even worsen the negative effects of excess nitrogen especially in windy climates like in Patagonia.

Globally, coastal lowlands are becoming more saline by the combined effects of sea level rise, land subsidence and altered hydrological and climatic conditions. Although salinization is known to have a great influence on biogeochemical processes, literature shows contrasting effects that challenge the prediction of future effects. In addition, the effects of fluctuating salinity levels, a more realistic scenario than constant levels, on nutrient cycling in coastal wetland sediments have hardly been examined. A better understanding is therefore crucial for the prediction of future effects and the definition of effective management. To test the effects of constantly brackish water (50 mmol Cl l -1 , 3.2 psu) or fluctuating salinity (5-50 mmol Cl l -1 ), versus constantly low salinity (5 mmol Cl l -1 , 0.32 psu) on nutrient biogeochemistry, we conducted a controlled laboratory experiment with either peat or clay sediments from coastal wetlands. Increased salinity showed to have a fast and large effect. Sediment cation exchange appeared to be the key process explaining both a decrease in phosphorus availability (through calcium mobilization) and an increase in nitrogen availability, their extent being strongly dependent on sediment type. Supply of brackish water decreased surface water turbidity and inhibited sediment methane production but did not affect CO 2 production. Constant and fluctuating salinity levels showed similar longer term effects on nutrient and carbon cycling. The contrasting effects of salinization found for nitrogen and phosphorus, and its effects on water turbidity indicate major ecological consequences for coastal wetlands and have important implications for water management and nature restoration.

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