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Sphagnum mosses are adapted to thrive in cool, nutrient-poor acidic environments. Nitrogen (N2)-fixing and methane (CH4)-oxidising microorganisms living as epiphytes and endophytes on and in Sphagnum mosses are known to support Sphagnum growth. We explored how these processes combined with the microbial community link to different Sphagnum moss species, sites and moss segments (capitulum or shoot), using Sphagnum palustre and Sphagnum fallax sampled from two field sites and separated into upper (capitulum; 0–3 cm) and lower (shoot; 3–6 cm) segments. The results of 16S rRNA gene amplicon sequencing showed that the microbial community composition is both site- and moss-specific. The microbial activity, measured as 15N-N2 fixation (6.5  0.9 µmol g-1 d-1) and 13C-CH4 oxidation (0.5  0.1 µmol g-1 d-1), differed between field sites but not between moss species. The CH4 oxidation was higher in the upper moss segments. On the basis of these results we hypothesise that the discrepancy between microbial activity and microbial community composition at moss-species level may be caused by functional redundancy; although microbial communities vary between Sphagnum species, they can function similarly when the mosses are growing in comparable environments.

Worldwide, drainage-based water management is applied to facilitate agricultural use of coastal peatland areas, leading to large-scale peat oxidation and land subsidence. Meanwhile, there is a strong call for a more sustainable use and management of drained peatlands. Drainage severely enhances greenhouse gas emission and land subsidence, which can be counteracted by rewetting. However, this advocated alternative may also induce unwanted processes affecting ecosystem functioning. In this overview we discuss the complex biogeochemical responses related to drainage and rewetting in peatlands and compare the biogeochemical effects of deep drainage to those of rewetting in order to identify the most sustainable management strategy for drained coastal peatlands. Due to drainage, oxidation of the iron sulphide pool leads to a highly underestimated source of sulphate for adjacent surface waters, indirectly enhancing eutrophication. Although rewetting also enhances eutrophication by mobilisation and discharge of phosphorus to surface waters, we propose this effect may be mitigated by co-discharged iron, binding phosphorus. As rewetting is expected to reduce societal costs and maximises ecosystem services of peatlands, we suggest to rewet drained agricultural peatlands wherever feasible. We discuss several promising, more sustainable, land use alternatives for rewetted coastal peatlands.

Sphagnum mosses are poikilohydric bryophytes, i.e. dependent on nearly-constant wet conditions. Exposure to mineral-enriched water has long been recognised as a threat to Sphagnum mosses and a driver of niche formation. Atrophy of Sphagnum is currently attributed to high pH in combination with high calcium concentration. Because the natural occurrence of high pH regularly auto-correlates with alkalinity and calcium concentration it remains unclear which of these factors is detrimental to Sphagnum. In a ten-week controlled laboratory experiment we measured the effects of high pH and bicarbonate concentration in combination with various cations (Ca2+, Na+, K+, Mg2+, Fe3+) on K+ leakage and survival in Sphagnum palustre L. Increased pH (7.2) combined with low (≤ 200 µmol L-1) bicarbonate concentration had no effect. In contrast, high bicarbonate levels (supplied or formed in solution) combined with pH values of 8.0 and higher produced signs of physiological stress (chlorosis and electrolyte leakage) within two weeks and were toxic in all treatments. Cations failed to modulate the adverse effects of high alkalinity; nor could additional potassium alleviate detrimental effects. This study shows that S. palustre is adversely affected by increased bicarbonate concentration and alkalinity, which both show a tight correlation with pH and often with calcium levels in water and bedrock. The management of groundwater and surface water sources for restoration of Sphagnum-dominated habitats and irrigation of Sphagnum farms should focus on lowering alkalinity levels (including pH), whereas cation concentrations may remain elevated.