Interactions of Sphagnum with Water and Air

Abstract

Peat-accumulating wetlands occupy 2–3% of the Earth’s land surface. Sphagnum, an important constituent of much of the peatland vegetation, is responsible for initiating acid conditions in ombrotrophic bogs and, because it decays disproportionately slowly, becomes over-represented in peat. Several features of Sphagnum physiology are important: (1) the plant produces polyuronic acids which, by cation exchange, release H⁺ into the bog watery (2) it is sensitive to the combination of high pH and high Ca²⁺ concentration together, though not to each separately; (3) it is sensitive to even moderate concentrations of o-phosphate, NO₃ ⁻ and NH₄ ⁺; and (4) it is sensitive to moderate concentrations of HSO₃ ⁻.

Cation exchange may be an important source of acidity in some bogs but is probably less important generally than was once thought. The role of coloured organic acids as primary sources of acid is not clear. Acid rain sensu stricto has not been shown to affect Sphagnum, but atmospheric pollution in the wide sense is responsible for its disappearance from badly polluted areas of the southern Pennines.

Since the last glaciation, peatlands have been a ‘sink’ for atmospheric carbon, but some bogs in Europe, at least, are becoming less effective as they approach the natural limit to their growth. Death of their vegetation, where it occurs, and mining of peat both contribute to increasing atmospheric CO₂ concentration, the extent of which can only be guessed. Nor do we know how peatlands would respond to increased concentrations of CO₂ in the atmosphere.