Ion Exchange in Sphagnum and its Relation to Bog Ecology

Abstract

In Sphagnum cuspidatum unesterified polyuronic acids form 12 per cent. of the dry weight; in S. acutifolium 25 per cent, of the dry weight. A good correlation has been found for Sphagna between the content of unesterified polyuronic acid and the cation exchange ability, and between cation exchange ability and height of normal habitat above the water table. Anion exchange ability in Sphagna is less than 0.0026 m.eq./g. D.W. compared with about 1.2 m.eq./g. D.W. for cations at pH values above 7. In natural conditions the exchange sites are, however, only partly dissociated. The production of new plant material in a bog dependent on rainwater for nutrients can be sufficient to maintain the pH below 4.5, but on average in England only of the order of 2 per cent, of the monovalent cations other than H+ could be retained in exchangeable form. A greater proportion of polyvalent cations could be retained. The kinetics of cation exchange are consistent with a heterogeneous exchange phase containing regions of high charge density and regions with lower charge density. At equilibrium the proportions of different cations in the exchange phase are largely explicable by a Donnan distribution, but there are notable exceptions. Two estimates based on Donnan distribution suggest that with low external pH and/or low cation concentration the apparent concentration of exchange sites may be 2-3 eq./l., falling with rise in pH and/or increase in cation concentration to 0-9-1 -5 eq./l. The apparent dissociation coefficient also varies in these conditions from 2 × 10−2 to 1 × 10−4.