The Mangrove Swamps of Keneba, Lower Gambia River Basin. II. Sulphur and pH in the Profiles of Swamp Soils

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Abstract
The concentrations of water-soluble sulphate and free sulphur have been determined in successive 6-m. steps down the profile in swamp soils under Rhizophora racemosa. Avicennia germinans (A. nitida) and Sesuvium portulacastrum, and in the soils of four barren flats associated with the mangrove complex. The concentrations of soluble sulphate in the air-dried soils were variable, being highest in the profile under Avicennia vegetation (9-15 mg SO4/g) and lowest in the profile of the Tebebe barren flat (3-4 mg SO4/g). The peak concentrations in a profile often corresponded to the peak concentrations of free sulphur. Three sources of soluble sulphate are suggested: bacterial oxidation of free sulphur, translocation from surrounding areas and accumulation from a saline water table, and flooding with salt water. Concentrations of free sulphur were largest in soils under Rhizophora mangrove with peak values of 10 and 15 mg S/g air-dry soil, corresponding with fibrous layers in the profile. Where Avicennia appeared as a secondary colonizer, high concentrations were found in that part of the profile which was associated with a former Rhizophoretum. Peak values in the profiles under Sesuvium vegetation and in the profiles of the barren flats corresponded with similar fibrous horizons. Two sources of free sulphur are suggested in these profiles: the chemical oxidation of sulphides and polysulphides and the reduction of sulphates translocated from neighbouring soils. pH measurements were made on both fresh and air-dried soils. The extent of change in pH on drying within an entire profile has been called the pH spectrum of the profile. Large falls in pH occurred on drying in soils under Rhizophora vegetation (>3 pH units) and in some horizons in profiles under Avicennia, Sesuvium and barren flats, in which fibrous muds indicated previous Rhizophora vegetation. pH values as low as 2.5-3 were recorded in some air-dried samples. Variations in the pH spectra of profiles under Rhizophora and Avicennia have been used to classify the soils with regard to the age of existing vegetation, whether primary or secondary, and drainage. The change in pH when soils were dried, and the degree of acidity produced on drying have been related to the fluctuation of the soil water table in the dry season, as well as to high sulphur concentrations associated with present or past Rhizophora vegetation. Movements in the water table in a barren flat (Burutu) are presented in relation to tidal cycle and climate. The pH of the water table fell from 6-7 at 12 in. to 3 at a depth of 24-30 in. as the water table fell during the dry season. At the onset of the rains the water table rose to a depth of 12-18 in. and the pH returned to 6-7. Acidity produced in the water table at depth was attributed to the vertical distribution and oxidation of sulphide-sulphur.

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