The mangrove swamps of Keneba. lower Gambia river basin. II. Sulphur and pH in the profiles of swamp soils.
See S. & F. XXVIII ). Soils under Rhizophora racemosa, Avicennia germinans and Sesuvium portulacastrum and soils of four barren flats associated with the swamp were studied. Water-soluble S in air-dry soil varied from 9-15 mg/g under Avicennia to 3-4 mg/g in one barren flat; in any profile the peak SO4 concentration often coincided with the peak free-S concentration. Suggested sources of SO4 are bacterial oxidation of free S, translocation from surrounding areas and accumulation from a saline water table, and flooding with salt water. Free-S concentrations were greatest under Rhizophora with peak values of 10 and 15 mg/g in the fibrous layers of the profile. In other profiles, most S occurred in fibrous zones associated with former Rhizophoreta. Suggested sources of free S are chemical oxidation of sulfides and polysulphides and reduction of SO4 translocated from neighbouring soils. Large (>3 units) falls in pH followed drying of Rhizophora soils and some horizons of other profiles previously under Rhizophora vegetation. The change in pH when soils were dried and the degree of acidity produced on drying were related to the fluctuation of the soil water table in the dry season and to high S concentrations associated with present and past Rhizophora vegetation.