Sulphur oxidation in tidal mangrove soils of Sierra Leone

Abstract

INTRODUCTIONThe development of mangrove soils for rice cultivation in Sierra Leone has been proceeding for about seventy years, and much of the rice now produced in the territory is grown on these soils. The system used is one of free tidal irrigatióll: during both wet and dry seasons tidal water is allowed to ftow freely on and off the land. In the dry season, when the water in the rivers becomes saline, high salt concentrations develop in the soils but these are quickly reduced at the onset of the Iollowing rainy season, before rice is transplanted. The soils contain large amounts of sulphur in various forms, and although fluctuations occur in the pH value of the surface soil (Tomlinson ~0), such fluctuations do not interIere with the growing of a rice crop during the rainy season.The tidal irrigation system is limited to those areas lying on main rivers or creeks which carry suIficient fresh water during the rains to push back the sea water and leach the accumulated salt Irom the soil. Future expansion of the rice-producing capacity of the territory will depend to some extent on the development of permanently saline Rhizophora mangrove swamps in coastal areas.Large tracts of this type of land are at present uncultivable owing to their high salinity and to the extremely acid conditions which develop in them if they are empoldered to keep out salt water (Dent 4, Tomlinson 19). Owing to the absence of any appreciable source of indigenous liming material in Sierra Leone, the use of heavy dressings of lime to neutralise the acid formed in ëmpoldered soils would be very expensive. On the other hand, experience has METHODSThe soil used in the experiments was collected from a silted drain on the Rokupr Experimental Farm, which is under tidal conditions throughout the year. Collections were always made from a depth of about one foot owing to seasonal variations in the sulphur content of the surface mud, which is well leached and may be Mtered in composition by the deposition of river-borne silt.In experiments on the effect of drying on soll acidity and other factors, the soll was brought into the laboratory and allowed to dry at room temperature in the air. It was thoroughly mixed every day throughout the experimental period.Soll pH v«lue was determined in 1 : 1 water suspension using a pH meter and glass electrode.Titrateable acid was determined by titrating 5 g soll in 15 ml distilled water with N/IO NaOH, the alkali being added in l-tal aliquots and the mixture stirred for 5 min before the pH value was determined.Sulphate was determined by the method of Chesnin and Yien 3, possible interference from colloidal organic marter being eliminated by co-precipiration with ferric hydroxide as recommended by Hesse s Soluble sutphate was extraeted with Morgan's reagent (sodium acetate -acetic acid mixture buffered to pH 4.5) and total sulphate with conc. I-IC1, which dissolves the basic ferric sulphates which are common constituents of dried mangrove soll.

Sediment biogeochemistry in an East African mangrove forest (Gazi Bay, Kenya)

“…Hart 1959;Hesse 1961). Soil acidification is thought to result from the oxidation of reduced sulfur compounds present in mangrove sediments.…”

Mangrove reforestation in Vietnam: the effect of sediment physicochemical properties on nutrient cycling

“…Conversion of mangroves to agricultural use is often not effective, because these wetlands exhibit a high potential for sediment acidification by pyrite (FeS 2 ) oxidation (Dent 1986). Moreover, agricultural systems of free tidal irrigation are limited to areas that carry sufficient fresh water during the rainy season to leach accumulated salt (Hart 1959). If large mangrove areas have been destroyed, as for example in Vietnam during the Second Indochina War, there is no alternative other than reforestation.…”

Rhizophora mangle (Mangue vermelho) em áreas contaminadas de manguezal na Baixada Santista

“…As bactérias decompõem a matéria orgânica sob condições anaeróbicas, reduzindo os óxidos e hidróxidos de Fe 3+ a Fe 2+ e íons sulfato a sulfeto, levando a formação da Pirita (FeS 2 ), estável sob condições anaeróbias, mas quando há drenagem, com conseqüente exposição ao ar, ocorre oxidação da Pirita formando o ácido sulfúrico. Se a quantidade de ácido produzida é maior que a capacidade de neutralização do solo, produz-se uma acidificação dos solos e águas, podendo alcançar pH inferiores a 4,0 (Hart, 1959 Em resumo, a acumulação de elementos traço é condicionada pela decomposição anaeróbica orgânica via sulfato redução que resulta na liberação de sulfetos e na precipitação dos metais, processos intimamente dependentes do equilíbrio das condições ambientais próprias do Manguezal que podem facilmente ser pertubadas por impactos antrópicos, revertendo os processos não sendo os metais mais precipitados (como Pirita), mas, ao contrário, são biodisponibilizados ao meio (Krauskopf, 1956). …”