The effects of in situ drying on sediment-phosphate interactions in sediments from an old wetland

We examined the effects of drying on sediment-P interactions for sediments from a shallow wetland ca. 6000 years old. Sediments from this wetland woul d have been subjected to numerous drying and wetting episodes during the li fe of the wetland. The factors affecting potential orthophosphate release w ere compared in surface sediments that had previously been desiccated for 3 months, surface sediments that had remained inundated and wet sediment 25- 30 cm below the surface. All sediments released small amounts of orthophosp hate once subjected to anaerobic conditions. Sulfate alone stimulated phosp hate release from the surface sediment, irrespective of the previous hydrol ogical status, whereas orthophosphate release from deep sediments was co-li mited by carbon and sulfate. Decreases in soluble Fe (II) were measured con currently with sulfate-stimulated P release, which is consistent with forma tion of iron sulfides. Similar numbers of culturable sulfate-reducing bacte ria were obtained from the wet and dry sediments, their tolerance to desicc ation explaining why release could occur after extensive drying of the sedi ment. Phosphate adsorption isotherms of the sediments showed that sediments from all sites had a relatively low affinity for ortho-phosphate. The adso rption data showed reasonable fit to the Freundlich adsorption isotherm. No difference in the maximum amount of ortho-phosphate uptake was observed if the sediment isotherm experiments were done under an inert atmosphere or, following exposure to air, suggesting that even anoxic sediments were not h ighly reduced. However, significant variations in the value of the Freundli ch constants were observed. The results presented in this study are consist ent with the hypothesis that repeated wetting/drying cycles select for bact eria that are tolerant to periods of desiccation and/or oxidation. Furtherm ore, repeated wetting and drying cycles may result in changes to sediment m ineralogy.