Anoxic Decomposition in Sediments from a Tropical Mangrove Forest and the Temperate Wadden Sea: Implications of N and P Addition Experiments

The anoxic decomposition processes in sediments from a tropical mangrove forest (Bangrong, Thailand) and a salt marsh in the temperate Wadden Sea (Denmark) were compared, and the effects of increased ammonium and/or phosphate concentrations on the anoxic decomposition were studied. Sediment was incubated in jars (20-ml glass vials) and changes in porewater solutes were followed in a time series of 3 weeks. Furthermore, the short-term fate (days) of dissolved inorganic phosphate (DIP) added to Bangrong mangrove sediment was evaluated from phosphorus fractionation. Although the organic matter at both study sites had relatively high C:N and C:P ratios, the anoxic sediment decomposition was not affected by nutrient enrichments at the level applied in this study. The sediment metabolism (TCO2and DOC production, and sulfate consumption) was 5–10 times higher in Wadden Sea sediment than in Bangrong mangrove sediment, probably due to the higher content of structural carbohydrates (e.g. cellulose) as indicated by higher C:N and C:P ratios in mangrove organic matter. This is substantiated by a lower nutrient release from Bangrong mangrove sediments and suggests a faster turnover of N and P by nutrient deficient bacteria in the mangrove sediment. DIP was released during anoxic decomposition in Wadden Sea sediment, but was retained in Bangrong mangrove sediment. Analysis of phosphorus fractions in Bangrong mangrove sediment revealed that added excess DIP was efficiently taken up by the sediment particles and primarily retrieved in the easily exchangeable and iron bound fractions. The studied mangrove forest sediment seems to be a phosphorus sink.