Benthic decomposition rates in mangroves of the Mekong Delta, Vietnam

Rates and pathways of organic matter decomposition were estimated in sediments of Rhizophora apiculata plantations (6, 8 and 35 years old) in the lower Mekong delta, Vietnam.Rates of total carbon oxidation were assessed. Sediment cores for pore water and solid-phase elements and for redox potential and pH were taken at each site. Sediment temperature, Eh, and pH were measured to a depth of 40 cm. The water content of samples was determined as difference in weight loss of wet sediment dried at 80°C for 16 h. The percentage of sand, silt and clay was determined by sieve and pipette analysis on duplicate bulk samples.Pore water samples were analyzed for sulphate, chlorine, Fe, Mn, DOC, phosphate, ammonium, nitrite, nitrate and hydrogen sulphide. Dissolved inorganic nutrients were determined by standard automated techniques. Solid-phase elements (TOC, total N, P, S, Fe, Mn, FeS2) were determined in the same sediment slices squeezed for pore water. Methane concentration in the pore water was determined.Above ground biomass and average net primary production for each forest was assessed. Net rates of iron and manganese reduction were estimated with a core-incubation method.Denitrification was measured from replicate cores. Fluxes of Total CO2, DOC, Fe, Mn, DON, DOP, ammonium, nitrite, nitrate and phosphate across the sediment-water interface were made. Three opaque-glass chambers per site were used to measure CO2, O2, and methane fluxes across the air-sediment interface.
To assess nutrient cycling in mangroves in Mekong Delta.