Benthic decomposition rates and pathways in plantations of the mangrove Rhizophora apiculata in the Mekong delta, Vietnam.

Rates and pathways of organic matter decomposition were estimated in sediments of 6, 8 and 35 yr old Rhizophora apiculata plantations in the lower Mekong delta, Vietnam. Rates of total carbon oxidation (T COX = average of CO 2 gas fluxes from exposed sediments + ΣCO 2 fluxes from submerged sediments) were slowest in the 8 yr old forest (mean T COX = 17.1 mmol C m -2 d -1 ), with higher rates in the 6 yr old (mean T COX = 48.1 mmol C m -2 d -1 ) and 35 yr old forests (mean T COX = 53.7 mmol C m -2 d -1 ). In all 3 forests, sediments to a depth of 40 cm were acidic, with mostly positive redox potential; free sulfides and methane were not measurable in the pore water or across the sediment/water-air interface. Oxic respiration was the major decomposition pathway, ranging from 63 to 64 % of T COX in the 2 older forests to 94 % of T COX in the 6 yr old stand. Budget calculations suggest that most of the O 2 flux was associated with chemical oxidation in sediments of the 2 youngest forests. Sulfate reduction was the second most important diagenetic pathway (range 0.2 to 13.0 mmol S m -2 d -1 ) and, on average, total rates increased with increasing forest age. Manganese reduction appeared to be a minor decomposition pathway in all 3 stands (range 1.0 to 2.8 mmol Mn m - d -1 ), and iron reduction was measurable only in the 6 yr old forest (0.9 ± 0.6 mmol Fe m -2 d -1 ). Denitrification was measurable only in the 35 yr old forest (2.2 ± 0.5 mmol N 2 m -2 d -1 ), but was the third largest C oxidation pathway at this site. Nitrogen fixation was most rapid in the 8 yr old forest (1425 ± 468 μmol N 2 m -2 d -1 ) and equivalent in the 6 yr old (245 ± 127 μmol N 2 m -2 d -1 ) and 35 yr old forests (444 ± 92 pmol N 2 m -2 d -1 ). The molar carbon ratio of sediment respiration to forest net primary production (R hetero /NPP) in the 6 and 35 yr old forests averaged 18 and 28%, respectively. These comparatively low mineralization losses, coupled with the lack of measurable denitrification at 2 of the 3 plantations, imply that these R. apiculata plantations are highly efficient at sequestering labile carbon and nitrogen into plant biomass and sediment pools.