Benthic decomposition rates and pathways in plantations of the mangrove Rhizophora apiculata in the Mekong delta, Vietnam.
2000
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.
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