MEPS 194:87-101 (2000)  -  doi:10.3354/meps194087

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

D. M. Alongi1,*, F. Tirendi1, L. A. Trott1, T. T. Xuan2

1Australian Institute of Marine Science, PMB No. 3, Townsville M.C., Queensland 4810, Australia
2Research Institute for Aquaculture No. 2, 116 Nguyen Dinh Chieu St., District 1, Ho Chi Minh City, Vietnam

ABSTRACT: 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 (TCOX = average of CO2 gas fluxes from exposed sediments + …CO2 fluxes from submerged sediments) were slowest in the 8 yr old forest (mean TCOX = 17.1 mmol C m-2 d-1), with higher rates in the 6 yr old (mean TCOX = 48.1 mmol C m-2 d-1) and 35 yr old forests (mean TCOX = 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 TCOX in the 2 older forests to 94% of TCOX in the 6 yr old stand. Budget calculations suggest that most of the O2 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-2 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 N2 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 N2 m-2 d-1) and equivalent in the 6 yr old (245 ± 127 µmol N2 m-2 d-1) and 35 yr old forests (444 ± 92 µmol N2 m-2 d-1). The molar carbon ratio of sediment respiration to forest net primary production (Rhetero/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.

KEY WORDS: Mangrove · Decomposition · Respiration · Sediment · Benthic · Rhizophora · Carbon · Nitrogen

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