Benthic decomposition rates and pathways in plantations of the mangrove Rhizophora apiculata in the Mekong delta, 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 ( T o , = average of CO, gas fluxes from exposed sediments + ZCO, fluxes from submerged sediments) were slowest in the 8 yr old forest (mean T o , = 17.1 mm01 C m-' d-'), with higher rates in the 6 yr old (mean To, = 48.1 mm01 C m-' d.') and 35 yr old forests (mean Tcox = 53.7 mm01 C m-2 d-'). 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 sedirnenwwater-air interface. Oxic respiration was the malor 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 mm01 S m-' d-l) and, on average, total rates increased with increasing forest age. Manganese reduction appeared to be a minor decornposition pathway in all 3 stands (range 1.0 to 2.8 mm01 Mn m-' d-l), and iron reduction was measurable only in the 6 yr old forest (0.9 * 0.6 mm01 Fe m-' d-'). Denitrification was measurable only in the 35 yr old forest (2.2 i 0.5 mm01 N2 m-' d-l), but was the thud largest C oxidation pathway at this s~t e .Nitrogen fixation was most rapid in the 8 yr old forest (1425 i 468 pm01 NZ m-' d-l) and equivalent in the 6 yr old (245 + 127 pm01 N2 m-' d-l) and 35 yr old forests (444 * 92 pm01 NZ d-l). The molar carbon ratio of sediment respiration to forest net primary production (Rh,,,,JNPP) 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.

Seasonal patterns of nitrogen fixation and denitrification in oceanic mangrove habitats

“…The rates of N 2 fixation and denitrification observed in Twin Cays microbial mats were comparable to rates of N cycling observed in other mangrove cyanobacterial mats and soils ( (Corredor et al 1999, Alongi et al 2000, which suggests that mangrove mats, particularly those in fringe and transition habitats, may naturally mitigate anthropogenic DIN inputs. Efforts aimed at conservation and restoration of mangrove forests should consider microbial processes such as those observed in cyanobacterial mats and soils (Holguin et al 2001, Rejmánková et al 2004, as these processes may influence the productivity and potential recovery of mangrove habitats.…”

Phylogenetic Diversity of Diazotrophs along an Experimental Nutrient Gradient in Mangrove Sediments

“…In Belize, no correlation was observed between SO4 2− depletion and dissolved organic carbon concentrations [75] supporting our findings that sulfate reducers are not only affected by the availability of organic carbon as suggested previously [83] but as well by specific pore-water parameters. The high number of microenvironments or niches observed for some potential nitrogen-fixing sulfate reducers ( Figure 5) reveals the complexity of microbial communities even within specific functional groups with varied roles in sediments.…”

Ethnobiology, socio-economics and management of mangrove forests: A review

“…Favorable sediment characteristics and high photosynthetic rates of many mangrove systems provide the basis for the biofilter function with high nutrient uptake levels (Rivera-Monroy et al, 1995;Robertson and Phillips, 1995;Alongi et al, 2000). Peri-urban coastal areas of the developing world receive extensive amounts of untreated sewage, and mangroves certainly filter this discharged wastewater, thereby limiting coastal sewage pollution.…”