Below-ground decomposition of organic matter in forests of the mangroves Rhizophora stylosa and Avicennia marina along the arid coast of Western Australia

Differences in biogeochemical characteristics between three Rhizophora stylosa and three Avicennia marina forests were examined in different coastal settings of and Western Australia. Decomposition rates of sediment organic matter correlated with temperature, but did not differ significantly between Rhizophora (range: 46.5-52.9 mmol C m(-2) per day) and Avicennia (range: 28.5-48.3 mmol C m(-2) per day) forests. There were, however, clear differences in the dominance of specific carbon oxidation pathways between forest types. Rates of sulfate reduction were significantly greater in the Rhizophora (range: 12.9-28.2 mmol S m(-2) per day) than in the Avicennia (range: 2.1-8.5 mmol S m(-2) per day) forests, accounting for 54-100 and 20-55% of total mineralization rates in both forest types, respectively. Sulfate reduction rates correlated significantly with live root biomass, above-ground biomass and forest net primary production. Rates of oxic respiration were, on average, greater in the Avicennia forests (range: 12.7-37.4 mmol m(-2) per day). Oxic respiration appeared to be a minor decomposition pathway in two of the three Rhizophora forests (range: 0.0-23.1 mmol m(-2) per day). It was estimated that, on average, 50% (Avicennia) to 87% (Rhizophora) of total oxygen uptake was consumed in oxidation of reduced metabolites. Methanogenesis was not detected in any of the forests, and Mn and Fe reduction were minor carbon oxidation pathways. Rates of dissolved nitrogen and phosphorus regeneration across the sediment-water interface were slow when measurable. The percentage ratios of total sediment respiration to forest net primary production (T-COX/NPP) were low, ranging among forests between 3 and 7%. Although there may be some carbon loss via tidal export, the low T-COX/NPP ratios suggest slow rates of organic matter decomposition in relation to tree productivity. A large, slowly decomposing, pool of wood and other plant detritus may be an adaptive response, as in other tropical forests, to enhance ecosystem stability and conserve essential nutrients. (C) 2000 Elsevier Science B.V. All rights reserved.