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 styl osa and three Avicennia marina forests were examined in different coastal s ettings of arid Western Australia. Decomposition rates of sediment organic matter correlated with temperature, but did not differ significantly betwee n 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 differe nces in the dominance of specific carbon oxidation pathways between forest types. Rates of sulfate reduction were significantly greater in the Rhizoph ora (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 t otal mineralization rates in both forest types, respectively. Sulfate reduc tion rates correlated significantly with live root biomass, above-ground bi omass 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 w as estimated that, on average, 50% (Avicennia) to 87% (Rhizophora) of total oxygen uptake was consumed in oxidation of reduced metabolites. Methanogen esis was not detected in any of the forests, and Mn and Fe reduction were m inor 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%. Al though there may be some carbon loss via tidal export, the low T-COX/NPP ra tios suggest slow rates of organic matter decomposition in relation to tree productivity. A large, slowly decomposing, pool of wood and other plant de tritus may be an adaptive response, as in other tropical forests, to enhanc e ecosystem stability and conserve essential nutrients. (C) 2000 Elsevier S cience B.V. All rights reserved.