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Dynamics of nutrients (carbon, nitrogen and phosphorus) in decomposing leaves was studied using litterbags in a subtropical mangrove dominated by Bruguiera gymnorrhiza (L.) Lamk and Kandelia candel (L.) Druce (Okinawa, Japan). In addition to fresh yellow leaves, degradation of treated leaves (dried at 50°C and 80°C) was also investigated. Comparison was made between species, locations (lower and upper mangroves) and among treatments. Results showed that K. candel leaves with higher initial N concentrations (low C:N ratio) decayed faster than B. gymnorrhiza leaves; decay constants were 0.062 and 0.022 per day. This finding implies that degradation rate is species specific and depends on the initial chemical composition of leaves, particularly the C:N ratio. Furthermore, B. gymnorrhiza leaves, with lower initial N levels, gained more N from external sources (i.e. bacteria) than K. candel leaves which exhibit a high initial N content. Leaves in the upper mangroves decayed much more slowly than in lower mangroves; decay constants were 0.019 and 0.022 per day. The overall decay rates of dried leaves were lower than those of fresh leaves, decreasing in the order: fresh leaves 0.022, 50°C 0.019 and 80°C 0.018. The t 0.5 values were 32, 36 and 38 days respectively. Drying of mangrove leaves prior to incubation in the field significantly altered the pattern of mass loss, nutrient loss and delayed microbial recolonization. A single exponential equation described losses of ash-free dry weight, C, and P for the two species, sites and treatments. Only N in K. candel leaves was described by the single exponential equation.

Changes in lipid content and fatty acid (FA) composition in decomposing mangrove leaves of Bruguiera gymnorrhiza (L.) Lamk and Kandelia candel (L.) were investigated in a subtropical mangrove forest on Okinawa Island, Japan (26.5°N, 128°E) by field experiments for 18 wk (July to November 2000), using yellow senescent leaves, and compared with FAs in the green leaves and mangrove sediments. We tested the hypothesis that changes in FA composition during decomposition can indicate the state of leaf decay and periods of high and low microbial activity, and that bacteria may rapidly degrade polyunsaturated fatty acids (PUFAs). During decay, FA composition in the yellow leaves changed in 2 wk from predominantly saturated FAs to monounsaturated FAs, and to the more branched FAs typical of bacteria, and lipid and N increased due to microbial colonization. However, the microbial decomposition of leaves did not alter the concentrations of long-chain fatty acids (LCFAs), suggesting that these vascular plant-markers remain unchanged in mangrove detritus and surface sediments for more than 4 mo. Furthermore, bacteria did not degrade PUFAs as we had hypothesized, indicating that during decomposition of mangrove leaves, bacteria tend to conserve PUFAs (as they do nitrogen), thus enriching the detritus with nutrients. Comparison of ω3 and ω6 PUFAs between the 2 species showed that nutritional quality varies greatly with the state of the leaf material, increasing through time in B. gymnorrhiza and decreasing through time in K. candel.

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