Nutrient dynamics and leaf litter decomposition in a subtropical mangrove forest at Oura Bay, Okinawa, Japan

Abstract

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.