A comparison of nitrogen fixation (acetylene reduction) among three species of mangrove litter, sediments, and pneumatophores in south Florida, USA

Assays of nitrogen fixation (acetylene reduction method) were performed on fresh leaf litter (yellow leaves recently fallen from the trees), aged leaf litter (brown leaves on the forest floor) of Rhizophora mangle, Avicennia germinans, and Laguncularia racemosa; and in addition rates were measured on pneumatophores of A. germinans and mangrove sediment from two different sites along the Shark River estuary in the Everglades National Park (south Florida, USA). Differences in sediment nitrogen content between sites were not important enough to determine statistically different C:N ratios for the leaf litter, and there was no effect of site on nitrogen fixation rates. Fresh leaf litter, sediment and pneumatophores showed very low ethylene production rates, ranging from 0 to 31.3 nmol C^sub 2^H^sub 4^ g dry wt^sup -1^ h^sup -1^.Aged leaf litter showed the highest ethylene production rates, ranging from7.3 to 538.8 nmol C^sub 2^H^sub 4^ g dry wt^sup -1^h^sup -1^. Ethylene production rates showed no apparent differences in species composition, but there was an effect by the stage of decomposition of the leaves. Fresh leaf litter and mangrove sediments represent initial and final stages in decomposition, respectively, and both have minimum rates of nitrogen fixation in the forest floor. New nitrogen to this forest by fixation in leaf litter is associated with the intermediate stages of litter decomposition.[PUBLICATION ABSTRACT]