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In situ decomposition of senescent leaves of two abundant mangrove species (Rhizophora mucronata Lamarck and Ceriops tagal (Perr) C.B. Rob), enrichment of nitrogen and activity of dinitrogen fixing bacteriaduring decomposition were investigated during both rainy and dry seasons in a tropical coastal lagoon (Gazi, Kenya). Rates of leaf decomposition were higher for R. mucronata than for C. tagal and were highest, for both species, during rainy season. Rates of decomposition, expressed as percentage dry mass loss, over a decomposition period of 50 days was: C. tagal (rainy season), 69%; C. tagal (dry season), 27%;R. mucronata (rainy season), 98%; and R. mucronata (dry season), 48%. High rainfall and diurnal tidal inundation appear to enhance the leaf decomposition process. Maximum rates of nitrogen fixation were 380nmol N2 h-1 g-1 dw for C. tagal (rainy season), 78 nmol N2 h-1 g-1 dw for C. tagal (dry season), 390 nmol N2 h-1 g-1 dw for R. mucronata (rainy season) and 189 nmol N2 h-1 g-1 dw for R. mucronata (dry season). Although N2 fixation rates were highest during rainy season,total nitrogen immobilised in the leaves was highest during the dry season. Biological nitrogen fixation can account for between 13 to 21% of the maximum nitrogen immobilised in the decaying mangrove leaves. Nitrogen fixation, as a source of allochthonous nitrogen, sustains a nitrogen input to the mangrove ecosystem, which adds significantly to the nitrogen input through leaf litterfall. [ABSTRACT FROM AUTHOR]