Leaf Litter Decomposition and Changes in Leaf C:N Ratio in the Mangals of Negombo Lagoon
V. PAHALAWATTARACHCHI and M.D. AMARASINGHE
The objective of the study was to characterize the mangrove areas in Negombo lagoon, as sources of detritus for the coastal foodwebs. Senescent Rhizophora mucronata leaves in 2 mm mesh bags placed in the innermost areas of a mangal in an island, lost weight faster (7-14 days) than those that were placed in the water-front areas, which took 14 - 28 days to lose 50% of the initial weight On the contrary, litter in the 13 mm mesh bags, lost half its weight within 3 days at water-front, 7-14 days at the middle area and 35-42 days in the innermost site. In the dry season, decomposition was slower and more than 46 days were taken for 50% weight loss. R. mucronata leaf litter placed in the tanks filled with lagoon water took the longest time (41 days) for decomposition, further substantiating the effect of tides, soil moisture as well as the macro- and micro- organisms on the process. Carbon represented 42% and 29% of the dry weight of senescent leaves of R. mucronata in the wet and dry seasons respectively. During 96 day i of decomposition, about 75% of this carbon was lost and about 25% transformed to particulate organic matter. An increase in the nitrogen content and a decrease in the carbon content were observed during the first 56 days of decomposition, indicating the nitrogen mobilization due to the microbial action. The remarkable decrease after 56 days may be due to the very low rainfall, thus low moisture content during this period. The nitrogen contribution to the ecosystem upto 56 days of the wet season was calculated to be 1.76 x 10' t/ha. The total increase of N in the wet season however is as twice as that in the dry season. The average C:N ratio of R. mucronata changed with decomposition from 199.7: Ito 69.98:1 in the dry season and from 60.97:1 to 16.06:1 in the wet season. The dietary requirement of protein for most animals is 16.5% of the dry weight of diet, which corresponds to a C:N intake of about 17:1. The detritus produced, particularly during the wet season therefore, is of immense importance to the foodwebs in these coastal waters.