Impact of mangrove litterfall on nitrogen dynamics of virgin and reclaimed islands of Sundarban mangrove ecosystem, India

The Sundarban mangrove ecosystem is one of the fragile and dynamic mangrove systems of the world. This ecosystem consists of many islands both virgin and reclaimed in nature. The present study compares the contribution of mangrove litterfall on nitrogen dynamics of Sagar Island (reclaimed) and Lothian Island (virgin). Though, both islands bear similar mangrove species, the dynamics of nitrogen in soil and water is different. Modeling of the virgin island is needed to determine the parameter values governing the transformation of nitrogen in soil and water. Here, a seven compartment nitrogen based model is constructed and applied to Lothian Island. The state variables, physical and chemical factors are retained same as in the reclaimed island model. The experiments and survey works were carried out on both islands for two consecutive years. Sensitivity analysis is performed followed by model calibration and validation. Chi square, root mean square error and average bias are calculated between simulated and observed results of both islands. Sensitivity analysis indicated that the loss rate of soil organic nitrogen as humic acid and fulvic acid is the most sensitive parameter for both systems. The model results indicated that microbial degradation rates are higher in the Lothian Island than Sagar Island while the leaching rates are lower in former and higher in the latter. The concentration of dissolved inorganic nitrogen in the creeks of Sagar Island is higher due to anthropogenic input. Detritus in the estuarine system of Sagar Island is decreased by 44%, if litter biomass is reduced to 50%. This effect is 55% in case of Lothian Island, when litter biomass is reduced to 50%. The model depicts that both systems are governed by nutrient and detritus derived from degradation of mangrove litterfall. If the litter biomass of Sagar and Lothian islands is reduced to 50%, DIN of the system is affected by 48% and 50%, respectively. This model will act as a tool for environment managers to take necessary steps regarding conservation of mangroves and restoration of creeks in the Sundarban mangrove ecosystem.

► The work is based on nitrogen cycling in mangrove ecosystem.
► This is the comparative study of nitrogen cycling of reclaimed and virgin island.
► Litter nitrogen to the adjacent estuary in the form of DIN is assessed.
► Different forms of nitrogen transformation are considered.
► Biological, chemical and physical factors are considered in these transformations.