Assessment of bioavailable nitrogen and phosphorus content in the sediments of Indian mangroves

Efficient nutrient cycling and adequate sediment bioavailable nutrient supply are considered to be the two most important factors regulating the high productivity and subsequent carbon sequestration by mangrove ecosystems. We assessed spatial variability and the possible regulating factors of sediment bioavailable nutrients (nitrogen (N) and phosphorus (P)) and surface water-dissolved nutrients (N, P and silicate (Si)) in the five ecologically important mangrove ecosystems along the east and west coast of India during dry season. Higher bioavailable nitrogen concentrations in the sediments were recorded in Coringa mangroves (36.27 ± 14.7 μg g −1 ) and Bhitarkanika (18.54 ± 5.9 μg g −1 ) mangroves in the east coast followed by Karnataka (15.51 ± 8.26 μg g −1 ), Goa, (10.18 ± 9.96 μg g −1 ) and Kerala (6.36 ± 5.05 μg g −1 ) mangroves in the west coast. The dissolved inorganic nutrients in the mangrove waters ranged between 5.1 and 220.9 μmol l −1 for N and 0.07 and 3.9 μmol l −1 for P. These results indicated that terrestrial inputs, in situ remineralization and prevalent anoxic conditions regulated sediment nutrient content in these ecosystems, whereas the higher ammonium in the sediments was attributed to the greater nutrient adsorption by finer particles. The stoichiometry of the bioavailable nutrients (N, P) in the mangrove sediments deviated drastically from the Redfield ratio, and strong P limitation was recorded in most of the ecosystems. The results highlighted the potential role of sediment particle size and physiochemical (salinity and pH) properties in regulating bioavailable nutrient dynamics in mangrove sediments.