Denitrification activity in mangrove sediments varies with associated vegetation

Abstract

To test the hypothesis that mangrove vegetation associated sediment characteristics could influence an ecologically important function such as denitrification, a study was carried out at the Divar mangrove ecosystem in Goa, India. The composition of intertidal mangroves was assessed based on which the down-core (0-10 cm) variation in environmental variables and denitrification activity (DNT) associated with dominant vegetation types was examined. Our observations revealed a distinct zonation pattern of mangroves with the dominance of Rhizophora mucronata (RM) in the lower intertidal zone followed by Avicennia marina (AM) in the mid region. Acanthus ilicifolius (AI) was restricted to the upper intertidal zone (UIZ) which was characterized by relatively lower sediment temperatures, higher porewater salinity and intense reducing conditions. A zonation pattern in occurrence of DNT was also observed. Denitrification activity generally decreased with depth and increased in intensity from the seaward to the landward side. Maximum DNT of 4.06 ± 0.44 nmol N₂O g^-1 h^-1 was recorded in the surficial AI sediments followed by AM and RM sediments. Surface plant litter content also increased from the lower to UIZ. A negative relationship of NH₄⁺ and total organic carbon content (n = 15, r = -0.388, p < 0.05) in AI sediments was observed. This suggested that re-mineralization of accumulated organic matter (up to 3.07 ± 0.58% at 0-2 cm) enhanced NH4+ availability in the porewater which in turn could support nitrification-denitrification. Thus, degradation of vegetation-derived organic matter was important in sustaining nutrients mainly towards the landward side proving our hypothesis that vegetation could have an influence on DNT, albeit indirectly