Exploration of N2 fixation and denitrification processes in the Sundarban mangrove ecosystem, India

Abstract

Seasonal variations in nitrogen (N₂) fixation and denitrification rates were estimated in the sediment of Sundarban mangrove ecosystem, India. The mean rate of microbial N₂ fixation (16.53 ηmol N fixed/ hr/ g dry wt of sediment) was relatively higher than the mean rate of denitrification (14.72 ηmol N₂O-N /hr/dry wt of sediment) in the mangrove sediment throughout the study, suggesting satisfactory fertility of this ecoregion in terms of sediment nitrogen. Sediment from the deeper parts of this ecosystem showed higher N₂ fixation rate (18.23 ηmol N fixed/ hr/ g dry wt of sediment) than the surface sediment (10.17 ηmol N fixed/ hr/ g dry wt of sediment). Similar trend was observed for the denitrification rate in all the seasons. The ratio of N₂fixation to denitrification rate showed an increasing trend with depth. The results depicted that under higher anoxic condition bacteria would prefer N₂ fixation over denitrification and would ultimately enhance the nitrogen storage capacity of this mangrove ecosystem. Although population of free living N₂ fixing bacteria decreased with increase in depth, the rate of N₂ fixation showed a reverse trend, during the study period. These results suggested that anoxic condition and low level of nitrate-nitrogen content could stimulate the enzyme activity responsible for N₂ fixation in this particular ecosystem.