Characteristics of Water Quality and Nitrogen-Associated Bacterial Functions in Mekong Delta Mangroves

抄録

Rapid changes, such as urbanization and artificial field expansion, are occurring in mangrove regions worldwide. To assess the characteristics of nitrogen-related processes in mangrove forests in the Mekong delta in relation to conditions in the surrounding environment, we measured water quality: inorganic nitrogen, inorganic phosphate and heavy metals; and nitrogen-associated bacterial activities: nitrogen-fixing activity and nitrogen-consuming activity. The water quality results suggest that key changes in Mekong delta man groves include a high input of ammonium, phosphorus and heavy metals from cities and aquaculture farms upstream. The current value of mangrove soil bacterial nitrogen fixation in the Mekong delta, 0.87-24.57 mg N m^-2 d^-1, has dropped to within the previously reported order of other mangrove regions. We determined that the addition of ammonium had reduced soil nitrogen fixation to less than half that of the control treatment. The addition of aquaculture pond water also suppressed soil nitrogen fixation. We postulated that if drainage from the artificial systems upstream continuously supplies the mangroves downstream, part of the mangrove soil nitrogen will shift from atmospherically-derived nitrogen to anthropogenically-derived nitrogen. If the nitrogen input from the upstream region exceeds the nitrogen-consuming capacity of the mangrove forest, effects on plant growth due to excessive ammonium and eutrophication of the surrounding ocean will be a concern.