活動備忘 Event Memo：
Mangrove forest is considered a nitrogen (N) limited environment having high potential of microbial N2-fixation, denitrification and plant N uptake. However, high anthropogenic N loading and saltwater intrusion may change the soil microbial community and alter the N dynamics in the mangrove ecosystems. This research was designed to evaluate three major N processes in a mangrove forest in Tamshui estuary: soil nitrogenase activity (NA), soil denitrification enzyme activity (DEA) and N assimilation of mangrove seedlings. We aimed to obtain a better understanding of how these processes relate to soil physiochemical factors such as temperature, salinity and organic carbon (OC), and how the processes reflect high anthropogenic N sources.
Results showed both the NA and DEA were positively correlated to soil temperature and availability of soil OC. DEA was inhibited by salinity but not the NA because more than 80% of the NA was attributed by the activity of sulfate-reducing bacteria in the mangrove soils. In addition, the potential DEA was still higher than the field DEA at the highest experimental salinity (60 psu), implying salinity has less effect on soil denitrification in the field. With increasing salinity from 0-3.3% of NaCl, N assimilation rate measured by the 15N stable isotope technique was significantly reduced from 2.2 to 0.9 mg N g-1 biomass hr-1. Mangrove favored nitrate (NO3-) than ammonium (NH4+) at 0% NaCl, but preferred NH4+ to NO3- at 1.75% and 3.30% NaCl.
Overall, this research demonstrated mangrove forests with high anthropogenic N loading and threatened by saltwater intrusion may maintain similar DEA and NA, while the N assimilation rate by mangrove seedlings was significantly decreased.
Keywords: Mangrove, N2-fixation, denitrification enzyme activity, sulfate-reducing bacteria, N assimilation, stable isotope