SEAWATER-GROUNDWATER EXCHANGE: POTENTIAL ASSOCIATION WITH NITROGEN CYCLING PROCESSES IN A MANGROVE SWAMP

Coastal mangrove swamps play an important role in nutrient cycling at the land-ocean boundary. However, little is known about role of periodic seawater-groundwater exchange in N-cycling process. Here an attempt integrating mangrove hydrology, N-cycling and microbiology was made to understand this role by examining a shore-perpendicular intertidal transect in Daya Bay, China. The transect comprises three hydrologic subzones (tidal creek, mangrove and bare mudflat zones). Potential rates of ammonia oxidation (0.10-0.62 μmol N g^-1 h^-1) and nitrite oxidation (0.10-0.57 μmol N g^-1 h^-1) were measured through a rapid and high-throughput method. Potential rates of denitrification (1.78-9.15 μmol N kg^-1 h^-1) and anammox (0.29-1.09 μmol N kg^-1 h^-1) were measured using modified MIMS method. The results showed that the mangrove zone experienced less hydrologic exchange than other areas but manifested the highest rates of nitrogen removal. Rates of nitrogen removal decrease with sediment depth and with distance to the high tide line. Seawater-groundwater exchange may facilitate high-DO seawater infiltration into the sediment, which enhances N-cycling, especially the coupling of nitrification and denitrification. In the mangrove swamp, denitrification accounts for 90% of the total nitrogen loss, and anammox accounts for the remaining 10%. The average nitrogen removal rate was about 2.07 g per day per cubic meter of mangrove sediments. Overall, coupling water exchange and N-cycling microbial activity can considerably provide new insights into how seawater-groundwater exchange contributes to nitrogen budgets on the scale of intertidal zone.

Acknowledgments: This research was supported by the National Basic Research Program of China (“973” Program, Grant No. 2015CB452902) and the Key Program of National Natural Science Foundation of China (Grant No. 41430641, 41576123). We would like to thank Yanman Li, Gang Li, Fengjie Guan, Meng Zhang, Manhua Luo, Tiao Guo and Liang Zhong for their field work.