Nitrogen Cycle And Hydrodynamics Of Macropores As Preferential Flow Conduits In The Intertidal Zone Of Coastal Wetlands

Coastal wetlands play an important role in the global hydrological cycle at the land-ocean boundary.However,little is known about the interrelation of periodic seawater-groundwater exchange,the spatial variation of microbial species and activity and chemical substance cycles.In view of this,this paper discussed the relationship between them and their impacts on the ecological environment in combination with wetland hydrology and microbial geochemistry.The main results of this study are summarized as follows:A shore-perpendicular intertidal transect in mangrove swamps in Daya Bay,China,was investigated by measuring physicochemical parameters?DO,pH and temperature?and concentrations of major ions(Na⁺,K⁺,Ca²⁺,Mg²⁺,SO₄^2-,HCO₃^-and Cl^-),nutrients?DIP,DIN and Dsi?and heavy metals?Cu,Zn,As,Hg,Cd,Pb and Cr?.The intertidal transect comprises three hydrologic subzones?tidal creek,mangrove and bare mudflat zones?,each with different physicochemical characteristics.The maximum seawater-groundwater exchange rate was located within the tidal creek zone,which facilitated high oxygen seawater infiltration and subsequent nitrification.In contrast,the lower exchange rate,occurred in the mangrove zone,caused over-loading of organic matter and longer groundwater residence times.This creates an anoxic environment conducive to nitrogen loss through anammox and denitrification processes.In the whole mangrove swamp,denitrification accounted for 90%of the total nitrogen loss,and anammox accounted for the remaining 10%.The average nitrogen removal rate of anammox and denitrification was 0.23 g d^-1 m^-3 and 1.84 g d^-1 m^-3,respectively.The groundwater in the intertidal zone was slightly alkaline.The ions Cl^-and Na⁺were approximately conservative ions and the linear relationship between the main ions and Cl^-was consistent with the local seawater dilution line.Heavy metals concentrations were higher in the mangrove zone and the creek zone than those in the mudflat zone.Except Cu,most of the heavy metal concentrations were below the water quality standard.The numerical simulation results showed that the local groundwater circulation was formed in the creek zone,which can enhance the groundwater flow and substance transport.Existence of crab burrows is a common feature in coastal wetlands.Crab burrows can play an important role as preferential flow conduits for enhancing pore water-surface water interaction and subsurface transports.Two methods included tension samplers and passive diffusion samplers were used to measure the salinity variation of pore water.The porewater removed by tension samplers was consistently more saline?5-10 g/kg?than the porewater measured by the passive diffusion samplers.This difference relates to sampling mechanism of two methods and the preferential flow associated with crab burrows.The tension samplers used suction to withdraw porewater from sediment matrix.Passive diffusion samplers did not rely on suction,which allowed the sample to more closely mixed pore water,which may represent to some extent the salinity of both macro-pores and matrix.The simulation results showed that:the crab burrow can increase tidally driven water exchange and soil aeration.Particularly,at low tides the salinity in the sediment matrix was more saline than those in crab burrows.This was consistent with our field observations.These effects of crab burrow may increase the marsh productivity.