Nitrogen Retention Capacity Of Different Types Of Wetlands In The Danjiang River Basin,China

Danjiang River basin is an important part of the water source area of the middle route of South-to-North water transfer project in China.There are many wetlands in the Danjiang River basin.Wetlands have a strong retention effect on the non-point source pollutants entering water body through surface runoff and infiltration process.Wetlands can effectively remove nitrogen pollution in water body through sediment nitrification and denitrification.At present,there are many studies focusing on the rivers,lakes and reservoirs in China,however,ditches and ponds have received relatively little attention over the years.On the basis of remote sensing interpretation of land use in Danjiang River basin,four common wetland types,including rivers,streams,ditches and ponds were selected.Potential nitrification rate,background denitrification rate and potential denitrification rate of sediments were compared,and nitrogen removal capacity of four types of wetlands were explored.The remote sensing images and DEM data of Danjiang River basin were collected and the basic information such as land use status,river network and watershed information were extracted.The remote sensing images were classified by unsupervised classification,supervised classification and object-oriented classification,respectively.Among them,the object-oriented classification method had the highest accuracy.The overall accuracy was 98.4394%,and the Kappa coefficient was 0.9104.Land use in the watershed was divided into six categories:reservoir/pond,river/stream/ditch,building,bare land,forest,and cropland/grassland.The area of the reservoir/pond is 401 km2,and that of river/stream/channel is 736 km2,accounting for 2.45%and 4.50%of the total area of Danjiang River basin,respectively.On the basis of remote sensing interpretation,four common wetland types(river,stream,ditch,and pond)were selected as the research objective.The nitrogen cycle rate of sediment,water quality,physical and chemical properties of sediment were measured in four seasons.The results showed that wetland type and season had significant effects.Potential denitrification rate and nitrification rate of different wetland types are as follows:Ditch>Pond>River>Stream.On the potential nitrification rate and denitrification rate of sediment.In March,the potential denitrification rate of ditch sediments was significantly higher than that of other three wetland types,while in September,the potential denitrification rate of ditch sediments was significantly higher than that of rivers and streams.In September,the potential nitrification,potential denitrification rate and background denitrification rate of sediments reached the annual maximum.The results of correlation analysis showed that denitrification rate was positively correlated with many environmental factors such as water temperature,TOC,soil moisture,carbon and nitrogen content.