Carbon And Nitrogen Isotopic Compositions Of Organic Matter In Dongjiang River And Its Application To Traceability

The organic matter can be a significant nutrient source to local and coastal marine ecosystems. The study for it could provide a detailed and integrated record of natural and anthropogenic activities within the drainage basin, and also reveal the vegetation types, soil erosion and water quality, etc.. Dongjiang was the main water supply source in Guangdong Province, and shoulder nearly 40 million population’s water supply of the Heyuan, Huizhou,Dongguan, Guangzhou, Shenzhen and Hong Kong. Some basic research about Dongjiang River has been carried out, but the the law of material recycling was unclear. Accurately grasp carbon and nitrogen’s sources may provide a very important reference to the Dongjiang river basin management.During the period of flood season(July 2013) and dry season(January 2014), the environmental status was studied, and the use of stable isotope tracer technique to discuss the source of carbon and nitrogen. Collecting suspended particles, surface sediment, dominant plant,soil and epiphytic algae in the main stream of Dongjiang River. Elemental Contents(C, N) and stable isotopic composition(δ13C、δ15N) were analyzed and then employed to trace the sources of organic matter. At last, an end-member mixing model was utilized to quantitatively assess the contribution of different sources in suspended particles and sedimentary organic matter. The main results are as fellows:1. Dongjiang River is a nitrogen-rich while phosphorus-poor river, the nitrogen pollution of which was severe. The range of TN concentrations was 0.75~4.23 mg·L-1. Except for the downstream region, the water quality of most regions met Environmental Quality Standards for Surface Water(GB3838-2002) Ⅱ and Ⅲ. Upstream and downstream were respectively the two high nitrogen value areas. The Nitrogen isotope analysis showed that domestic sewage and livestock wastewater was the main source of upstream, while in the downstream industrial production as the main source2. In flood and dry season, the δ13C values of suspended particles were-30.2‰~-28.4‰and-29.7‰~-28.1‰, respectively. The δ15N values were 0.3‰~8.6‰ and-4.7‰~13.9‰,respectively. The C/N values were 10.1~21.7 and 11.8~23.0, respectively. And the δ13C values of surface sediment in flood and dry season were-29.5‰~-25.6‰ and-28.0‰~-23.1‰,respectively. The δ15N values were 3.8‰~9.7‰ and 4.1‰~25.8‰.The C/N values were11.2~18.1 and 9.7~16.5, respectively. There were no significant correlation in the isotopic composition between POM and surface sediment, which showed that suspended particles and surface sediment without inheritance. The water quality of Dongjiang River had influence onisotopic composition and distribution to a certain extent.3. End-member analysis and end-member mixing model showed that the main source of POM was C3 plant and soil organic matter in headstream and upstream. While the organic materials of river gradually become the main source in the middle to lower reaches since the raw mix of detrital terrestrial and aquatic organic materials into give priority to aquatic organic materials. Compared with POM, surface sediment was to give priority to soil organic matter,since the contribution of epiphytic algae could not be ignored. The dam and reservoir made the percentages of plankton to POM increased. The organic matter into the sea was mainly from aquatic materials because of hydraulic engineering.4. We concluded that surface sediment organic matter could represent the comprehensive situation of Dongjiang in recent years, while the POM reflected the short-term character of Dongjiang River. The change of natural and geographic conditions, a large number of plantation cultivation made good vegetation restoration, reduced the soil erosion. But eutrophication level intensified in middle and downstream due to human activities intensified, made the. The contribution of aquatic organic materials significantly increased, even become the main source of downstream.Overall, the present study demonstrated that the δ13C values of suspended particles and C/N values decreased along the river, as the contribution of aquatic organic materials increased. In headstream and upstream, external source input was the main source of POM. While the organic materials of river gradually become the main source in the middle to lower reach, eventually became the main source of POM. The soil organic matter was the main source of surface sediment.