| Urban river as major inland waters, plays an important role in the city. In recent years, the social and economic rapid development makes a big rising in urbanization level. And water environment pollution also highlighted at the same time, water pollution become more obvious. River sediments get a lot of nitrogen pollution from the city, and it makes river sediments a potential source of pollution. When nitrogen released into the river water, it will causes water pollution in cities. Denitrification can permanently remove N from sediment, so it is the most effective way to reduce river NO3-. The special environment of river sediment makes its denitrification different from other soil or sediment. Sediment denitrification has its own characteristics. Shanghai has a large water area, and there are different characteristics of polluted river. And river pollution situation is directly related to the ecological security of the whole city. So the study of river sediment denitrification and its main influencing factors is of great significance. After one year of study, the main conclusions include the following aspects:(1)The average water nitrate concentration, Ammonium nitrogen and TOC showed a certain spatial distribution features, which ranged from 1.00 to 1.70mg/L,0 to 10.54±0.10mg/L and 0 to 112.12mg/L. The average nitrate concentration and Ammonium nitrogen in summer was higher than spring, autumn and winter. The seasonal variation of TOC showed winter> summer> autumn> spring. The water content and Bulk density changed from 49% to 189% and 0.84 to 2.11g/cm3. pH ranged between 5.86 and 7.14, and characterized by weak acid. Eh ranged between-36.50 and -190.23mv. The SOC varied from 42.74 to 208.19mg/g, while in spring and summer were higher and in autumn and winter were lower. Sulfide was higher in summer and autumn, and average were 25.60ng/g and 32.24ng/g, the lowest was 6.31ng/g in winter. NO3--N and NH4+-N were ranged from ND to 17.46mg/kg and 67.65 to 1642.42mg/kg, NO3--N of some samples were not checked out. On vertical distribution, NO3--N was as depth increases gradually reduce, NH4+-N had no obvious regularity.(2)The denitrification rate of river sediment in Shanghai varied from 1.33±0.29 to 3.59±1.16mgN·m-2·h-1, which had obvious regularity on time and space distribution and vertical distribution. The seasonal variation showed summer and autumn were high and winter and spring were low. The denitrification rate in summer and autumn were 2.62±0.36mgN·m-2·h-1 and 3.03±0.29mgN·m-2·h-1; denitrification rate in winter and spring were 2.40±0.13mgN·m-2h·-1 and 2.41±0.37mgN·m-2·h-1. On spatial distribution, different sediment denitrification rate exist significant regional difference, showed FX, JS, PT, SJ and PD were higher than JD, CM and QP. The denitrification rate in the 0-5 cm ranged from 2.72±0.57 to 18.78±2.47ngN·g-1·h-1, and decreased from the soil surface to the deeper layer and was higher in the top 0-2cm. The correlation analysis showed that the sediment denitrification rate was positively correlated with pH, SOC, water content and nitrate nitrogen (P<0.01), and was negatively with Eh, sulfide (P<0.01). But the denitrification rate had no correlation with Bulk density.(3)Represented by QP and PT, the denitrification potential showed the regularity on temporal and spatial and vertical variation. The denitrification potential of PT was higher than QP. The spring had the highest denitrification potential. In spring, the denitrification potential was 800.21±75.42mgN·m-2·h-1 in PT,162.89 ±28.11mgN·m-2·h-1 in QP; in summer, the denitrification potential of PT was 120.96±23.32mgN·m-2·h-1, QP was 21.42±4.33 mgN·m-2·h-1; in autumn, the denitrification potential of PT was 169.40±29.03 mgN·m-2·h-1, QP was 52.92±13.31mgN·m-2·h-1; in winter, they were almost the same. On vertical variation, the denitrification potential ranged from 5.34±1.74ngN·g-1·h-1 to 1964.32±176.36ngN·g-1·h-1. It showed a large span, which had the maximum in the top sediment in spring and the minimum in the bottom sediment in summer. The denitrification potential of PT was positively correlated with sulfide (P<0.05). The denitrification potential of QP was negatively correlated with Eh and SOC (P<0.05).(4)The N2O natural rate of river sediment in Shanghai varied from 0.66±0.10 to 2.68±0.58 mgN·m-2·h-1, which spring and summer were higher than autumn and winter. The average N2O natural rate was 1.87±0.30mgN·m-2·h-1 in summer,1.69±0.31mgN·m-2·h-1 in spring, 1.28±0.17mgN·m-2·h-1 in autumn and 0.99±0.12mgN·m-2·h-1 in winter. N2O natural rate in PD was the highest, average rate was 1.88±0.35mgN·m-2·h-1. QP and JD were the lowest, others were in the middle.The N2O natural rate was also positively correlated with pH, SOC, water content and nitrate nitrogen (P<0.01), and was negatively with Eh (P<0.01), but had no correlation with Bulk density and sulfide.(5)Shanghai river sediment had large denitrification ability. The difference between denitrification rate and water area make the difference amount of denitrification. The summer and autumn had a large amount of denitrification, which was 54.01% of the annual total; the spring and winter was 45.99%. Shanghai river sediment had large denitrification potential ability, Spring was 69.22% of the annual total, summer and autumn was 26.21%, winter was only 4.58%. The top 2cm of sediment had larger denitrification potential than the bottom. The river sediment in Shanghai had a huge denitrification amount of 100Gg, which was equal to the nitrogen input 70Gg. So river sediment was a huge nitrogen sink and N2O emission source. In combination with the seasonal and vertical regular, reasonable governance can alleviate N pollution in Shanghai rivers. |
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