| Riparian zone is to be the effective sink of removing the nitrate into the water body (rivers and streams), and is also an important place to prevent pollutants from entering into the aquatic environment. Plant absorption, denitrification, microbial fixation are the main three ways to remove the nitrogen in riparian zones, in which the soil denitrification can permanently remove most of the nitrogen, so denitrification is seen as the most effective way. Strongly influenced by human activities, urban riparian zone is different from the rural and suburban riparian zones. So the soil denitrification has unique characteristic in urban riparian zone.This paper was supported by the project of "National Major Water Special Topic", which was approved by China’s ministry of environmental protection, funded by National Natural Science Fund:The study of nitrogen retention efficiency and transfer mechanism in riparian zone in coastal city, and the Project of Science and Technology Commission of Shanghai. The research selected the riparian soil in Shanghai as the object, using the multi-inter disciplinary means (physical geography, environmental geochemistry, environmental science and engineering). The research was based on the combination method of field sampling, laboratory analysis and indoor simulation. The soil denitrification rate in Shanghai riparian zone was systematically studied, including the space-temporal variation and vertical characteristics. The soil sampling in the urban riparian zone was also collected to study the denitrification rate with different vegetation types. The denitrification potential of the top soil (2-5cm) was studied in the rural riparian zone. At last the annual average removal rate in Shanghai riparian zone was also calculated. The main conclusions are as follows:(1)The soil denitrification rate in Shanghai urban riparian zone varied from0.07ngN2O-N·g-1·h-1to33.23ngN2O-N·g-1·h-1, which had significant space-temporal variation. And the nitrogen annual average removal rate due to the soil denitrification was27.51mgN2O-N·g-1·h-1.The average soil denitrification rate in August has the highest rate in all of the seasons with6.63ngN2O-N·g-1·h-1; While the soil denitrification rate in spring in April and winter in December were low with0.74ngN2O-N·g-1·h-1and0.75ngN2O-N·g-1·h-1, respectively. The denitrification rate in riparian soil near the farmland (PD and JD) are higher than those in bushland (JS and CM) and grassland (QP, MH and PT). The JD sampling site has the highest denitrification rate with5.10ngN2O-N·g-1·h-1，and QP has the lowest denitrification rate with0.87ngN2O-N·g-1·h-1.(2)Denitrification rate decreases from the soil surface to the deeper layer and is higher in the top2-5cm. The soil denitrification rate in the2-5cm,12～15cm,22～25cm,32～35cm layer were4.18ngN2O-N·g-1·h-1,2.26ngN2O-N·g-1·h-1，2.19ngN2O-N·g-1·h-1and1.61ngN2O-N·g-1·h-1, respectively. The capacity of removing nitrogen in the top2-5cm accounted for about41%, and that in other three layers were22%,21%,16%, respectively.(3)The soil denitrification rate in urban riparian zone with different vegetation types had significant difference. The mean soil denitrification rate in high bushwood, low bushwood, grassland were0.76ngN2O-N·g-1·h-1,0.93ngN2O-N·g-1·h-1,0.54ngN2O-N·g-1·h-1, respectively. The mean soil denitrification in low bushwood was the highest, and that in the grassland was the lowest, which was agree to the investigated results in the Shanghai riparian zones.(4)The correlation analysis showed that the soil denitrification rate was positively correlated with temperature, ammonia nitrogen (NH4+), nitrate nitrogen (NO3-)(p<0.01), and was negatively with water content (p<0.01). But the denitrification rate had no correlation with pH value, soil organic carbon (SOC), soil total nitrogen (STN), carbon and nitrogen ratio (C:N).(5)The incubation temperature in Shanghai riparian zone changed from8℃to30℃with low in winter and spring and high in summer and autumn; The soil pH varied from6.70to8.39, and overall in neutral partial level. The soil water content varied from11.58%to60.38%, and was higher in spring and winter, lower in autumn and summer. The SOC and STN content were from0.22%to2.23%and from0.03%to0.18%, respectively. The soil carbon and nitrogen ratio varied from5.69to27.11, with the mean value was9.84. The extraction ammonia nitrogen changed from0.03mg·kg-1to3.98mg·kg-1and the extraction nitrate were from none detected to7.03 mg·kg-1. The extraction nitrate in QP and MH was so low that can not detect.(6)The average nitrate concentration in PD, JS, JD were0.68mgN·kg-1,1.31mgN·kg-1,1.33mgN·kg-1, which can be seen as the rich nitrogen systems. While those in QP, MH, CM were0.12mgN·kg-1,0.21mgN·kg-1,0.21mg N·kg-1, which were the poor nitrogen systems. Soil denitrification rate has positive linear correlation with nitrate concentration in the relatively rich nitrogen systems (PD and JS). But in the nitrogen restrictived systems, the soil denitrification rate was obvious lower than the rich nitrogen systems. Nitrogen concentration was the main factor limiting soil denitrification process.(7)The experiments conducted in the laboratory for the nitrate restrictive riparian systems show that the situ soil denitrifying in the nature state was very weak without nitrate input. When the nitrate was adding into the soil, denitrification rate was obviously higher. The higher of the temperature, the more greater of the denitrification rate, which can be confirmed from the simulation experiments in QP, MH, CM in August. The results showed that temperature and nitrate concentration were the two important factors influencing riparian soil denitrification.(8) The controllable factors of laboratory simulation showed that the best temperature range was25～35℃for the denitrification. When the temperature reached to40℃, the denitrifying process was inhibited; The riparian soil denitrification potential needed a best water condition. When the soil water content was about25%, the denitrification rate was in the peak. Too high or too low moisture content would inhibit denitrification process.The simulation experiments of ammonia nitrogen and nitrate nitrogen influencing soil denitrification potential showed that adding nitrate nitrogen into the soil can significantly improve the denitrification potential. But when the nitrate nitrogen concentration was high enough, the soil denitrification potential can be inhibited. In all of the incubation time series, the soil denitrification potential was the highest in24hours. The simulation experiments of different carbon sources influencing soil denitrification potential showed that glucose was the most effective carbon source, but it needed to consume a long time to affect the soil denitrification microbial. Methanol was limited to the denitrifying process because methanol only has one carbon atom. The role for the ethanol and sodium acetate in promoting the soil denitrification were between the glucose and methanol.(9)The riparian soil potential in Shanghai is huge. If the soil denitrification potential of removing nitrogen is fully to be explored, the nitrogen pollution load of urban non-point sources will be relieved to a large extent. |
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