The Preliminary Study On Effect Of Vegetation On The Denitrification Process In Sediments Of The Estuarine Tidal Flat Wetland

In recent years, with the rapid developments of industry, agriculture and animal husbandry, the highly intense human activity has given rise to the great wastewater discharge including a large quantity of reactive nitrogen into estuaries and coastal zones, consequently leading to serious eutrophication and frequent occurrence of red tide. Denitrification is one of the main pathways to remove reactive nitrogen from estuarine wetlands. Previous research on nitrogen removal had mainly focused on N2O emission, the mechanism of the coupling nitrification and denitrification, plant uptake and microbiological assimilation. However, there are few studies concerning about interaction between denitrification and environmental factors such as sulfur, active iron, organic acids in the plant habitats. Also, it needs further study to enhance understanding effects of roots on denitrification.In the present study, the eastern tidal flat of Chongming Island as a typical study area was selected to study the denitrification process using the slurry experiments combined with a15NO3-tracing technique. The main objective was to explore the effect of plants on the denitrification process and identify the prevalence pathways of removing nitrogen, by comparing the differences in the relationships of denitrification rates with environmental factors between the vegetation habitats and bare habitats. In addition, the present study estimated the contribution of denitrification to the removal of nitrogen from the estuarine wetland which could offer some reference for restoring estuarine ecosystem. The main findings of this work were as follows:(1) Based on the denitrification rates in the surface sediments from wetland, it was shown that temperature was one of the main factors affecting denitrification. And it was found that the most suitable temperature for denitrification was22℃in the bare sediments while it was25℃in the vegetation sediments. For the vegetation habitat, denitrification rates in the freshwater areas were far higher than in the brackish and saline areas, with the values of10.27±0.23μ mol N kg-1.h-1,2.51±0.24μmol N. kg-1.h-1and7.17±0.29μmol N. kg-1.h-1, respectively. In contrast, there is no considerable variability about denitrification rates in bare habitats.(2) The depth behaviors of the denitrification process showed that the integrated depth denitrification rates were relatively higher in the vegetation sediments profiles than in the bare sediment profile. At the study area, denitrification rates in the bare sediments generally increased with depth, although they also showed a great fluctuation. The rates of denitrification in the sediment cores were0.0008～1.81μmol N kg-1h-1,0.009～6.33μmol N kg-1h-1and0.13～10.38μmol N kg-1h-1in the high, middle and low tidal flats, respectively. However, the denitrification rates in the vegetation sediments cores generally decreased with depth, in the ranges of0.03～11.83μmol N kg-1h-1in the Scripus triqueter habitat,0.03～21.25μmol N kg-1h-1in the Spartina alterniflora habitat and0.007～4.32μ mol N kg-1h-1in the Phraqmites australis habitat.(3) By comparing the correlations of the denitrification with environmental factors obtained from the bare sediments and the vegetation sediments, it was found that the dominant factors affecting the denitrification process were different between the bare sediments and the vegetation sediments. According to the statistic analyses, the denitrification rates in the bare sediments were influenced positively by water content(R=0.31, P<0.01), exchangeable manganese(R=0.30, P<0.01), sulfides (R=0.24, P<0.01), total nitrogen (R=0.37, P<0.01) and negatively by pH (R=-0.50, P<0.01), Eh (R=-0.38, P<0.01), Fe3+/Fe2+(R=-0.25, P<0.01), reductive manganese (R=-0.27, P<0.01). However, in the vegetation sediments, the denitrification rates were correlated positively with water content (R=0.25, P<0.01), Fe2+(R=0.23, P<0.01), TOC (R=0.31, P<0.01), total nitrogen (R=0.22, P<0.01), and organic acids (R=0.26, P<0.01). (4) By comparing the nitrogen removal efficiency of dentirification between the bare and vegetation sediments, the average amount of nitrogen removal by denitrification was significantly higher in the vegetation sediments (3.86×103t/a) than in the bare sediments (1.58×103t/a). This comparison showed that plants in estuarine wetlands played a key role in the process of nitrogen removal.