| Two long-term groundwater monitoring networks were setup in their hyporeic zones for a representative rural tributary and a typical wetland in Zhushan Bay,Taihu in this study.The temporal changes of various nitrogen species,DO content,and Eh in groundwater,and the spatial distribution of soil denitrification potentials in two hyporeic profiles were measured.The results show that denitrification may occur in hyporeic aquifers near river banks;that there exist regions with higher denitrification potentials in the hyporeic aquifers near river banks,which are the so-called denitrifying hot-spots;and that the region with more plant species share higher denitrification potentials.Two different planted forests in Zhushan Bay,in the buffering zone of Taihu were chosen as the vegetation buffer strips for the comparison of their soil denitrification.One is a composite forest composed of poplar trees,shrubs and herbs,named the Poplar and Shrub forest(PSF),and the other is composed of poplar and herbaceous plants,named the Poplar forest(PF).Their floor surface runoff,the soil water and groundwater below their floor were monitored for one-year,and their soil denitrification potentials were compared.The research results show that the vertical distributions of their soil physical chemistry and biological properties,the organic matter content,and the content of various species of nitrogen at depth less than 1 meter are basically the same;that there are no significant increase trends for nitrate contents for groundwater in both forests for one year.This means that the denitrification in their soils reduces efficiently the nitrate in their soil water,and prevents the nitrate from being transported into their groundwater;that there is a maximum value for the measured denitrification potentials at depth of 40 cm in the vertical soil profiles for both planted forests,coincided with lower values for DO concentration and Eh at the same depth,proving the existence of a coupled nitrification-denitrification layer;that in the vertical soil profiles for both planted forests,all the maximum values for the soil denitrification potentials and the numbers of denitrification bacteria in these two forests occur at the depth of 40 cm,demonstrating the existence of an active denitrification layer around the depth of 40 cm,which is closely related to the root system of grass vegetation for both forests;and that at the same depth,the soil denitrification potentials for PSF are twice of those for PF,however,there is no significant difference for the actual denitrification rates for the soils in both forests,which are limited by the concentration of nitrate in soil water..Decomposition of leaf litter and dead root of plants releases dissolved organic carbon(DOC),which has important influences on soil denitrification.Six kinds of plants in Zhushan Bay,in the buffering zone of Taihu were chosen for the DOC leaching experiments of plant leaf and root.The plants respectively are Poplar,Ophiopogon japonicus,Annual Fleabane,Orthodon formosanum Kudo,Veronica persica and Achyranthes bidentata Blume.The research results show that the leached maximum DOC contents of the six plants are respectively:60.99mg/g in the water and 88.44mg/g in the soil for Poplar leaf;14.18mg/g in the water and 17.59mg/g in the soil for Poplar root;50.16mg/g in the water and 58.75mg/g in the soil for Ophiopogon japonicus leaf;38.29mg/g in the water and 45.12mg/g in the soil for Ophiopogon japonicus root;57.16mg/g in the water and 98.18mg/g in the soil for Annual Fleabane leaf;48.42mg/g in the water and 100.67mg/g in the soil for Annual Fleabane root;58.64mg/g in the water and 58.55mg/g in the soil for Orthodon formosanum Kudo leaf;18.28mg/g in the soil for Orthodon formosanum Kudo root;46.75417mg/g in the water and 78.83042mg/g in the soil for Veronica persica Kudo leaf;44.825mg/g in the water and 52.85125mg/g in the soil for Achyranthes bidentata Blume leaf;45.88042mg/g in the water and 67.23875mg/g in the soil for Achyranthes bidentata Blume root.There are found five patterns in which the leached DOC contents for the six plant species vary with time,including the types of decreasing,increasing after the first period of decreasing,impulse,stabilization and oscillation. |
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