Study On Source Of Nitrogen And Nitrogen Removal Ability In The Xixi Wetland Based On Stable Isotopes

As a national wetland park,the Xixi Wetland is facing a serious risk of eutrophication due to the progress of society and the expansion of the city.In this study,the transformations and sources of nitrogen were analyzed by the monthly water quality and the nitrogen and oxygen isotopes(?¹⁵N-NO₃^-and ?¹⁸O-NO₃^-).Denitrification and nitrogen removal of the wetland were researched and discussed in particular.The main contents and conclusions are as follows:?1?The value of ?¹⁵N-NO-3 and ?¹⁸O-NO-3 in the Xixi Wetland ranged from +3.7‰ to+9.0‰ and from+1.1‰ to+13.7‰.Combined with principal component analysis and water quality parameter analysis,the nitrogen and oxygen isotopes were used to analyze the source of nitrogen in the wetland.It was revealed that sewage/manure,chemical fertilizer and soil nitrogen were the main sources of nitrogen in the wetland.?2?Nitrification and denitrification were the most important biochemical processes of nitrogen in the Xixi Wetland.Nitrification was widespread throughout the wetland,but it might be inhibited in the cold season due to the weak microbial activities.Denitrification was the key process of nitrogen in the wetland,especially in summer when the intensity of denitrification is extremely high.However,it might be greatly suppressed in winter and early spring because of the low water temperature and the high DO concentration.?3?Quantitative calculation of wetland nitrate source using SIAR model.Among them,the contribution sewage/manure?29.3%?40.0%?to the wetland nitrate was the largest,especially in autumn.Chemical fertilizers?19.5%?38.0%?and soil nitrogen?22.7%?35.0%?also contributed significantly to wetland nitrate.Atmospheric nitrogen has the smallest impact on nitrate of the wetland,with a contribution rate of5.5%to 14.8%.?4?The result of denitrification experiment in laboratory showed that the nitrogen isotope fractionation factor???of denitrification was-1.6‰.The natural isotope fractionation factors of the wetland were affected by other nitrogen biochemical processes such as nitrification and assimilation,and the values were ranged from-0.7‰to-1.0‰,of which the absolute values were significantly smaller than the result in laboratory.The results of quantification by Rayleigh curve which used nitrogen isotope fractionation factors of laboratory denitrification and natural processes showed that denitrification contributed most to nitrogen removal in the wetland,and its contribution rate ranged from 62%to 78%,with an average of 69%.Using the laboratory denitrification and natural processes results to estimate the amount of nitrogen removal via denitrifification of the Xixi Wetland.It was revealed that the results varied widely,which were from 22 kg N·ha^-1y^-1 to 798 kg N·ha^-1y^-1.And its maximum nitrogen removal efficiency was in the middle-upper levels in the same type of researches,indicating that the potential for nitrogen removal in the wetland was great.