Nitrogen Mineralization Of Lake Surface Sediments And Its Relationship With The Source And Composition Of Organic Matter

Internal pollution is one of the main reasons for the eutrophication of lakes in our country.Nitrogen mineralization occurring in surface sediment is an important form of internal pollution,which can impact the eutrophic state of a lake.In this study,surface sediments and overlying water at 16 sites in Lake Taihu were collected each season in a year.Nitrogen mineralization and immobilization were investigated by conducting indoor incubations and using the isotope dilution technique.Combining the characterization of sediment organic matter,we analyzed the relationship between nitrogen mineralization potential and the source and composition of sediment organic matter.The risk of nitrogen release from the mineralization of organic nitrogen in surface sediments of Lake Taihu was evaluated.Rates of gross nitrogen mineralization（GNM）,gross ammonium immobilization（GAI）and net nitrogen mineralization（NNM）in the surface sediments of Lake Taihu showed significant seasonal and spatial variations.During the investigation period,GNM rates of surface sediments ranged from 0 to 26.67 mg N/（kg·d）with a mean of 3.72±5.15 mg N/（kg·d）,GAI rates ranged from 0 to 15.79 mg N/（kg·d）with a mean of 2.06±3.01 mg N/（kg·d）,and NNM rates ranged from-4.10 to 14.64 mg N/（kg·d）with a mean of 1.65±2.98 mg N/（kg·d）.Season-averaged GNM and NNM rates of surface sediments both showed the pattern of summer>spring>autumn>winter.During the summer,nitrogen mineralization occurring in surface sediments in Meiliang Bay was high,with GNM and NNM rates at 18.19±7.53 mg N/（kg·d）and 9.22±4.94 mg N/（kg·d）,respectively.Correlation analyses showed that GNM,GAI,and NNM rates in surface sediments of Lake Taihu were significantly and positively correlated with temperature and water column p H,and were significantly and negatively correlated with the oxidation-reduction potential of water column and surface sediment as well as with the p H of surface sediment.The absorption spectroscopy characteristics of the extractable chromophoric dissolved organic matter（CDOM）in the surface sediments of Lake Taihu showed significant seasonal variations.The relative abundance of CDOM was the highest in spring,followed by summer and winter,and the lowest in autumn.The humification degree and molecular weight of extractable CDOM were the highest in spring while the lowest in winter.Among the three fluorescent components of CDOM,the relative abundance and proportion of the tyrosine-like component C2 was the highest,followed by the terrestrial humus-like component C1,and the tryptophan-like component C3 the lowest.Based on the natural abundances of stable carbon and nitrogen isotope(δ¹³C andδ¹⁵N)using the mixed isotope model（Mix SIAR）,the source contributions to sediment organic matter showed that sediment porewater contributed the most at Meiliang Bay,Gonghu Bay,and the lake center;inflow streams contributed the most at Zhushan Bay,Western Lake area,and Southern Lake area;domestic sewage contributed the most at Eastern Lake area and Eastern Lake area including Xuhu Bay.Correlation analyses between the nitrogen mineralization and immobilization rates and the composition of extractable CDOM in surface sediments showed that GNM,GAI,and NNM rates were significantly and positively correlated with the relative abundance of CDOM（a（254））（p<0.01）,and the GNM and NNM rates were significantly and negatively correlated with the molecular weight of surface sediment CDOM（p<0.05）.The GNM,GAI,and NNM rates during summer seemed to be most strongly influenced by the composition of extractable CDOM in surface sediment,which were all significantly and positively correlated with the relative abundance of CDOM and the relative abundance of each fluorescent component of CDOM.And the rates of GNM and GAI were also influenced by the molecular weight of CDOM.These analyses suggested that,during the summer“nitrogen limitation”period,nitrogen mineralization occurring in surface sediments is one of the important pathways for nitrogen supply into the overlying water.The highest sediment GNM rate occurred in Meiliang Bay during summer,where sediment organic matter is mainly contributed by sediment porewater,inflow streams,domestic sewage,and algae degradation.The results of this study will provide basic data and analyses for identifying the source and contribution of internal nitrogen in Lake Taihu.This study is also meaningful for the control of internal pollution in Lake Taihu.