Temporal And Spatial Differences Of Nitrogen Removal Rates And Their Controlling Factors In Tianmuhu Lake Basin

Eutrophication affects the water ecological security of lake and reservoir seriously.Nitrogen is one of the main causes as it is an essential element for biological growth,so nitrogen control is considered as an important ways to control eutrophication.In lake and its basin,denitrification and anammox are microbial reactions which can transform inorganic nitrogen into N₂ and separate from water.It is great significance for maintaining nitrogen balance and alleviating eutrophication.Nitrogen-fixing microorganisms,especially nitrogen-fixing cyanobacteria,can convert N₂ into bioavailable nitrogen when nitrogen is insufficient in water.Ponds and ditches are widely distributed in agricultural basin,which is one of the important nitrogen removal hot spots.It plays an important role in reducing non-point source nitrogen pollution and lake and reservoir nitrogen.In this study,the seasonal variation of nitrogen in the Tianmuhu Lake and the dissolved nitrogen in the reservoir were monitored,the nitrogen balance of the reservoir and the potential nitrogen removal rates of the reservoir sediments were studied,and the nitrogen removal rate of the ponds and ditches in the reservoir basin were investigated.The main results are as follows:（1）The water quality of Tianmhu Lake was monitored for a whole year from February2021 to January 2022,and the N₂:Ar saturation（ratio of actual N₂:Ar to theoretical N₂:Ar）of water was measured.The results showed that the water quality of Tianmuhu Lake was better in winter and spring,and the abnormal proliferation of algae appeared in late summer and early autumn.The TN concentration in the reservoir is relatively stable throughout the year,with an annual value of 0.87mg/L,the peak value was monitored in August after the rainstorm.The annual nitrogen and argon saturation（the ratio of measured N₂:Ar to theoretical N₂:Ar）ranges from 0.95 to 1.07,with an annual mean of 0.99,indicating that dissolved nitrogen（N₂）in Tianmu Lake is almost saturated.In spatially,the N₂:Ar saturation in the downstream is higher than that in the upstream,indicating that the nitrogen removal rates is faster in the downstream.In vertically,hydrothermal stratification occurs in the middle and lower reaches in summer,and anoxic or anaerobic environment gradually forms in the bottom layer,which provides a suitable reaction environment for denitrification and anammox,while stable hydrothermal stratification does not take advantage of nitrogen removal.The relationship between high temperature and nitrogen-fixing cyanobacteria is small,and nitrogen removal reaction is greatly limited in winter,resulting in unsaturated nitrogen in winter.（2）The potential denitrification and anammox rates of Tianmuhu Lake in autumn（September）and winter（December）were measured by isotope mud method.The results showed that the potential nitrogen rates were（4.29±0.37）to（10.93±0.93）nmol/（g·h）and（0.74±0.02）and（4.87±0.10）nmol/（g·h）,respectively.The TM6（middle reaches）is the hot spot of nitrogen removal reactions in the whole reservoir,followed by the TM1（lower reaches）,and the TM10（upper reaches）has poor nitrogen removal ability.Denitrification is the main approaches of nitrogen removal in reservoir,and anammox is more sensitive to temperature drop,resulting its contribution to nitrogen removal decreased from 21.31%to 5.19%.Due to the characteristics of mud method,nitrogen concentration and dissolved oxygen concentration are not the main limiting factors of nitrogen removal reaction.And higher water content of sediment and lower chlorophyll-a concentration of water are conducive to nitrogen removal reactions.The linear fitting between N₂:Ar saturation in the upper layer of sediment and potential nitrogen removal rate of sediment showed high fitting effect（R²=0.55）,indicating that the measurement of N₂:Ar saturation in the upper layer of sediment can explain the nitrogen removal rate of sediment to a certain extent.（3）Undisturbed sediment column cores of ponds and ditches with different typical land use types were collected in the spring（March）and summer（June）of 2021,in the Tianmuhu Lake Basin,China.The denitrification and anammox rates of different ponds and ditches were determined through the isotope matching technology with indoor continuous-flow incubations.The results showed that,the ponds and ditches had a high nitrogen removal rate,ranging from10.59 to 107.65μmol/（m2·h）in spring,and from 32.07-150.10μmol/（m2·h）in summer.The orchard ditch showed the highest nitrogen removal rate in spring and the tea garden retreat pond showed the highest nitrogen removal rate in summer.The average contribution of anammox to nitrogen removal was 45.45%in spring and 36.26%in summer.In summer,the contribution of anammox to nitrogen removal in domestic sewage discharge pond and tea garden retreat pond decreased 26.82%and 14.98%,respectively.The contribution of anammox in orchard ditch and river discharging to Tiamuhu Lake had little change between spring and summer,the mean value were 21.17%and 48.99%,respectively.Statistical analysis shows that the concentration of dissolved inorganic nitrogen（DIN）in water and the content of organic carbon in sediment were significantly correlated with the sediment nitrogen removal rate.In addition,the nitrogen removal rate of ditches and ponds was higher under the conditions of high temperature and low dissolved oxygen.This study investigated the nitrogen form and seasonal variation of dissolved nitrogen in Tianmuhu Lake,a water-source reservoir,as well as the nitrogen removal rate of sediments in reservoir and ponds and ditches in basins were measured,and revealed the limiting factors.This will contribute to a better understanding of the nitrogen cycle in lake and reservoir,and to a better management of nitrogen to reduce eutrophication from the perspective of lake,reservoir and its basin.