| The technique of using nitrogen and oxygen isotopic compositions to identify nitrogen sources and assess the nitrogen cycle process has been extensively applied in fresh water research in the last few decades. Nitrate pollution in fresh water has caused concern worldwide:if nitrate sources could be identified, this major form of pollution could be combated. Thus, this technique provides much-needed scientific help with the remediation of fresh water. As the method of determining nitrogen and oxygen isotopic ratios has become more precise, many deficiencies in earlier methods of determination have been overcome. Moreover, each kind of nitrate source and some isotopic fractionation processes have also been well studied; all these factors play an important role in research into the use of isotopic tools in tracing nitrate sources. The joint use dual isotopes with water chemistry are indeed providing very detailed and useful information relating to the identification of nitrate sources and the tracking of nitrate transformation in freshwater systems. Surface water samples were collected from Taihu Lake and Chaohu Lake in three different periods. The current study can be divided into three parts to discuss, such as Taihu Lake (collected in December.2007); Taihu Lake (collected in January and July of2009, respectively); Chaohu Lake (collected in January and July of2009. respectively). The δ15N and δ18O-NO3-values.as well as major ions tracers, from collected water samples in Taihu Lake and Chaohu Lake were investigated to identify the primary nitrate sources and assess nitrogen biogeochemical process in the present study. The results of Taihu Lake (collected in December,2007) showed that the nitrate concentration in North Taihu Lake (WTL) was generally higher than those in East Taihu Lake (ETL) and its upstream inflow rivers. The NO3-/Cl-value combined with mapping of δ15N-NO3-and NO3-concentration [There was a positive correlations (R2=0.83,0.63respectively) between Cl-and NO3-levels can be observed in WTL] suggested that mixing process should play a major effect in WTL, and denitrification was the dominant nitrogen transformation process in WTL. A linear relationship of close to~1:2was observed between δ15N-NO3-and δ18O-NO3-values in NTL. confirming the occurrence of denitrification in NTL. The nitrogen isotopic composition of local nitrate sources were comparable with previously published values. The δ15N-NO3-in ETL ranged from3.32%o to6.l7‰(n=5. mean=5.1‰). and the δ15N-NO3-in WTL and rivers ranged from5.69%o to10.35%o (n=7, mean=8.79‰) and from7.33‰to10.1l%o (n=5, mean=9.35‰). respectively. The δ15N-NO3-data implied that sewage and manure were the principal nitrate sources in WTL and its feeder rivers, while the nitrate in ETL might derive from soil organic nitrogen and atmospheric deposition. The range of determined δ188O-NO3-value in the current study varied from4.0‰to12.4‰(n=17, mean=7.9‰). The δ18O-NO3-data indicated most of nitrate from microbial nitrification of organic nitrogen matter could possibly make a significant contribution to the lake.In January and July of2009, the Cl-concentrations in Taihu Lake basin were found to range from58.33to85.72mg/L in winter and from30.52to69.22mg/L in summer. with average values of66.68and56.21mg/L. respectively. This seasonal variation was also observed in SO42-and NO3-concentrations, in which the mean values of SO42-and NO3-in summer (73.54and3.87mg/L, respectively) were lower than those in winter (84.49and5.53mg/L, respectively), which clearly suggested a dilution effect by rain water in summer months. The relationship between NO3" and Cl-suggested that a mixing effect dominated the nitrate transformation process in Taihu Lake in winter. The δ15N-NO3-values in watersheds varied from+5.3%o to+22.2‰(n=6, mean value=+15.l‰) in winter, and from+0.5‰to+20.6‰(n=8, mean value=+9.2%o) in summer. The δ15N-NO3-values in NTL varied from+4.0‰to+17.8‰(n=7, mean value=+8.7‰) in winter, and from+5.2‰to+11.1‰(n=7. mean value=+9.7%o) in summer. Moreover, the δ15N-NO3-values in ETL varied from+1.8‰to+5.9‰(N=5, mean value=+4.4‰) in winter, and from+8.1%o to+10.2‰(n=7. mean value=+9.4‰) in summer. The δ15N-NO3-values suggested that: In winter, sewage and manure were the primary nitrate sources in major inflow rivers and NTL. while nitrate sources in ETL probably derived from soil organic nitrogen. In summer, atmospheric deposition and swage/manure inputs appeared to play an important role in controlling the distribution of nitrates in the whole Lake. The δ18O-NO3-values in Taihu Lake basin ranged from+0.4%o to+17.9%o in winter and from+0.4‰to+17.9‰in summer, with average value of+6.3‰and+6.6‰. respectively, suggesting that the nitrate produced from microbial nitrification was another major nitrate sources during both winter and summer months. The variations in isotopic values in nitrate suggested that denitrification enriched the heavier isotopes of nitrate in NTL in winter and in ETL in summer. About the results of Chaohu Lake (collected in January and July of2009, respectively). The nitrate concentrations and their nitrogen and oxygen isotopic compositions in the water samples were analyzed. The nitrate concentration ranged from0.30to13.94mg/L in winter and from0.08to5.14mg/L in summer, with average values for6.72and2.10mg/L, respectively, which exhibited a large seasonal variation, suggesting the dilution effect of rainwater. There was a positive correlation (y=0.2x+4.0. R2=0.74) between NO3-and Cl-in WCL in summer, indicating the occurrence of mixing process in WCL in summer. In the current study, the δ15N-NO3-values in the water samples ranged from+5.8to+16.1‰in winter with a mean value of+10.9%o (n=8), and from+5.2to+9.9‰in summer with a mean value of+6.7‰(n=15), and the δ15N-NO3-values for the rivers in summer (mean value=13.7%o, n=5) were evidently lower than those in winter (mean value=6.9‰, n=5). The data concerning δ15N-NO3-values in winter suggested that the nitrate sources of inflow rivers derived mainly from sewage and manure inputs, while the nitrate sources in the southern part of Chaohu Lake were possibly influenced by chemical fertilizer or plant derived nitrogen; in summer, seasonal fertilizer application in the lake watersheds and rainwater probably contributed more nitrates into the rivers and the lake. The δ18O-NO3-values did not exhibit distinct seasonal variations in rivers and Chaohu Lake, and the δ18O-NO3-values of collected water samples in both winter and summer scatted from+0.7to+13.5‰. with a mean value+7.1‰(n=23).The δ18O-NO3-values suggested that nitrate produced from microbial nitrification could also be a significant nitrate source in the Chaohu Lake basin. The slope of the linear regression between δ15N and δ18O-NO3-values supported the hypothesis that the denitrification process may control the isotopic composition of nitrates in East Chaohu Lake in summer. |
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