| Nitrate is one of the vital forms of biological available nitrogen.In the last 30 years,the study of nitrate nitrogen and oxygen isotope has gradually become one of the most important methods when studying nitrogen cycle,providing many new insights for the research of nitrate source and process in atmosphere,water,soil and biological procedures.The source and distribution characteristics of nitrate in waters are the key factor to evaluate the quality of environment and the nitrogen cycle.However,the present methods for analyzing the nitrogen and oxygen isotopic composition of nitrate in waters still have procedures to be adapted,such as improving conversion efficiency,shortening reaction time and automatic sampling.In this study,the chemical conversion method which determines the nitrogen and oxygen isotopic composition in nitrate is adapted by replacing the nitrate reduction method,modifying the instruments used to measure N2O and deteming the efficiency of azid reaction;established the adapted mehod and procedure which is suitable for freshwater and seawater,simplify the experiment,shorten the measurement time and improved the determination efficiency,then the adapted method is used to determine the nitrogen and oxygen isotopic composition of nitrate in the water samples from Xiamen coastal areas,the main results were obtained as follows:(1)The adaptation of nitrate reduction procedure is replacing the Cadmium column reduction method with Ammonia-Zinc-Cadmium method.The optimum pH range for Ammonia-Zinc-Cadmium method is 6-8.If the sample is in this range,then there is no need for adjusting the pH.The simplified method for reducting nitrate is:adding 0.8 mL cadmium chloride solution(20g/L),0.8 mL ammonium chloride solution(250g/L)and 3cm*10cm of clean Zinc sheets(99.9%)to water samples,and then oscillate under the condition of 220r/min for 15min.This method can shorten the reduction procedure and improve the efficiency.(2)In the range of nitrate concentration from 5 to 100?mol/L,the efficiency of azid reaction is above 99%and the maximium efficiency of Ammonia-Zinc-Cadmium method is 92.9%;by comparing the difference between those two efficiencies,can speculate that the azid reaction would not cause nitrogen and oxygen fractionation,but could be casused by the adapted nitrate reduction procedure.(3)The modification of Gas-Bench continuous gas introducing device includes redesigning the sample tray,double the length of Nafion water trap and optimizing the column temperature to improve the separation outcomes,then built a device which use the 60mL headspace vials as sample reactor,automatically sampling without Pre-Con device and can determine the N2O,the instrument structure is simplified and the liquid nitrogen is no longer needed.(4)The nitrogen and oxygen isotope calibration curve in this study have the slope of 0.47 and 0.89,which matched the reported results.The slope of oxygen calibration curve is 0.89,which is more closer to the theoretical value of 1 than reported results,indicating using Ammonia-Zinc-Cadmium method may inhibit the oxygen exchange during reduction process and could get more accurate results of the samples.The results of using Ammonia-Zinc-Cadmium method and Cadmium column method to determine the same batch of water samples have no significant differences within the range of error,indicating the adapted method is reliable,and from the aspect of simplifying the determine procedure and shorten the reaction time,Ammonia-Zinc-Cadmium method is worthy of popularization.(5)The results of nitrogen and oxygen isotopic composition of nitrate from Xiamen coastal area samples(?15NAir value is +4.9-?+6.1‰ and ?18Ov-smow value is-2.1?+14.29‰)match the reported results.According to the results from two sections/six stations,the nitrate in Xiamen coastal areas is mainly from Jiulong River,nitrate from this source is affected by insutrial wastewater and domestic sewage emmisions then used by biological absorption and assimilation,which results in high?15N and low ?18O,meanwhile the coastal areas are affected by the tidal current from Taiwan Strait,nitrate from this source is affected by biological nitrogen fixation and nitrification,which results in low ?15N and high ?18O.Therefore,the nitrate in Xiamen coastal areas is the result of the mixation of river runoff and tidal currents. |
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