基于同位素技术的菜地土壤淋溶硝酸盐溯源与氮转化过程研究

In our country, nitrate contamination is very serious and ubiquitous in rural groundwater, especially in areas of high agricultural activity. To date, nitrate source identification has been a central topic to rural groundwater quality studies to reduce nitrate occurrence. One of the commonly use methods is stable isotope analyses, which investigates 15N/14N and 18O/16O ratios in dissolved NO3-(referred to as δ15Nnitrate and δ18Onitrate values respectively ). However, overlaps in nitrate δ values may occur between N source types early in the leaching process, because multiple biochemical reactions from nitrate transformation and complex isotope exchange can change the original δ15Nnitrate and δ18Onitrate values. Unfortunately, nitrate transformation and isotope exchange mechanism are not clear, so it is difficult to accurately identify the nitrate sources by isotope methods. Therefore, it is very urgent to carry out related research for guiding the identification of nitrate sources by isotope analyses. .Based on the application of advanced analytical tools such as the denitrifier method, tracegas-isotope ratio mass spectrometer, the intramolecular 15N site preference of N2O, this project is accomplished in three aspects for the research of soil water in vegetable field under varying N sources types: .Firstly, to obtain the δ15Nnitrate and δ18Onitrate values by monitoring the quantitative dynamic of nitrate and to make clear the change regulation of nitrate isotope in soil water under different nitrogen fertilizer sources..Secondly, to discriminate the sources of nitrate by analyzing and comparing the relationship between water ions and δ15Nnitrate and δ18Onitrate values, meanwhile, to define the main nitrogen transformation process by identifying the N2O biological sources during nitrate leaching in soil..Thirdly, to make clear the exchange and fractionation process of nitrogen and oxygen isotopes between nitrite, nitrate, nitrous oxide and water oxygen by statistical analysis of their relationship, then to explore the mechanism of isotope exchange during leaching process. .The research results of this project can not only shed more light on the accurate identification of nitrate sources in groundwater by isotope methods, but also can improve the theoretic and technological basis to control the nitrate contamination in water in our country.

近年来氮氧同位素分析已成为识别地下水硝酸盐来源的常用手段，然而硝酸盐淋溶过程的氮转化规律及同位素交换过程与机制尚不明确，导致因不同类型氮源具重叠的同位素值而无法准确区分来源，这已成为硝酸盐稳定同位素溯源的瓶颈问题。针对这一问题，本项目拟以不同氮源处理的菜地原状土柱土壤水为研究对象，基于反硝化细菌法、Tracegas同位素比质谱仪、位嗜值测定等先进仪器及分析手段，通过分析硝酸盐氮氧同位素值，查明其变化规律；通过比较研究水质离子、同位素特征以及土壤N2O产生途径，阐明硝酸盐来源及主要的氮转化过程；通过统计分析硝酸盐、亚硝酸盐、N2O的氮氧同位素和水氧同位素值及其相关关系，明确同位素交换过程，进而揭示同位素交换机制。本项目旨在通过明确不同氮源在土壤淋溶过程中氮氧同位素变化规律和交换机制，为应用稳定同位素法准确识别地下水硝酸盐来源提供理论基础，进而使我国地下水硝酸盐污染控制与治理技术达到有的放矢。

氮氧同位素分析已成为目前识别地下水硝酸盐来源的常用手段，然而硝酸盐淋溶过程中的氮转化规律及同位素交换过程与机制尚不明确，导致因不同类型氮源具重叠的同位素值而无法准确区分来源。针对这一问题，本项目设置了有机肥、化肥、有机肥与化肥配施处理，安装了小型田间渗漏计，收集了渗漏水、土壤、气体样品，利用反硝化细菌法、同位素比质谱仪、位嗜值测定等先进仪器及分析手段，分析了硝态氮、铵态氮、N2O含量和氮氧同位素变化特征，结果表明：（1）不同肥料处理的土壤硝态氮氮同位素表现出明显的规律性，有机肥处理其土壤硝态氮氮同位素呈现有机肥来源的δ15N特征，表明土壤硝态氮的δ15N倾向于表现高的同位素组成值；有机肥与化肥配施处理，δ15N低于有机肥处理，但是仍然呈现有机肥来源的δ15N特征，化肥处理呈现化肥来源的δ15N值，该同位素规律不但出现在0-20cm表层土，同样在20-40cm，40-60cm，60-80cm土层硝态氮也出现了高的δ15N值，也可以初步判断肥料产生的硝酸盐迁移动态。（2）在初期的渗滤液中硝态氮δ15N与各处理的来源基本吻合，但是，在中期开始，渗滤液中的硝态氮已经发生了转化，其δ15N与δ18O已经不能完全反应初始的氮肥来源，这说明随着时间推移，土壤硝酸盐淋溶过程中发生了系列化学反应，已经不能很好的反应初始来源，采用稳定同位素区分硝酸盐来源具有一定的局限性。（3）根据土壤N2O微生物产生过程表明，不同肥料施入土壤后产生的氮转化过程不同，化肥以硝化作用为主导，有机肥以硝化作用和反硝化作用共同作用为主。然而，以硝态氮氮氧同位素比率来揭示氮转化过程有局限性。（4）在土壤硝酸盐淋溶过程中硝酸盐中的氧与空气、硝酸盐、H2O发生了交换，使得O同位素偏高。另外，氨挥发也可能引起同位素分馏，使剩余NH4+富集15N，结果导致NH4+硝化产生的δ15Nnitrate升高，有机肥施入土壤后出现较高的δ15Nnitrate值。本项目研究结果为采用稳定同位素研究水体硝酸盐溯源及土壤氮迁移转化过程提供了基础数据和理论依据。