含微量氮样品的氮同位素分析方法开发及在古环境中的应用

In recent years, we have confronted a series of tough environmental problems, which were caused by excessive release of reactive nitrogen (Nr) from the processes of over-fertilization in agriculture, combustion of fossil fuel and other human activities. The influences on ecology and environment exerted by the Nr cannot be effectively controlled until we fully understand the process and mechanism of the nitrogen cycle and its response to environmental changes and human perturbation. Stable nitrogen isotope analysis can serve as a powerful tool to constrain nitrogen cycling processes and has received a wide application. However, we can't accurately analyze nitrogen isotopes in the samples containing trace amount of nitrogen (such as loess and paleosol) yet due to the limitation of current analytical method. This has greatly limited our understanding of the processes of nitrogen cycle in terrestrial ecosystems. In this project, we attempt to realize high precision N isotopic measurement on the samples containing trace amount of nitrogen through reforming the available element analyzer - isotope ratio mass spectrometer (EA-IRMS) by incorporation of the eight-port valco valve and replacement of the interface between EA and IRMS. Meanwhile, as a vivid example, we plan to choose Lijiayuan section located in the west part of the Chinese Loess Plateau as our study site and measure nitrogen isotopes in loess and paleosol samples using the above reformed instrument, with objective to reconstruct the processes of nitrogen cycle in arid and semi-arid region since the Last Glacial Maximum. Moreover, we will also examine the impacts of past human activities on the nitrogen cycle since 5ka. The analytical method generated in this project will boost a broad application of nitrogen isotope analysis to study N cycle in terrestrial ecosystems, which would ultimately provide important scientific data for environmental management to counteract Nr.

近年来，农业过度施肥、化石燃料燃烧等人类活动导致了活性氮的大量释放，产生了一系列环境问题。只有正确认识生态系统氮循环过程及其对环境变化和人类活动的响应机制，才能有效控制其产生的环境效应。氮同位素研究是了解生态系统氮循环过程的重要手段。然而，利用现有仪器我们尚无法精确分析含微量氮样品（如黄土、古土壤等）的氮同位素组成（δ15N），极大限制了我们对陆地生态系统氮循环过程的理解。本项目拟对元素分析仪-同位素比值质谱仪（EA-IRMS）进行改造，通过增加八通阀和更换EA与IRMS的接口等方式提高样品收集效率，实现含微量氮样品的δ15N高精度测定。同时，我们拟选择黄土高原李家塬剖面，通过分析黄土、古土壤中δ15N值，初步探讨末次盛冰期以来气候变暖背景下，干旱-半干旱区植被生态系统的氮循环演化过程，以及5ka以来区域人类活动对氮循环的影响。此项方法将会推动氮同位素在陆地生态系统氮循环研究中的广泛应用。

本项目建立了微量氮同位素的高精度分析系统。首先，对仪器EA-IRMS的接口ConFlo进行了改造，通过引入八通阀和带有分子筛的样品环的过渡，保证经由EA产生的样品气能够完全收集在样品环中，并由氦气带入程序升温的气相色谱，再经小流量的open split将气体顺利带入质谱仪测定，大大提高了离子流强度。其次，成功将系统的氮本底由原有的548nmol N降至10.7nmol N，并给出了较为稳定的氮同位素值（δ15NBlank =-4.54 ± 0.36 ‰ AIR, n=32, 1SD）。通过上述仪器的改造和本底的控制，实现了微量氮同位素的高精度测定，并且利用标准样品验证了系统的可行性，将氮同位素测定的检测限由原有的5000nmol N降至40nmol N，精度优于0.26‰。此外，我们利用此套微量氮同位素分析系统测定了岱海湖泊沉积物中黑碳的氮同位素，重建了过去1万年以来我国北方生态系统中氮同位素的演化历史，揭示了在7400年前我国北方的原始农业就显著地改变了当地土壤-植物氮循环过程。