用硝酸盐氮氧同位素自然丰度量化我国南方森林土壤硝化和反硝化作用速率

With the globalization of nitrogen deposition, it is critical to understand how and to what extent nitrogen deposition affects nitrogen cycling of forest ecosystems, particularly on processes and rates of soil nitrificaiton and denitrification. Quantifying rates of nitrification is labor-intensive and time- consuming, and the losses of nitrogenous gases via denitrification are difficult to measure directly. In this proposed project, we attempt to develop two new isotope-balance approaches to quantify net rates of soil nitrification and dentirification, respectively, in forested watersheds using nitrate isotopes (δ15N and △17O). Our isotope approaches are applicable to both natural or managed ecosystems. Eight long-protected broadleaved forests at five sites across southern China from tropical (Hainan) to north subtropical regions (Hubei) will be chosen. These five sites encompass a climate gradient with decreasing annual mean temperature and annual precipitation from the south to the north. These selected forests are also supposed to be largely different in nitrogen status because of different climates and nitroegn deposition rates. We will measure: 1) nitrogen inputs via atmospheric deposition, and nitrogen loss via stream leaching; 2) stable nitrogen and oxygen isotopes of nitrate in precipitation, throughfall and stream water; 3) stable nitrogen isotopes of nitrate produced from soil nitrification, and 4) isotope effects of denitrification on nitrate. Nitrate isotopes will be analyzed by the denitrifier method. In so doing, we can calculate annual rates of nitrification and denitrification from the study forests using the isotopic approaches we developed and link these rates to other nitrogen variables, such as nitrogen deposition and soil nitrogen status. The results from this project will privide important insights from stable isotopes into ecosystem nitrogen status and nitrogen balance in southern China in the context of increased nitrogen deposition.

随着氮沉降全球化，研究森林生态系统氮素循环特征，特别是土壤硝化和反硝化作用速率，对于预测全球气候变化下森林生态系统功能及其反馈非常关键。然而，传统方法测定土壤硝化速率需要频繁野外采样和大量化学分析，目前也没有在生态系统尺度上直接测定反硝化作用速率的方法。本申请项目拟以我国热带到北亚热带五个森林站点的8个常绿阔叶林集水区为对象，测定1）大气沉降（降水和穿透雨）氮输入和溪水氮流失速率；2）大气沉降和溪水硝酸盐氮氧同位素组成特征（δ15N和△17O）；3）硝化作用产生的硝酸盐氮同位素组成特征；4）反硝化作用过程氮同位素分馏系数，从而根据质量平衡和同位素质量平衡的原理实现在生态系统尺度上量化土壤硝化和反硝化作用速率。通过本项目的实施，将为利用同位素技术量化各种森林生态系统通过反硝化作用的气态氮损失提供一个新思路和快捷方法，增加在氮沉降增加的背景下对我国南方森林生态系统氮状态和氮收支平衡的认识。

氮沉降全球化使得研究森林生态系统氮素循环特征，特别是土壤硝化和反硝化作用速率变得至关重要，对于我们有效预测全球气候变化下森林生态系统功能及其反馈非常关键。然而，传统方法测定土壤硝化速率需要频繁野外采样和大量化学分析，目前也缺乏生态系统尺度上直接测定硝化、反硝化作用速率的方法。为此本项目（1）创立和优化了一系列氨氮、硝氮同位素分析方法；并（2）采用国际首创的硝酸盐氮氧同位素自然丰度法量化了中国南方热带森林、日本中部温带森林的多个集水区的氮收支；我们将此方法进一步扩大应用范围，选取了跨越我国温带、亚热带、热带的典型森林类型多个集水区，结合野外观测和室内同位素分析的方法，在4年的项目执行期间，量化了（3）以上森林集水区的大气氮沉降氮输入时间动态、沉降量和沉降氮形式；（4）以上森林集水区的溪水氮流失的时间动态、流失量和流失氮形式；（5）根据同位素混合模型，结合硝酸盐Δ17O的测定，区分了温带清原森林和亚热带会同森林土壤硝酸盐来源；（6）根据质量平衡和同位素质量平衡的原理，在生态系统尺度上量化了土壤硝化和反硝化作用速率。本项目的实施为利用同位素技术量化森林生态系统反硝化作用导致的气态氮损失提供了一个新思路和快捷方法，在氮沉降加剧背景下，增加了我们对我国多区域森林生态系统氮状态和氮收支平衡的认识。项目执行期间共发表相关SCI论文9篇于PNAS，EST，Analytical Chemistry等国际主流刊物上，超额完成既定研究任务和计划。