中文摘要：

氮素（N）是陆地生态系统尤其是草地生态系统第一生产力的重要限制性养分之一.陆地生态系统N素的可利用性由土壤N素的转化速率决定,其中包括氨化和硝化2个重要过程.准确测定N素在各转化过程中的量,对于估算陆地生态系统N转化非常重要.稳定性同位素15N由于其安全、准确且不干扰自然生态系统等特点,近年来在生态系统N循环研究方面得到广泛应用,常用方法包括15N自然丰度法、15N还原法、15N库稀释法和15N示踪法.在查阅大量文献的基础上,搜集整理了15N库稀释法和15N示踪法的详细操作流程并综述了其在草地生态系统应用的最新进展,分别从不同草地管理方式（增施氮肥、放牧、火烧和刈割等）和全球气候变化（增温、增雨、大气氮沉降和CO2浓度升高等）对草地生态系统N转化过程的影响进行论述.同位素15N在草地生态系统应用的方法同样适用于森林、农田以及其他陆地生态系统.

英文摘要：

Nitrogen （N） is the growth-limiting nutrient of plants in terrestrial ecosystems,especially in grassland ecosystems.The availability of N is determined by N cycling processes in terrestrial ecosystems.To better understand the N cycles,a key way is to exactly determine the rates of N transformation.To date,15N stable isotope technique is highly recommended for quantifying the processes of N transformation.The isotope technique is generally classified into four categories as 15 N natural abundance,15N reduction,15N pool dilution and l5N tracer techniques.In this paper,the progresses of researches on N transformation using 15 N technique with a focus of 15N pool dilution and 15N tracer techniques in grassland ecosystems were summarized.Firstly,two detailed protocols were summarized from published papers for l5N pool dilution and l5N tracer techniques,respectively.Secondly,the recent findings of N transformation in response to human activities （fertilization,grazing,fire and mowing） and global change （global warming,increasing precipitation,atmospheric N deposition and increased C02 in atmosphere） using the above two N isotope techniques in natural grassland ecosystems were reviewed.The isotope techniques described here were also applicable to other terrestrial ecosystems,such as forest ecosystems and agro-ecosystems.