本研究以旱地农田潮棕壤（粘土矿物以伊利石和蛭石为主）为研究对象，以中国科学院沈阳生态实验站不同施肥模式长期定位试验为平台，以同位素交叉标记技术和灭菌技术为主要手段，以培养试验和盆栽试验为主要技术途径，围绕土壤铵的固定和微生物固持这一核心，系统研究不同外源氮添加对铵的矿物固定及其时效性的影响；分析"新固定的"固定态铵在不同粒级中的分布及其循环周转速率；阐明土壤微生物在铵固定与释放过程中的作用；探讨固定态铵和微生物量氮对不同外源氮的响应和转化机制及其对土壤保氮和肥料利用率的贡献；寻找不同肥力水平土壤"新-旧"固定态铵释放的生态阈值，相关成果既可以丰富土壤氮素循环的研究，也可以为农田氮素优化管理和科学运筹提供理论依据和技术支撑。

Using meadow brown soil collected from a long-term experiment in Shenyang Agricultural Station, we aim to study the effect of different kinds of external nitrogen addition on soil fixed ammonium and the changes of this effect with the time. In both incubation and pot experiments, multiple techniques were adopted, including cross label of isotope and sterilization technology. The specific aims of the research were to: (1) analyze the distribution of recently fixed ammonium in different particle fractions and their turnover rates; (2) elucidate the roles of soil microbe in ammonium fixing and releasing; (3) characterize the responses and transforming mechanisms of soil fixed ammonium and microbial biomass nitrogen to different kinds of external nitrogen and discuss contributions of these processes to holding soil nitrogen and upgrading fertilizer use efficiency. This study intended to seek the threshold values of soil fixed ammonium releasing in contrasting soil fertility levels. This study could enrich knowledge of the soil nitrogen cycling and supply the theoretical foundation and technical support for reasonable nitrogen managements and operations in farmlands.