氮沉降可通过影响植物碳输入的化学组成、微生物群落结构、土壤酶活性、有机质物理保护等过程，影响土壤有机碳的组成及含量。 但由于这些过程的研究相对独立，缺乏对各过程间关联机理的综合探讨，我们对氮沉降如何影响土壤有机碳形成和周转的认识还尚待深入。本项目拟以中国北方典型草地为研究对象，通过设置多梯度氮添加野外试验,结合土壤碳同位素添加室内模拟，对土壤酸碱度、阳离子、植物化学组份、微生物碳利用效率、酶动力学过程、微生物群落以及土壤有机碳的各密度分级进行分析。分离和量化植物化学组成、微生物分解、土壤物理保护三个过程对氮沉降下土壤碳储量及不同稳定性分碳库的影响，揭示氮沉降调控土壤碳库形成和周转的机理，建立对氮沉降下植物化学特性与微生物群落的生物学特性之间的响应关系，为氮沉降下土壤碳循环的响应和反馈提供植物化学和微生物过程的模型参数及机理解释。

Nitrogen deposition has been found to alter the chemistry of plant tissues, microbial community composition, soil enzyme activity and the physical protection of soil organic carbon (SOC). Those changes could have profoundly impacts on both the composition and the content of SOC. However, because limited studies have explored the linkages among those processes, we still do not know very much about the mechanisms that drive soil carbon turnover and storage under increased nitrogen deposition. The proposed research will be conducted at a long-term multiple-level nitrogen addition experiment site in a steppe in Inner Mongolia. We plan to measure soil acidification, base cations, plant chemical structure, enzyme kinetics, microbial taxonomic composition, and soil carbon content in different density fractions under different nitrogen treatments. In addition, a lab-incubated experiment using 13C-labeleld carbon will be performed to assess microbial carbon use efficiency. The project aims to quantify the effects of plant chemical structure, microbial decomposition and soil physical protection. In addition, this proposed study will link plant chemistry traits to microbial community physiological characteristics under N deposition. By integrating plant chemical, microbial biological and soil physical processes, the project will help to reveal the mechanisms driving soil carbon turnover and formation under long-term N deposition.