土壤有机碳(SOC)是全球碳循环过程中的重要碳库，其储量变化和转化机制是当前国际研究的热点。过去三十年，我国农田SOC含量发生了很大变化，且不同地区土壤之间存在较大差异。此间，氮肥施加持续增长对农田SOC含量变化的影响和潜在贡献不容忽视，然而有关施氮对SOC转化过程的影响及其相关机制目前尚不清楚。本申请拟采用C-14同位素示踪技术研究氮肥施加对我国典型农田SOC转化过程的影响及相关机制，包括：1)SOC及其组分的C-14全标记和选择性标记制备；2)氮肥施加对SOC及其组分土壤中矿化、降解、赋存形态及归趋的影响；3)施氮引起的微生物群落和活性改变与SOC及其组分转化之间的关系；4)氮肥施加与农田SOC变化之间的潜在定量关系。研究结果对于揭示氮肥施加对我国农田SOC含量变化的潜在贡献，全面掌握我国农田SOC的转化和变化规律具有重要的理论和现实意义。

Soil organic carbon (SOC) holds more than twice as much C as either the atmosphere or terrestrial vegetation,and plays crucial role in the global C cycling. Storage change, transformation and stabilization mechanisms of SOC are presently the research "hotspot". Agricultural SOC stock of China has been changed dramatically in the last thirty years, mainly attributed to the change of cropland management (such as fertilization and cultivation) and climate warming, in which continuously increase application of synthetic N-fertilizer exerts a powerful influence. However, mechanisms of N-fertilizer on transformation of SOC and the potential contribution of N-fertilization to the change of agricultural SOC are still obscure. In the present project, response of SOC and its components to N-fertilizer will be investigated by means of 14C tracer, including: 1) Synthesis of uniformly and specifically labeled SOC; 2) Mineralization, transformation and residue distribution of 14C-SOC as affected by N-fertilizer in soils; 3) Effects of N-fertilizer on microbial activity and community related to transformation of SOC; 4) Establishing quantitative relationship between N-fertilizer application and SOC stock change. Results of this investigation are not only helpful to elucidate the potential contribution of N-fertilizer application to agricultural SOC change, but also have crucial implications for deeply understanding the transformation mechanisms of SOC.