在氮沉降背景下，土壤氮可利用性变化已成为碳循环的主要调控因子。由于土壤中生物、化学、物理等过程的频繁变化，硝化和反硝化作用、铵态氮和硝态氮挥发、淋溶/地表径流和土壤侵蚀以及植物根系养分吸收等过程都将造成土壤中严重的氮损失，然而，极少有研究考虑到氮损失过程。此外，氮可利用性的变化和温度、水分之间存在显著的交互作用，但其作用强度和方向仍存在很大不确定性。本项目拟对北方农牧交错带(蒙辽、京北、内蒙和宁陕)60个样地240个样方的表层土壤，利用离子交换膜吸收土壤中的无机氮模拟氮损失，结合氮添加处理和不同温度、水分梯度的设置，进行室内长期培养实验，通过测定土壤微生物呼吸速率、不同碳库分解速率以及生物和非生物因子的变化，开展氮可利用性改变对土壤有机碳分解的影响以及机制的研究。本研究将有助于陆地生态系统的碳过程模拟和理论完善，准确预测土壤有机碳动态和氮、水以及温度的耦合关系及其对气候变化的反馈。

Under the scenarios of nitrogen (N) deposition, soil N availability becomes the main regulator of carbon (C) cycle. Frequent changes of biological, chemical, and physical processes in the soil, such as nitrification and denitrification, volatilization of ammonium and nitrate N, leaching/surface runoff and soil erosion, and nutrient uptake by plant roots, may cause serious N loss. However, the process of N loss was rarely considered in previous studies. In addition, there was a significant interaction between changes in N availability and temperature and moisture, but there is still considerable uncertainty in the strength and direction of this interaction. The project will be conducted to collect 240 soil samples from 60 plots in the agro-pastoral ecotone of northern China (Mengliao, Jingbei, Inner Mongolia and Ningshan), and examine the effect of N availability changes on soil organic C decomposition and its involving mechanism. Soil samples will be subject to long-term indoor incubation experiment, which combined with N addition and N loss treatment and different temperature and water gradients. An ion exchange membrane technique will be used to absorb inorganic N in the soil to simulate N loss process. Variations of soil microbial respiration rate, decomposition rate of different carbon pools, and biotic and abiotic factors will be measured. This study will contribute to the C process simulation and theoretical improvement of terrestrial ecosystems, accurately predict the relationships among soil organic C dynamics and N, water and temperature and their feedback on climate change.