植物光合碳是土壤有机质（SOM）的主要来源。在全球变化背景下，明确树木光合碳向地下部分的输入及其对SOM组成、功能及产-失平衡的影响是评价土壤碳库源汇功能的关键。然而，光合碳向SOM及关键组分（氨基酸、氨基糖、中性糖、木质素）的转化动态和分配特征仍不清楚，如何受CO2浓度升高和氮沉降的影响也亟待探讨。因此，本项目以杉木-土壤生态系统为研究对象，利用不同氮沉降处理的土壤和不同浓度的13CO2供给杉木生长，采用13C示踪和GC-IRMS技术区分SOM新老化合物，通过研究树木光合13C向土壤及其关键分子组分中的转化和富集动态，阐明光合碳向土壤有机碳库的转化特征及其对CO2浓度升高和氮沉降的响应；通过研究不同CO2浓度和氮沉降对SOM及其关键组分在轻组和重组有机质间分配动态的影响，揭示全球变化背景下森林SOM转化循环过程及调控机制，为科学评价森林土壤碳库的源汇功能、预测其未来变化奠定坚实的基础。

Photosynthetic carbon is the main source of soil organic matter in forest ecosystems. Ascertaining the belowground input of tree photosynthetic carbon and its influence on the composition, function and gain-loss balance of soil organic matter is the key issue of assessing the source or sink of soil carbon pool in forest ecosystems. However, dynamical transformation and allocation of photosynthetic carbon to soil organic matter and its key fractions (e.g., amino acid, amino sugar, neutral sugars, and lignin) is still not clear. Furthermore, how CO2 elevation and nitrogen deposition affect the transformation and allocation of photosynthetic carbon is need to be explored. Thus, the project will address this key issue using 13C labeled method and Gas chromatography-combustion -isotope ratio mass spectrometry technique (GC-IRMS). In this experiment, seedlings of Chinese fir will be planted in the soils collected from long-term nitrogen deposition plots and grow under different 13CO2 concentrations. GC-IRMS technology is often used to distinguish the sources of new and old soil organic matter and its fractions. We will examine the allocation and enrichment of tree photosynthetic carbon to light and heavy fractions of soil organic matter and its key fractions through root deposition and their dynamics under 13CO2 concentrations and nitrogen deposition. This will elucidate the transformation characteristics of photosynthetic carbon to soil organic matter and its response to CO2 elevation and nitrogen deposition. We also examine the effect of CO2 elevation and nitrogen deposition on the allocation of soil organic carbon and its key fraction to light and heavy of soil organic matter. In the context of global change, findings from the project will reveal the process of soil organic matter transformation and cycle and the control mechanism, and also can provide basic data for assessing the carbon source/ sink of forest soils and predicting the future change.