用双同位素标记法研究凋落物分解中碳氮流通机制

The carbon and nitrogen flux during litter decomposition play important roles in carbon and nitrogen cycling in terrestrial ecosystem and in regulating the revolution of ecosystem. However, the pathways of carbon and nitrogen flux during litter decomposition were not well understood. In this project, we try to partition the pathways of carbon and nitrogen flux during litter decomposition by using dual isotope tracing and in site decomposing. We try to measure the CO2 and N2O emitted from litter and soils, the DOC and DON released from litter and soils, the litter derived carbon and nitrogen incorporated into soil particles. We will partition the litter derived carbon and nitrogen during these processes by measuring 13C and 15N in CO2 and N2O, DOC and DON, and in soil organic matter associated with soil aggregates. We also try to understand the dynamics of these processes during litter decomposition. The effects of precipitation changing patterns will be considered in this project. Through these studies, we aim to clarify how litter decomposition affect soil carbon and nitrogen flux and how soil affect litter carbon flux, to understand the distribution of litter carbon and nitrogen in soil particles and their dynamics during litter decompositon. The objects were to understand the pathways of carbon and nitrogen flux during litter decomposition, the coupling of carbon flux and nitrogen flux, the feedbacks between litter carbon and nitrogen flux and soil carbon and nitrogen flux during litter decomposition, and the response of these processes to precipitation changes. This project would provide scientific evidences for understanding the carbon and nitrogen flux during litter decomposition and for precise predicting carbon and nitrogen cycles terrestrial ecosystem.

凋落物分解中碳氮流通途径的辨识是认识凋落物在陆地生态系统物质循环中的作用和准确预测陆地生态系统碳氮通量的基础。本项目针对目前凋落物分解研究中的热点和难点，拟采用稳定性13C和15N双重标记法，研究凋落物在土壤中的原位分解过程，通过长期定位监测标记凋落物分解中释放出的CO2和N2O、淋溶释放出的DOC和DON、转移到土壤中的有机碳和氮的动态特征，辨识凋落物来源碳氮在分解中的流通途径（包括向大气释放、淋溶、向土壤转移）和各个流通途径的动力学特征，查明凋落物碳氮流通对土壤碳氮过程的影响以及土壤因子对凋落物碳氮流通的影响，获得凋落物来源的碳氮在土壤组分中的分布及其动态过程，初步获得凋落物分解中碳氮流通的耦合作用，并且分析凋落物分解中碳氮流通对降水变化的响应特征，为正确认识凋落物分解中碳氮流通特征和准确预测陆地生态系统碳氮循环提供科学依据。

项目围绕凋落物分解过程中的碳氮流通过程和对环境变化的响应机理，利用双同位素示踪法开展研究。探索了双同位素示踪法在研究碳氮流通过程的可行性，分析了凋落物分解过程中凋落物和分解介质中碳氮含量、同位素丰度的分布特征和动态特征，揭示了凋落物分解过程中碳氮流通的耦合特征，并确立了这些过程对降水变化的响应。项目得到如下主要结论：（1）通过碳氮双同位素标记法，获得的刺槐标记材料的δ15N和δ13C分别达到3604‰和55.6‰，杨树标记材料的δ15N和δ13C分别达为8115‰和452‰，标记材料的量和同位素丰度满足标记的要求。（2）刺槐和杨树叶凋落物在沙子介质中分解时残余物碳氮含量显著高于在土壤介质中分解时的含量，随分解进行，凋落物中碳含量逐渐降低，刺槐叶凋落物氮含量也逐渐降低，但杨树叶凋落物氮含量变化不大。分解过程中，刺槐叶凋落物13C的丰度显著低于杨树叶凋落物，而且随分解的进行变化较小；刺槐叶凋落物15N丰度逐渐增加，杨树叶凋落物15N丰度有所降低。（3）凋落物分解造成了有机碳和全氮在分解介质表层的积累，沙子介质中刺槐叶凋落物的影响大于杨树叶凋落物，而土壤介质中两种凋落物的影响相近；土壤碳氮含量均以600 mm降水处理较高，400和800 mm降水处理较低，这种影响趋势与凋落物来源无关。随剖面深度的增加，碳氮同位素丰度逐渐降低，土壤的碳氮同位素丰度及其剖面差异显著高于沙子介质；凋落物分解显著改变了分解介质中碳氮同位素丰度的剖面分布特征。（4）随土层的加深，来源于凋落物的碳氮比例显著降低，原有碳氮比例逐渐升高，而且来源于刺槐的碳氮比例高于来源于杨树的碳氮比例；土壤中凋落物碳的比例显著大于氮的比例，原有的碳的比例则显著小于氮的比例；凋落物分解后土壤碳氮来源受降水影响。（5）凋落物分解中碳氮流通过程相互耦合，并受分解介质和降水共同影响，杨树叶凋落物碳氮耦合关系对这两个因素响应敏感，而刺槐叶凋落物受影响较小。项目研究结果确立了利用双同位素研究凋落物分解中碳氮流通的可行性，并探索了凋落物分解过程中碳氮流通的耦合特征及其对降水的响应，为全球变化背景下凋落物分解的研究提供了新的思路。