青藏高原季节性冻土区湿地冻融期甲烷排放高峰的形成机制

Seasonal freeze-thaw cycles of soil highly affect the biogeochemical cycle at high altitudes and latitudes. A growing body of evidence suggests methane emission burst occurs in high-altitude and -latitude wetlands during spring thaw or autumn freezing. To date, the mechanisms underlying are still being debated and not be incorporated into process-based models of terrestrial ecosystems, which led to the uncertainty in estimating carbon emission from wetlands. Our previous study showed that methane emission burst occurs during spring thaw in a permanently flooded wetland on the Tibetan Plateau. We proposed that methane emission burst during spring thaw is a delayed release of biogenic methane production in deep soil layers during the previous fall. In this project, we aim to verify the hypothesis by using field observations of the freeze-thaw process, methane concentrations at different soil depths and the land-atmosphere methane flux in a permanently flooded wetland and an occasionally flooded wetland in seasonally frozen regions on the Tibetan Plateau. We will explore the mechanisms underlying the autumn methane production in deep soil layers by observing the community composition and activity of methanogen and methane-oxidizing bacteria at different soil depths. Furthermore, we will investigate the effects of treatments of changing the rate of soil freezing and the surface ice cover during autumn freezing on methane emissions during spring thaw by using a manipulative experiment of intact peat monoliths. This project will help us further understand the mechanisms of carbon cycle in alpine wetlands, and improve the framework of process-based models of terrestrial ecosystems.

土壤季节性冻融交替对高海拔和高纬度地区生物地球化学循环具有重要影响。越来越多的证据显示，高海拔和高纬度湿地在土壤春季融化期或秋季冻结期出现甲烷排放高峰。目前这种现象的形成机制尚存在争议，且没有被大多数陆地生态系统过程模型所考虑，从而引起湿地碳排放估算的不确定性。本项目是对我们以前发现的“青藏高原常年淹水湿地春季融化期甲烷排放出现高峰”的现象继续深入研究。通过监测青藏高原季节性冻土区常年淹水湿地和季节性淹水湿地冻融过程、土壤剖面甲烷气体浓度和地表甲烷通量，验证春季融化期甲烷排放高峰是秋季冻结期深层土壤生成甲烷延迟释放的假说。通过监测土壤剖面产甲烷菌和甲烷氧化菌组成及活性，探讨秋季冻结期深层土壤甲烷产生机制。同时结合室内原状土柱控制实验，研究秋季土壤冻结快慢和地表冰层覆盖有无对春季融化期甲烷排放的影响。本研究将进一步从机理上揭示高寒湿地碳循环过程，同时为陆地生态系统过程模型的改进提供理论依据。

甲烷（CH4）是继二氧化碳之后对工业革命以来人为温室效应贡献最大的温室气体。越来越多的证据表明，高海拔和高纬度地区湿地在非生长季的CH4排放不可忽视，且土壤季节性冻融循环可能是形成非生长季CH4排放高峰的关键。然而，目前人们对土壤季节性冻融循环如何影响高寒湿地CH4排放的理解仍然有限。为此，本项目聚焦于青藏高原高寒湿地春融期甲烷排放的形成机制，结合土壤冻融循环过程和地表甲烷通量的野外监测以及土柱冻融循环的室内控制实验，探究了高寒湿地土壤季节性冻融循环特征及其对甲烷通量的影响。主要取得了以下发现：（1）高寒湿地土壤季节性冻融循环包括自上而下的冻结过程与自上而下、自下而上的双向融化过程，且冻结和融化的开始时间与生态系统地表能量的盈亏时间点基本一致；此外，冬季最大土壤冻结深度仅为78.4 cm，而未冻结的深层土壤为秋冬季CH4的产生和累积提供了条件；（2）高寒湿地在非生长季的CH4通量主要集中在春融期（71.9%），这不同于高寒草甸（48.8%）和高寒沼泽化草甸（43.9%）CH4通量主要发生在冬季冻结期；（3）水位降低和土壤冻结减缓处理均对高寒湿地在冻结期和完全冻结期的CH4通量无显著影响，但对融化期CH4排放具有交互作用；土壤冻结减缓处理在对照水位下提高了CH4排放（156.0%），但在降低水位下并没有对CH4排放产生影响。这些研究发现支持了“高寒湿地春融期CH4排放高峰是上一年秋冻期深层土壤产生CH4的延迟释放，且地表冰层覆盖和土壤微生物活动时长是影响融化期CH4排放大小的关键因素”的假说，并将为生态系统CH4过程模型框架的改进提供理论依据。