三峡库区消落带湿地甲烷排放及其微生物机制研究

The construction of hydropower station increased the area of reservoir and formed lots of drawdown wetlands which undergo flooding and exposing periodically. Reservoir and drawdown wetland are important sources of CH4, especially drawdown wetland ecosystem. However, there are still debates about the influence of hydropower station's construction on CH4 emission, and what is the microbiological mechanism for methane production. In this study, the paddy field, newly created wetlands and reservoir in the typical drawdown area, and the paddy field in the non-drawdown area of three gorges reservoir are slected for the measurement of CH4 fluxes. The objectives of this study to (1) understand the CH4 emission intensity of reservoir, paddy field and newly created wetlands along the different elevation, and (2) evaluate the effect of reservoir water level fluctuation on CH4 emission in the drawdown area by comparing with that in the paddy field of non-drawdown area. At the same time, we will measure the temporal variation of soil redox, dissolved CH4 concentration in pore water of soil, dissolved organic carbon (DOC), dissolved oxygen and plant characteristics, in order to reveal the key factors controlling the variation of CH4 emission rates. Laboratory incubation experiments,by using 13C isotope trace techinque and molecular biotechnology, are carried out to investigate the association of dynamic variation of CH4 production potential, CH4 oxidation potential with functional or dominant communities of methanogens and methanotrophs with the fluctuation of water level. This will explain the microbiological mechanism on CH4 emissions along different elevation in drawdown area of the three gorges reservoir. This study can provide scientific evidence for evaluation of CH4 emisions in the drawdown area of three gorges reservoir, and supply basic data for the compilation of national greenhouse gases emission inventory.

大型水电站修建在增大水库面积的同时，也形成了周期性淹水的消落带湿地。水库和湿地是大气CH4的重要排放源，消落带湿地尤其如此。然而，库区消落带湿地的形成如何影响区域CH4排放，微生物机制如何，目前尚无定论。本项目拟选取三峡库区典型消落带稻田、新生湿地和水体以及非消落带的常规稻田为研究对象，连续监测不同类型湿地CH4排放通量，定量估算消落带稻田、新生湿地和水体CH4排放强度，通过与非消落带稻田比对，明确水位波动对库区CH4排放的影响；同步测定湿地水中CH4和可溶性有机碳浓度、土壤氧化还原电位、植物性状等的动态变化，甄别湿地CH4排放季节性变化的关键驱动因素；运用13C示踪和分子生物学等技术，研究不同湿地CH4产生和氧化潜力、产甲烷优势（功能）菌和甲烷氧化优势（功能）菌群落结构特征，阐明消落带湿地CH4排放的微生物机制，为三峡水库消落带形成对区域CH4排放量影响的评估提供科学依据。

为了满足对新能源的需求，修建水电站正经历一场全球性的热潮，未来20年水电装机容量将增加170%。水电站的修建在增大水库面积的同时，也形成了周期性淹水的消落带湿地，水库水体和消落带湿地都是大气CH4的重要排放源。三峡大坝是世界上最大的水电工程，大坝的修建使库区长江水位从海拔66 m提高至最大为175 m，水库最大面积达1080 km2，同时形成了落差30米，面积349 km2的消落带。经过3年的连续观测，我们发现三峡库区消落带稻田和新生湿地甲烷平均排放系数分别为108和3.85 kg ha-1，显著低于非淹没区典型稻田（248 kg ha-1）。相关分析表明，CH4排放通量与土壤溶解有机碳（DOC）含量呈显著正相关关系。同时，随着海拔高度的降低，消落带土壤优势产甲烷菌从氢营养型产甲烷菌（Methanobacteriales）为主转变为兼性产甲烷菌（Methanosarcinaceae）为主。这表明周期性淹水导致消落带土壤DOC含量降低，不仅使得土壤产甲烷底物减少，也导致土壤产甲烷菌群落发生演替。结合三峡大坝修建前后，不同高程土地利用类型面积和实测的CH4排放通量，我们计算得出，三峡水库修建后整个淹没区年CH4排放量为0.73 Gg，而修建前为2.07 Gg，即三峡大坝的修建使水库淹没区每年减少1.34 Gg CH4排放。