青藏高原中南部全新世气候变化及其与冻土湿地甲烷循环微生物的关系

An understanding of soil microbial communities in this low-temperature and carbon-rich ecological critical zone is central to our ability to assess terrestrial carbon cycle-climate feedbacks. Tibetan permafrost wetlands with a high altitude of 4500m are critical and vulnerable. The lacking of microbial communities and carbon cycling mediated by microbes in low-mid latitude, high altitude (>4500m) permafrost wetland ecosystems is largely limited to understand the role of permafrost microorganisms in global change as well as carbon cycling permafrost ecosystems under climate change. Its CH4 biogeochemical cycling and microbial population dynamics have been poorly understood. This proposed research will characterize Tibetan permafrost wetland dataset of microbial community composition and climate changes during the Holocene using cutting edge approaches exploiting gas chromatography (GC), GC-mass spectrometry, high performance liquid chromatography-mass spectrometry, GC-isotope ratio mass spectrometry (IRMS), GC-thermal conversion-IRMS and high‐throughput investigations of microbial communities. We will establish the Tibetan dataset of peat-forming plant δD values, a key precipitation indicator; reconstruct mean annual air temperature (MAAT) using the recently developed peat-specific MAATpeat temperature calibration based on the distribution of bacterial branched glycerol dialkyl glycerol tetraethers; quantify and isotopically characterize microbial biomarkers and especially those derived from organisms involved with methane cycling; evaluate the link between precipitation, vegetation, temperature, pH and microbial community. This dataset seeks to understand the relationships between microbial communities and climate changes and will be used to develop higher resolution and longer-term microbial organisms involved with methane cycling records over the Holocene and to better understand the impact of climate change on ancient CH4 biogeochemistry. Critically we will reveal functional relationship between methane-cycling microorganisms and climate variations and learn how these relations are affected by climatic/environmental changes. This work will be one of the very first applications of these novel methodologies to the study of past changes in permafrost peatland biogeochemistry in the mid-south Tibetan Plateau. It will improve our understanding of biogeochemical consequences of microbial community dynamics, permafrost wetland carbon cycling and CH4 emissions and will lead the development of permafrost wetland microorganisms and function in global climatic change and carbon budget. It will provide the mechanism understanding of CH4 biogeochemical cycling and predict carbon budgets in permafrost wetland ecosystems.

海拔4500m以上的青藏高原冻土湿地在全球碳平衡和气候变化中占有特殊、重要的地位，深入认识这些高寒富碳生态脆弱带土壤微生物群落是评价陆地碳循环-气候反馈关键的一环。本项目以青藏高原中南部全新世冻土湿地为研究对象，重点应用分子有机地球化学技术高分辨率地研究类脂物及其同位素组成，揭示植物叶蜡单体氢同位素的降水演化模式，基于MBT′5me/CBTpeat指标定量重建温度变化，发展高分辨率、长序列的甲烷循环微生物类脂分子记录；结合高通量测序的分子微生物分析，率先解析全新世冻土湿地生境中甲烷循环微生物菌群组成、分布及多样性等时空演变，查明全新世不同时期温度、降水、植被等与甲烷循环微生物菌群之间的关系，探讨甲烷菌群及其介导的生物地球化学过程如何受气候变化所影响，分析其主要控制因素，更好地理解这些参与甲烷代谢的微生物对气候变化的反馈程度，深化高寒环境微生物调控的碳循环及甲烷释放方面的研究。

青藏高原冻土湿地富碳生态系统气候变化-微生物群落在全球碳收支中起着举足轻重的作用。 本项目在AMS14C可靠年代标尺的基础上，以青藏高原中南部全新世冻土湿地为研究对象，重点应用分子有机地球化学和高通量测序技术，重建了全新世季风气侯演化，获取了全新世冻土湿地生境中甲烷循环微生物菌群组成、分布及多样性等时空演变，评价了全新世不同时期温度、降水、植被等与甲烷循环微生物菌群之间的关系，分析了其主要控制因素。 研究显示，基于MBT′5me /CBTpeat指标定量重建的青藏高原中南部全新世温度在约-11.8～8.9℃之间变化，远低于研究区现今年均温；研究区有效湿度在早中全新世显著高于晚全新世，其中约6-4ka期间达到最盛，不同于受东亚季风影响青藏高原东北地区，提出印度夏季风对该区水文变化具有重要的控制作用。在此气候背景下，早中全新世产甲烷菌菌群丰度与多样性较高；在约4ka之后，产甲烷菌种群丰度降低，甲烷氧化菌群丰度和多样性相对增加。宏基因组分析表明，甲烷生成的基因种类和关键基因在早中全新世特别是6-4ka期间明显增加。结合原位气体观测，发现青藏高原中南部冻土湿地沿融化梯度甲烷循环菌群增加、甲烷循环过程增强，甲烷释放量高。相关性分析进一步显示，甲烷循环菌群以及mcrA基因丰度与土壤温度无明显关联，但与土壤含水量显著正相关，与pH值负相关；提出全新世不同时期甲烷循环菌群主要受控于季风降水变化，亦即降水多，土壤含水量高、腐殖酸高、pH低，产甲烷循环菌群、关键基因增加；然而，在空间上随融化程度加强，温度会显著改变甲烷循环微生物群落结构。该成果不仅对青藏高原不同区域降水趋势与水文格局等具有十分重要的指导意义，而且扩展了中低纬度高海拔冻土湿地甲烷循环微生物群落相关研究，对深入认识这些参与甲烷代谢的微生物对气候变化的反馈及其调控的碳释放方面的研究有重要的参考价值和科学意义。