长白山北坡岳桦林线形成的土壤微生物生态学机制研究

Alpine treeline is the transition zone between canopy closed forest and alpine tundra vegetation. This ecotone is highly sensitive to climate changes and anthropogenic activities, and consequently, becomes the hot-spot for the topic of global warming. The formation mechanisms of the alpine treeline are not fully addressed at current time. Most of the previous studies explained the treeline formation from the perspectives of physical and environmental conditions (such as mean annual temperature and precipitation), and the ecology and physiology of aboveground plant communities. To our known, the belowground soil nutrients availability and biological properties, in particular the soil microbial communities, at the treeline ecotone had rarely been studied. In the present study, we focused on the typical Betula Ermanii treeline ecotone system on the northern slope of Changbai Mountain. Two field experiments would be established: first, natural transects along elevational gradients across the closed forest, timber line, treeline, tree species line, up to the alpine tundra would be chosen; and second, soil transplantation among the closed forest, transition zone, and alpine tundra would be set up. The plant community composition, plant leaf elements, soil properties, enzymatic activities, and temperature sensitivity of carbon and nitrogen mineralization would be detected; soil microbial community structure and functional gene composition would be analyzed by using ribosomal RNA gene amplicon sequencing in combination with metagenome sequencing. The “climate-plant-soil-microorganisms” data would be integrated to explore the linkages between soil microbial community compositions and potential ecological functions, and soil biogeocycling processes and the treeline vegetation. The proposed project will contribute to explaining the formation and patterns of treeline at Changbai Mountain from perspective of soil microbial ecology. Also, it would be helpful for understanding and predicting future treeline shifts under global climate change and restoration of alpine vegetation.

高山林线作为郁闭森林和高山苔原之间的生态过渡带，对气候变化和人类活动十分敏感，因此成为全球变暖研究的热点区域。目前林线形成机制仍存在较大争议，大多研究从地上植被生理生态和温度、降水等物理气候条件出发，对地下土壤养分和生物学特性的研究相对匮乏，特别是对林线交错带土壤微生物群落的研究更鲜见报道。本项目将以长白山北坡岳桦林线交错带（郁闭岳桦林—过渡带—高山苔原）为典例，基于天然样带和土壤置换实验，分析林线交错带植被组成和植物营养元素；土壤理化性质、土壤酶、碳氮矿化和温度敏感性等指标；采用核糖体基因扩增子测序、宏基因组全测序等技术，检测土壤微生物群落结构和功能基因组成；综合 “气候—植被—土壤—微生物群落”数据，重点探讨微生物群落结构和潜在生态功能与土壤地化循环和林线植被的关系，研究结果对于从土壤微生物生态学角度阐释长白山林线形成机制、理解和预测全球气候变化下林线动态和高山植被恢复等具有重要意义。

高山林线作为郁闭森林和高山苔原之间的生态过渡带，对气候变化和人类活动十分敏感，因此成为全球变暖研究的热点区域。目前林线形成机制仍存在较大争议，对地下土壤养分和生物学特性的研究相对匮乏，特别是对林线过渡带土壤微生物群落的研究更鲜见报道。本项目以长白山北坡岳桦林线过渡带为研究对象，沿海拔梯度采集过渡带不同生境土壤样品，分析主要理化性质、微生物群落组成和生物地化循环功能基因丰度，以探讨土壤微生物群落结构和功能与林线动态的关系及其驱动机制。结果表明，随着郁闭森林向苔原生境过渡，土壤碳有效性随海拔升高逐渐增加，土壤呼吸温度敏感性（Q10）逐渐降低，微生物群落向r-策略主导转变。具体表现为，以Beta-, Gamma-变形菌和放线菌为代表的富营养细菌相对丰度随海拔升高逐渐增加，而以Alpha-, Delta-变形菌和酸杆菌为代表的寡营养细菌则表现出相反的海拔分布趋势。参与难降解碳（例如木质素）分解的功能基因和土壤酶活性随海拔升高逐渐降低，参与易降解碳（半纤维素等）分解的功能基因和土壤酶活性增加。上述结果暗示，随着未来气候变暖和林线上迁，土壤碳生物有效性下降，微生物群落向K-策略占主导转变，利于有机碳存储。但土壤有机碳矿化对增温将更加敏感，导致碳排放和增温之间的反馈程度更强烈。我们进一步通过“郁闭森林—过渡带—苔原带”三个生境之间的土壤置换实验模拟温度变化对土壤养分和微生物群落的影响规律，尝试提出能够作为预测未来气候变化条件下林线动态的土壤理化因子和微生物学指标。研究发现，土壤从高海拔下移至低海拔，短期内土壤速效养分增加，土壤碳有效性增加，微生物群落向r-策略转变，包括群落中富营养微生物相对丰度增加，简单碳水化合物分解能力增强，微生物固持碳的能力下降等。而土壤从低海拔移至高海拔，则刚好相反。这也预示着未来增温条件下，土壤养分和微生物活性增加可能是林线上移的先决条件。本项目从土壤养分特征和微生物生态学视角为林线形成机制和动态预测提供了重要科学参考。