长白山林线过渡带树岛土壤微生物群落结构和生态功能

Abstract

林线过渡带是指从郁闭森林上限到树种分布上限之间的区域，过渡带内生物多样性丰富，生态系统结构、功能和生态过程在很小的海拔梯度内发生剧烈变化，因此对全球气候变化和人类活动极为敏感。树岛是在林线过渡带内出现的斑块状或条带形不连续分布的树木集群，树岛内生存的树木通常能达到与较低海拔郁闭森林同样的高度和胸径，因此揭示树岛这一特殊生境的生态特征及其形成机制，对于预测未来气候变化下林线动态具有重要意义。以长白山岳桦林线过渡带一大型树岛作为研究对象，测定了土壤理化性质和土壤酶活性，采用宏基因组测序技术分析了微生物群落结构组成和功能基因丰度，通过与同海拔的开阔区生境进行对比，揭示了树岛这一特殊生境的土壤微生物群落结构特征和潜在生态功能，从土壤养分和土壤微生物学角度，阐明树岛形成的可能驱动机制。结果表明，树岛土壤的含水量、总碳、总氮和微生物生物量显著高于同海拔开阔区（P<0.05），与微生物r-策略相关的生理生化和遗传学指标，包括纤维素酶活性、放线菌相对丰度、与转录、防御、控制细胞周期相关的基因丰度、小分子碳降解基因丰度，均高于开阔区（P<0.05）。相反的，与微生物K-策略相关的指标，包括酸杆菌相对丰度、大分子碳降解基因相对丰度低于开阔区。揭示了树岛土壤微生物学特征，并从土壤微生物组学角度探讨了树岛形成的潜在机制，认为树岛内土壤养分增加并导致微生物群落r-策略倾向，这种变化反过来也可能促进树岛进一步扩大，进而影响林线动态。;The transition from the uppermost closed tall forests to the tree species line is often called the treeline ecotone. The treeline ecotone is one of the most important climate driven ecological boundaries and a potential habitat for diverse plant species. The ecosystem structure, function and ecological process can change over short elevational gradient in this specific habitat, and thus, the treeline ecotone ecosystem is thought to be extremely vulnerable to global climate change. The tree island is characterized by clumped patches or linear strips of krummholz or trees above the continuous forest limit. The trees in the tree island can sometimes grow to a relatively large size, with similar tree height and diameter breast height to the trees in the closed forest. To reveal the ecological characteristics and formation process of the specific habitat is of great significance in predicting the treeline dynamics under future climate change scenario. Research of the treeline ecotone has mostly focused on the aboveground vegetation cover. In this paper, soil samples were collected at the treeline ecotone of the birch forest line in the north slope of Changbai Mountain, within the tree island and the open area located at the same altitude, respectively. Soil physicochemical properties and enzymatic activities were measured. Soil microbial community structure and functional genes were analyzed by shot-gun metagenomic sequencing. The results showed that the soil moisture content, total carbon, total nitrogen and microbial biomass of the tree island were significantly higher than those of the open area (P<0.05). The microbial r-strategy indicators, including cellulase activity, relative abundance of Actinobacteria, gene abundance related to transcription, defense mechanisms, cell cycle control, cell division, chromosome partitioning and simple carbon compounds degradation, were much higher in the tree island than those in the open area. On the contrary, the indicators related to the microbial K-strategy, including the relative abundance of Acidobacteria and the abundance of complex carbon compounds degradation genes, were higher in the open area. The study, for the first time, explores the microbial community structure and ecological function in a tree island ecosystem, and explains the mechanisms of tree island formation based on microbial genomic data. We proposed that the soil nutrients were enriched, and the microbial community was shifted towards r-strategy in tree island. The changes in soil nutrients and microbes would in turn regulate the tree growth and thus might enlarge the tree island area and influence the dynamics of treeline.