北极冻土区活跃层与永冻层土壤微生物组的空间分异

Soil microbiomes modulate distinct patterns of soil respiration and methane oxidation in arctic active layer and permafrost

摘要:

[目的] 研究北极地区表层季节性融解冻土（活跃层）及埋藏于其下深层永久冻土（永冻层）的土壤呼吸速率、土壤微生物组差异和活性甲烷氧化微生物。[方法] 在相距2700 km的挪威斯瓦尔巴群岛和俄罗斯西伯利亚典型冻土区，共获得4个活跃层及4个永冻层土壤。模拟北极夏季近原位温度（10℃）培养土壤样品，测定土壤呼吸强度；利用稳定性同位素¹³CH₄示踪土壤甲烷氧化微生物核酸DNA；结合高通量测序16S rRNA基因，实时荧光定量qPCR及土壤理化性质分析，研究活跃层和永冻层土壤微生物群落差异及其对土壤呼吸的影响，揭示活性甲烷氧化微生物的群落组成。[结果] 西伯利亚冻土区土壤呼吸速率明显高于挪威斯瓦尔巴岛地区，其平均速率相差高达17倍。冻土区活跃层呼吸速率高于永冻层，活跃层约为61-7293 nmol CO₂/（g dws·d），而永冻层约为47-523 nmol CO₂/（g dws·d）。相应的，在所有活跃层中均发现变形菌和酸杆菌门共计10个微生物科的丰度显著高于永冻层，其中Hyphomicrobiaceae、Solibacteraceae和Sinobacteraceae是优势科，在活跃层中的相对丰度约为4.3%-18.6%，是永冻层的2.6-23.7倍，这些微生物可能是活跃层土壤呼吸强度较高的主要原因。稳定性同位素¹³CH₄示踪仅发现西伯利亚冻土活跃层能够氧化高浓度甲烷，其中的活性甲烷碳同化微生物为Methylobacterium和Crenothrix。[结论] 北极冻土区土壤微生物组存在明显的空间分异规律，并能较好解释土壤呼吸强度变化特征，而活跃层和永冻层垂直深度及其可能引起的物理化学因子可能是冻土区微生物组演替的主要环境驱动力。未来全球变暖情景下，永冻层逐渐融解并形成活跃层，其中的功能微生物将可能经历定向演替，并在北极冻土碳转化中发挥重要作用。

Abstract:

[Objective] This study was aimed to study the difference in respiration, bacterial diversity and community composition of permafrost layer and active layer, and methane consumption potential, and to find the active methylotrophs in soil that has high methane consumption potential. [Methods] Soils were sampled from Adventdalen (Svalbard), Ny-Alesund (Svalbard), Kurungnakh Third terrace (Lena delta) and Lena delta First terrace (Lena delta), including active layers and their corresponding permafrost layers, coming from one intact core at each site in 2013 and they were incubated at 10 ℃ in darkness for 26 days, with or without isotope 13-C labeled methane. Miseq sequencing and quantitative PCR of 16S ribosomal RNA gene and particulate methane monooxygenase pmoA gene were performed to assess difference of bacterial abundance, bacterial diversity and community composition of permafrost layer and active layer. Stable isotope probing experiment with 13-C labeled methane as carbon source was performed to reveal the active methanotrophs in geographically distinct arctic soils. [Results] Active layer soil had higher respiration rate than its permafrost soil, which was positively correlated with the difference of bacterial communities. The respiration rate of active layer ranged from 61 to 623 nmol CO₂/(g dws·d), while exceptionally high respiration rate of 7293/(g dws·d) was observed in LF active layer, and the respiration rate of permafrost layer varied from 47 to 523 nmol CO₂/(g dws·d). The average respiration of Lena delta was 17 times higher than that of Svalbard. Accordingly, active layer had higher bacterial diversity and higher abundance than permafrost soil where the relative abundance of 10 families in active layer were significantly higher than its permafrost. These bacterial families were mainly affiliated with Proteobacteria and Acidobacteria, such as Hyphomicrobiaceae, Solibacteraceae, Sinobacteraceae and their relative abundance was between from 4.3% to 18.6%, 2.6 and 23.7 times higher than that in permafrost layer soil. In Lena delta First terrace active layer with extremely high respiration rate, 6 families (mainly Bacteroidales and Gracilibacteraceae) were found with significant higher relative abundance (26.9%) than in other soils. Only active layer soil from Kurungnakh was observed to have strong methane oxidation potential. The methane oxidation rate in Kurungnakh active layer soil was 55.9 nmol CH₄/(g dws·d). And dominant active methylotrophs of Kurungnakh active layer were phylogenetically affiliated with Crenothrix and type I methane oxidizers (Methylobacterium). [Conclusion] The respiration rate of active layer was higher than its permafrost and with high heterogeneity and this could be well explained by difference of bacterial community composition and bacterial abundance in permafrost and active layer. Repeated thawing and refreezing of active layer has developed characteristic community structure that are key drivers for the turnover of the carbon pool, mainly including Hyphomicrobiaceae, Solibacteraceae and Sinobacteraceae. All these results suggest that microbial community structure in permafrost would go through a succession with a direction to that in its active layer, which might play important role in fixed carbon transformation in permafrost. These results provide a mechanistic understanding towards better optimization of soil carbon emission model under global change scenarios.