High mountainous environments are of particular interest as they play an essential role for life and human societies, while being environments which are highly vulnerable to climate change and land use intensification. Despite this, our knowledge of high mountain soils in South America and their microbial community structure is strikingly scarce, which is of more concern considering the large population that depends on the ecosystem services provided by these areas. Conversely, the Central Andes, located in the Mediterranean region of Chile, has long been studied for its singular flora, whose diversity and endemism has been attributed to the particular geological history and pronounced environmental gradients in short distances. Here, we explore soil properties and microbial community structure depending on drainage class in a well-preserved Andean valley on the lower alpine vegetation belt (~2500 m a.s.l.) at 33.5˚S. This presents an opportunity to determine changes in the overall bacterial community structure across different types of soils and their distinct layers in a soil depth profile of a highly heterogeneous environment. Five sites closely located (<1.5 km) and distributed in a well preserved Andean valley on the lower alpine vegetation belt (~2500 m a.s.l.) at 33.5˚S were selected based on a pedological approach taking into account soil types, drainage classes and horizons. We analyzed 113 soil samples using high-throughput sequencing of the 16S rRNA gene to describe bacterial abundance, taxonomic composition, and co-occurrence networks. Almost 18,427 Amplicon Sequence Variant (ASVs) affiliated to 55 phyla were detected. The bacterial community structure within the same horizons were very similar validating the pedological sampling approach. Bray-Curtis dissimilarity analysis revealed that the structure of bacterial communities in superficial horizons (topsoil) differed from those found in deep horizons (subsoil) in a site-specific manner. However, an overall closer relationship was observed between topsoil as opposed to between subsoil microbial communities. Alpha diversity of soil bacterial communities was higher in topsoil, which also showed more bacterial members interacting and with higher average connectivity compared to subsoils. Finally, abundances of specific taxa could be considered as biological markers in the transition from topsoil to subsoil horizons, like Fibrobacterota, Proteobacteria, Bacteroidota for shallower soils and Chloroflexi, Latescibacterota and Nitrospirota for deeper soils. The results indicate the importance of the soil drainage conditions for the bacterial community composition, suggesting that information of both structure and their possible ecological relationships, might be useful in clarifying the location of the edge of the topsoil-subsoil transition in mountainous environments.

中文翻译：

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山区环境中不同排水条件下的表土和底土细菌群落组合

高山环境特别令人感兴趣，因为它们对生命和人类社会发挥着至关重要的作用，同时也是极易受到气候变化和土地利用集约化影响的环境。尽管如此，我们对南美洲高山土壤及其微生物群落结构的了解却极其匮乏，考虑到依赖这些地区提供的生态系统服务的大量人口，这一点更令人担忧。相反，位于智利地中海地区的安第斯山脉中部，长期以来因其独特的植物群而受到研究，其多样性和特有性归因于特定的地质历史和短距离内明显的环境梯度。这里，我们根据南纬 33.5 度低高山植被带（海拔约 2500 米）保存完好的安第斯山谷的排水等级，探索土壤特性和微生物群落结构。这提供了一个机会来确定不同类型土壤及其在高度异质环境的土壤深度剖面中的不同层的整体细菌群落结构的变化。根据土壤学方法，考虑到土壤类型、排水类别和地平线，选择了五个位置紧密（<1.5 公里）且分布在南纬 33.5° 较低高山植被带（海拔约 2500 米）上保存完好的安第斯山谷的地点。 。我们使用 16S rRNA 基因的高通量测序分析了 113 个土壤样本，以描述细菌丰度、分类组成和共现网络。快18岁了，检测到属于 55 个门的 427 个扩增子序列变异体 (ASV)。同一视野内的细菌群落结构非常相似，验证了土壤采样方法。Bray-Curtis 相异性分析表明，浅层（表土）中的细菌群落结构与深层（下土）中的细菌群落结构存在一定的差异。然而，与下土微生物群落之间相比，表土之间的总体关系更为密切。表土中土壤细菌群落的α多样性较高，这也表明与底土相比，有更多的细菌成员相互作用且平均连通性更高。最后，丰富的特定类群可以被视为从表层土到地下层土层过渡的生物标记，例如纤维杆菌门，Proteobacteria、Bacteroidota 适用于较浅的土壤， Chloroflexi、Latescibacterota 和 Nitrospirota 适用于较深的土壤。结果表明土壤排水条件对细菌群落组成的重要性，表明结构及其可能的生态关系的信息可能有助于阐明山区环境中表土-底土过渡边缘的位置。