Abstract:

As the main decomposers in terrestrial ecosystems, soil microorganisms play indispensable roles in the global material cycle and energy flow. Investigating the distribution pattern of soil microorganism diversity at elevation and its influencing factors is of great significance for understanding the mechanism of microbial diversity formation and maintenance. Soil physicochemical properties and vegetation characteristics are considered to be the main driving factors that dominate the distribution pattern of microbial α-diversity in mountain soils. Mycorrhizal associations play an important role in regulating forest population structure as mutually beneficial symbiotic biological interactions; however, mycorrhizal effects on soil microbial distribution have received little attention. Soil microbial alpha diversity reflects the richness of microbial species and complexity of communities in an ecosystem. It provides a direct indication of the health and biodiversity of an ecosystem and is a priority for biodiversity conservation. This study focused on exploring the distribution patterns and factors influencing soil microbial alpha diversity along elevational gradient. The relevant results are crucial for understanding aboveground and underground ecological processes. To reveal the elevational distribution pattern and influencing factors of soil microbial diversity in forests in the subtropical-warm temperate transition zone, the present study used 15 fixed monitoring sample plots at an elevational gradient of 894-2200 m in the Lishan Nature Reserve as a platform for the study. Within each plot, the midpoint of the plot and the midpoint of each small plot were used as sampling points. Five sampling points were within each plot, 75 soil samples were collected from the 15 plots. Using high-throughput sequencing technology, combined with the investigation of the composition of mycorrhizal tree species, plant diversity, and soil physicochemical properties of the sample plots, we conducted comprehensive research on the α-diversity of soil microbial (bacterial and fungal) species (Chao1 and Shannon indicies) and phylogenetic α-diversity (Faith’s PD index). The main results are as follows: 1) Bacteria were identified as 29 phyla, 82 classes, 177 orders, 264 families, and 372 genera. The dominant phyla of the soil bacterial communities in the forests of the Lishan Mountains included Proteobacteria, Actinobacteria, and Acidobacteriota, with an overall proportion ranging from 72.4% to 83.1% at all elevations. The Chao1 index and Shannon index of soil bacteria showed an increasing trend (R²=0.38, p<0.001; R²=0.18, p<0.001), and the Faith’PD index showed a decreasing trend under the increasing of elevation (R²=0.13, p<0.001). Fungi were identified as belonging to 8 phyla, 22 classes, 55 orders, 95 families, and 148 genera. The dominant phyla of the soil fungal communities were Ascomycota and Basidiomycota, with an overall proportion ranging from 66.1% to 96.8% at all altitudes. The Chao1, Shannon, and Faith’PD indices of soil fungi in the forests of the Lishan Mountains showed increasing trends with increasing elevation (R²=0.13, p<0.01; R²=0.21, p<0.001; R²=0.06, p<0.05). 2) Most environmental factors changed significantly with elevation. The plant evenness index showed a significant increasing trend along the elevation gradient (r=0.49, p<0.05), whereas the proportion of shrubs decreased significantly with increasing elevation (r= −0.38, p<0.05). The characteristics of the mycorrhizal tree species showed significantly changing trends at different elevation gradients. The species abundance of ectomycorrhizal trees (ECM Abundance) showed a significant downward trend with increasing altitude (r= −0.21, p<0.001). In contrast, the basal area of the ectomycorrhizal trees (ECM Basal Area) increased with elevation. A significant increase was observed (r=0.23, p<0.001). Simultaneously, the ratio of ectomycorrhizal tree species to arbuscular mycorrhizal tree species (ECM/AM) increased significantly along the elevation gradient (r=0.42, p<0.001). Spearman’s correlation analysis showed that most soil factors showed significantly changing trends along the elevational gradient. Soil bulk density showed a significant decreasing trend with increasing elevation (p<0.05). The soil moisture content, porosity, organic carbon, total phosphorus, available phosphorus, total nitrogen, nitrate nitrogen, and available potassium increased significantly along the elevational gradient (p<0.05). Other soil factors, including pH, ammonium nitrogen, and total potassium did not change significantly with elevation. 3) The results of the random forest analysis showed that environmental factors on the elevational gradient had different explanatory effects on soil bacterial and fungal α-diversity. For soil bacterial diversity, the environmental factors explained 63%, 60%, and 4% of the Chao1, Shannon, and Faith’PD indices, respectively. The degrees of explanation of environmental factors for the fungal Chao1, Shannon, and Faith’PD indices were 43%, 43%, and 36%, respectively. Except for the bacterial Faith’PD index, which has a high overall explanatory power, environmental factors have a good explanatory effect on the α diversity of bacterial and fungal species Chao1 index, Shannon index, and the alpha diversity of the fungal Faith’PD index, whereas the explanatory power for the α diversity of the bacterial Faith’PD index was relatively low. The factors with the highest relative importance for bacterial species α-diversity were soil nitrate nitrogen, pH, and water content. Factors with higher relative importance for bacterial phylogenetic α-diversity were the Pielou index, ECM tree species/AM tree species, and ECM tree abundance. The factors with higher relative importance for fungal species α-diversity were nitrate-nitrogen, pH, and ECM tree abundance, and the factors with higher relative importance for fungal phylogenetic α-diversity were nitrate-nitrogen, pH, and total phosphorus. In summary, the soil bacterial and fungal species α-diversity, as well as fungal phylogenetic α-diversity of the Shanxi Lishan Mountains, showed an increasing elevational gradient pattern. In contrast, the bacterial phylogenetic α-diversity of the Shanxi Lishan Mountains showed a decreasing elevation gradient. The impact of environmental factors on the elevational gradient pattern of soil bacterial and fungal α-diversity was different. The composition of mycorrhizal tree species had a significant effect on the α-diversity and phylogenetic diversity of the bacterial and fungal species. This study found that the changes as well as the underlying drivers of α-diversity in warm temperate forests along the elevational gradient differed between soil bacteria and fungi, highlighting the importance of mycorrhizal tree species composition in the study of the mechanisms of soil microbial community assemblage.

Key words: elevational gradient, bacteria and fungi, species diversity, phylogenetic diversity, warm-temperate forests

摘要： 明确影响土壤微生物多样性海拔梯度的驱动因素是微生物生态学中尚未解决的一个基本问题。为揭示亚热带-暖温带过渡区森林土壤微生物α多样性形成及维持的潜在生态学机制，以历山国家级自然保护区894－2200 m海拔梯度固定监测样地为研究平台，综合研究了山地森林土壤微生物多样性的海拔分布格局及影响因素。结果表明：土壤细菌Chao1和Shannon指数随海拔升高而上升，Faith’PD指数则下降，而土壤真菌Chao1、Shannon和Faith’PD指数均呈上升趋势。环境因子对细菌Chao1、Shannon和Faith’PD指数解释度分别为63%、60%和4%。对细菌物种α多样性而言，相对重要性较高的因子是土壤硝态氮、pH、土壤含水量，而Pielou指数、外生菌根/丛枝菌根树种比、外生菌根树种多度对细菌系统发育α多样性相对重要性较高。环境因子对真菌Chao1、Shannon和Faith’PD指数解释度分别为43%、43%和36%。真菌物种α多样性相对重要性较高的因子是硝态氮、pH、外生菌根树种多度，真菌系统发育α多样性相对重要性较高的是硝态氮、pH、全磷。该研究发现暖温带森林土壤细菌和真菌α多样性随海拔梯度格局变化及其驱动因子的差异，并突出强调菌根树种组成在土壤微生物群落构建机制研究中的重要性。

关键词: 海拔梯度, 细菌和真菌, 物种多样性, 系统发育多样性, 暖温带森林