天目山银杏根区土壤固氮微生物群落沿海拔梯度的分布特征

土壤微生物多样性的海拔分布规律已被广泛研究，然而单一木本植物根区土壤固氮微生物的海拔分布特征尚不清楚。本研究采集了浙江省天目山308米至1236米海拔范围内的161棵野生银杏树的根区土壤，利用Illumina高通量测序研究了土壤固氮微生物的多样性、群落组成及其空间分布。结果表明：固氮微生物多样性（Shannon与Richness指数）随海拔高度增加而降低，群落异质性随海拔差值增大而增加。慢生根瘤菌属、根瘤菌属、疣微菌属等优势属的相对丰度与海拔显著正相关，而地杆菌属、固氮菌属、伯克霍尔德菌属等优势属的相对丰度与海拔显著负相关。进一步分析发现，固氮微生物多样性和群落组成与土壤N:P均呈最显著相关，上述优势属的相对丰度同样与土壤N:P有着极强的相关性。结构方程模型显示海拔可以直接或通过影响土壤N:P间接作用于固氮微生物多样性和群落组成。通过随机森林模型分别鉴定出与海拔、N:P紧密相关的物种，发现这两类物种具有较大比例（45%）的重合。综上，土壤N:P是天目山银杏根区土壤固氮微生物群落沿海拔分布的关键驱动因子，N:P的变化直接影响银杏固氮微生物群落。我们的发现从土壤微生物学角度为天目山野生银杏的保护提供了科学依据。

Elevational Distribution of Soil Diazotrophic Community in Root Zone of Ginkgo biloba in Tianmu Mountain

The elevational distribution of soil microbial diversity has been extensively studied, however, the distribution pattern of soil diazotrophic community along elevational gradients in the root zone of a single woody plant is still unclear. Based on the Illumina high-throughput sequencing, we investigated the soil diazotrophic diversity, community composition and spatial distribution in the root zone of 161 Ginkgo biloba trees at the elevations ranging from 308 m to 1236 m in Tianmu Mountain, Zhejiang Province. The diazotrophic diversity (Shannon and Richness indexes) significantly decreased along elevation. The diazotrophic community dissimilarity increased with the increase in elevation distance, and the relative abundances of the dominant genera such as Bradyrhizobium, Rhizobium and Verrucomicrobium were significantly and positively correlated with elevation, while the relative abundances of the genera such as Geobacter, Azotobacter and Burkholderia were significantly and negatively correlated with elevation. Besides, both the diazotrophic diversity and community composition were mostly correlated with soil N:P ratio. In addition, the relative abundances of dominant diazotrophic genera were significantly correlated with soil N:P ratio. Structural equation modelling analyses showed that elevation could directly or indirectly affect the diazotrophic diversity and community composition by affecting soil N:P ratio. Random forest models analyese identified the species clusters closely related to the elevation and the N:P ratio, respectively, and these two clusters had a large proportion (45%) of overlap. In summary, soil N:P ratio was the key factor in determining the elevational distribution of soil diazotrophs in the root zone of G. biloba in Tianmu Mountain, and the change in soil N:P ratio could directly affect the diazotrophic community of G. biloba. Our findings provide scientific basis in the conservation of wild G. biloba in the Tianmu Mountain from the view point of soil microbiology.