岷江干旱河谷优势灌丛对土壤微生物群落组成的影响

Effects of dominant shrubs on soil microbial composition in the semi-arid Minjiang River valley

通过调查岷江干旱河谷两河口、飞虹、撮箕和牟托4个样地优势灌丛及其灌丛间空地的表土土壤物理化学性质和微生物群落组成,探讨植物灌丛群落对土壤微生物群落组成的影响。研究发现不同灌丛种类对土壤微生物群落组成以及土壤物理化学性质并没有显著影响,而同一样地灌丛与空地间的差异却较为显著。灌丛下比空地土壤中具有更高的有机质、养分含量,更高的土壤含水量和更低的容重,而灌丛下相对富集的养分资源是造成灌丛与空地间微生物群落组成差异的主要原因。不同样地影响微生物群落的主要因子存在一定差异,但与氮相关的因子(总氮、有效氮、碳/氮比)对土壤微生物群落着非常重要的影响,特别是对土壤微生物群落总生物量和细菌类群(革兰氏阳性菌、革兰氏阴性菌、细菌等)。虽然不同灌丛和空地下土壤中细菌群落都没有显著地变化,但真菌和菌根真菌却明显的在灌丛下富集。在飞虹和牟托样地,总磷和碳/磷比与真菌类群,主要指真菌和菌根真菌,表现出显著正相关性,这或许反映了真菌类群对于该区域磷循环的重要作用。研究结果揭示了灌丛植被在干旱河谷地区地下生态系统中的重要作用,以及氮、磷这两种养分元素对土壤微生物群落的重要影响。同时,未来对于干旱河谷地区植物-土壤关系的研究应该关注真菌和菌根真菌类群的作用。

Although soil microorganisms play a critical role in soil organic matter decomposition, nutrient cycling and ecosystem productivity, we still know very little about how microbial communities are affected by environmental factors and how the structure and function of soil microbial communities influence key soil processes, especially for the arid and semi- arid river valley district. Studies focusing on soil microbial communities under this region are still under-represented when compared to moist forest and grassland ecosystems. Thus, the objectives of this work were to quantify the effects of shrubs on soil physicochemical properties and microbial composition （Phospholipid fatty acid, PLFA）, and to find out soil factors that significantly affect soil microbial composition. To achieve this, we investigated the variations of soil properties between dominant shrubs and shrub-interspaces at a set of sites along the mountainous semi-arid valley of the upper Minjiang River, Sichuan Province. The dominant shrubs had significant effects on soil physicochemical properties and microbial composition, while the effects of shrub species seem to be trivial. The soil under shrub canopies had more organic carbon and nutrient resources, higher water content and lower bulk density than those of the shrub-interspaces. The bacterial communities＇ abundance did not vary significantly between the two types of soil, while fungi and＇ arbuscular mycorrhizal fungi （AMF） tended to dominate under shrubs. Furthermore, the correlation based Principal Component Analysis （PCA） demonstrated a clear dissimilarity of microbial community composition and physicoehemical properties between shrub and shrub-interspace soils. Besides, the PCA revealed that the microbial community composition was a more sensitive and effective indicator than soil physioehemical properties in this study. Backward selection procedure and the combined redundancy analysis （RDA） were used to select the most important soil physiochemical variables affecting soil microbial communities. Though the main factors varied among the four sites, the nitrogen （N） associated factors （Total N, Available N and carbon-to-nitrogen ratio） always played a pivotal role in structuring the microbial groups, especially total microbial biomass （total PLFA） and the bacterial groups （Gran-positive （G＋）, Gran-negative （G-）, general bacteria （B）, and cyclo bacteria （cyclo））. The correlation between fungi groups （fungi, AMF） and N associated factors was relatively weak. Total phosphorus and carbon- to-phosphorus ratio strongly affected the fungi and AMF groups in Feihong and Mutuo. This may indicate the importance of phosphorus in shaping fungi and AMF communities, as well as the indispensable role that fungi and AMF communities may play in influencing phosphorus availability and cycling. In addition, soil water content exerted effects on the microbial composition in the two relative humid sites, Cuoji and Mutuo. Moreover, pH did not significantly affect microbial communities, though previous studies have pointed out its important role in affecting microbial composition. This may be due to the small variation of pH in this semi-arid river valley. Collectively, the enrichment of soil nutrients and better physical condition under shrub canopies were the main reasons for the difference of microbial community composition between shrubs and shrub-interspaces. Our work demonstrated the important role of shrubs in the semi-arid valley of upper Minjiang River, provided insights into the nutrient cycling for bacterial and fungal communities, respecti of semi-arid valley. It also revealed the importance of nitrogen and phosphorus rely. Future studies on the plant-soil relationship in dryland ecosystems should pay more attention to fungi and AMF communities.