On The Spatial Variability Of Soil Microbial Communities Beneath The Canopies Of Haloxylon Ammodendron In Two Soils

In arid and semi-arid ecosystems, ―fertile islands‖ or ―resource islands‖ around desert shrubs have great impacts on vegetation productivity, distribution pattern and ecosystem process. Soil microorganisms are important contributor for the formation and development of ―fertile islands‖ due to their key roles in litter decomposition and nutrient mineralization. Generally, soil microorganisms are closely related to soil water and nutrients. However, it remains uncertain whether the spatial pattern of soil microbial biomass and diversity around shrubs is linked to higher soil water and nutrient contents at corresponding locations. Furthermore, less is known is the spatial distribution pattern of microbial groups with different environmental sensitivity or tolerance around a shrub.In this study, soils beneath canopies of Haloxylon ammodendron, a dominant spieces of the arid zone in Center Asia, were studied. We selected two typical habitates of Haloxylon ammodendron: the Gurbantonggut Desert and the alluvial plain. The zonal soils in both study sites are the aeolian sandy soil and the grey desert soil, with very different mechanical composition. The sand content in the aeolian sandy soil is 2.0-2.3 times higher than that in the grey desert soil, and the silt and clay contents are 3.2-3.5 and 2.8-5.2 times higher than those in the grey desert soil, respectively. According to soil taxonomy of United States Department of Agriculture(USDA), the aeolian sandy soil and the grey desert soil are corresponding to Torripsamments and Haplocalcids, respectively. In the current studies, we compared the spatial pattern of soil water, nutrients, fine root, and microbial communities in two soils. The objective of this study is to test whether the spatial variability of microbial communities around shrubs is mediated by soil types. The main results are as follows:(1) Soil water and nutrient contents decreased with increasing distances outward from the taproot in two soils. However, compared with the grey desert soil, soil water and nutrients had a stronger spatial variability and higher enrichment ratios in the aeolian sandy soil. At soil depth of 0-20 cm, the enrichment ratios of soil water, organic matter, available nitrogen and available phosphorus were 4.7-11.9, 3.5-8.7, 3.1-4.8, 2.2-2.7 times higher in the aeolian sandy soil than those in the grey desert soil. Namely, the resource island in aeolian sandy soil is not only stronger, but also bigger.(2) Outward from the taproot, the spatial variability of microbial biomass and diversity was closely related to the variability of soil water, nutrients and fine root density. Soil microorganisms depend on the habitats, and tend to flourish in resource-rich soils beneath shrub canopies, forming a discontinuous patchy distribution pattern. Soil types may determine the intensity of spatial variability of soil microbial communities.(3) From 5 to 200 cm(distance from the taproot) in the aeolian sandy soil, the biomass of bacteria and fungi reduced by 50.3-53.7% and 74.7-84.1%, respectively. However, in the grey desert soil, that reduction was far less. This illustrated that the spatial variation of different microbial groups was not only mediated by soil types, but also is determined by their sensitivity or resistance to environmental variables.(4) From 5 to 200 cm, the biomass of Gram-positive and Gram-negative bacteria reduced by 50.8-80.5%, 92.6-98.0% in the aeolian sandy soil, and by 14.7-65.6%, 63.1-85.3% in the grey desert soil. The indicated that soil types mediated the spatial variability of Gram-positive than Gram-negative bacteria. This spatial variability is much stronger in the aeolian sandy soil than in grey desert soil.