PS 28-137
Simultaneous analysis of fungal and bacterial composition across a soil edaphic gradient in Southeast Asian dipterocarp rain forest
A major goal in microbial ecology is to determine the extent to which microorganisms conform to well-established ecological patterns observed in macroorganisms. In diverse tropical forests, soil edaphic variation has been posited as a major factor structuring tropical tree communities. However, the extent to which soil microbes track these patterns or contribute to tree edaphic specialization is not fully understood. Here, we examine fungal and bacterial composition across a soil edaphic gradient in Borneo (Lambir Hills), which is one of the most diverse lowland tropical forests ever documented. The soils in this region are divided into two major formations (clay vs. sandy loam) and six minor divisions. Numerous tree species display edaphic specialization across these soil divisions, but only two studies to date have examined microbial communities. We aimed to address the following questions: 1) How are tropical soil microbial communities structured across a soil edaphic gradient? and 2) How are soil microbial communities structured vertically within and among soil types?. We collected soil cores from four of the minor soil divisions and separated them into four sampling depths: litter, 0-2cm, 2-10cm, and 10-20cm. Fungi and bacteria were then sequenced using the Illumina HiSeq platform.
Results/Conclusions
Both fungal and bacterial communities were significantly clustered across soil types (P < 0.01), but were also strongly structured across sampling depths (P <0.001). Within the bacterial communities, Proteobacteria were more abundant on the sandy loam soils and in the upper soil horizons, while Actinobacteria, Verrucomicrobia, and Firmicutes were more abundant on the clay-derived soils. Ectomycorrhizal fungi, which associate with the dominant Dipterocarpaceae trees in this region, were also compositionally distinct in each soil type and showed increasing abundance with soil depth, with nearly 50% of the detected taxa occurring in the deepest sampling depth (10-20 cm). ECM fungal abundance was also negatively correlated with total soil nutrients and individual genera responded differently to soil N and P, indicating that resource specialization may contribute to the observed patterns of community clustering across soils and horizons. Together, these results indicate that soil microbial are structured across soil edaphic gradients both horizontally and vertically, but that there is a continuum of soil specialist and generalist taxa, as has been observed for trees in this system. The mechanisms related to plant-microbial feedbacks remain to be identified, but likely play a significant role in structuring these communities.