The development and decline dynamics of two ectomycorrhizal fungal mat soil microbial communities in response to a reciprocal soil transfer experiment in old-growth Douglas-fir forests Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/pn89db27m

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  • Ectomycorrhizal fungi (EmF) form symbioses with trees. These symbioses profoundly influence forest ecology. Certain EmF form specialized profusions of hyphae, known as ectomycorrhizal fungal mats (mats) which are visible to the naked eye, alter forest soil biogeochemistry, substantially contribute to soil microbial biomass/respiration and support unique microbial communities. Piloderma and Ramaria mats stratify in organic and upper mineral soil, respectively, and are the dominant mat-forming fungi of old-growth Douglas-fir forests of the Pacific Northwest, USA. The importance of Piloderma and Ramaria mats to forest ecosystem processes has driven the need to better understand their associated microbial communities, particularly development (birth) and decline (death) dynamics. To explore these dynamics, a reciprocal soil transplant experiment was established at seven old-growth Douglas-fir sites in the H.J. Andrews Experimental Forest. At each site Piloderma, Ramaria and non-mat soils underwent birth (non-mat into mat enclosed in 2-mm mesh), death (mat into non-mat enclosed in PVC pipe), disturbance control (core non-mat soil, replace in 2-mm mesh) and background (no manipulation) treatments. After 51 months, treatments were harvested and three microbial community components were assessed through molecular analyses: active EmF root-tips using Sanger sequencing and soil fungi and bacteria using 454-pyrosequencing. Results from this study revealed differential persistence of mats formed by Piloderma and Ramaria. In the mineral horizon, we found few microbial community differences. Originally unique Ramaria mat microbiota were no different from non-mat soils after 51 months, and the mat-forming genus, Ramaria, was notably missing from fungal sequences; these data support the ephemeral nature of Ramaria mats where their hydrophobic powdery structure may, at times, be a visual legacy of mat presence, a remnant of physical alteration of the soil environment. In the organic horizon, Piloderma mat fungal communities persisted for 51 months and remained distinct from non-mat soils; this permitted birth and death treatment analysis. Our data indicate strong development of Piloderma mat fungal communities in birth treatments, beyond colonization by Piloderma, making them indistinguishable from Piloderma mats; mat development can take many years. Death treatments were dissimilar to Piloderma mats and contained similar fungal communities to non-mat soils. Enclosure in PVC pipe, thereby removing roots and EmF from the system, significantly shifted the soil fungal community toward saprotrophic dominance. To compliment Piloderma, the EmF genus Russula was a robust indicator of non-mat organic soils; there was strong evidence for the competitive exclusion of Russula in Piloderma mats, though it may take many years for exclusion to occur. For organic horizon bacterial communities, only death treatments differed from others. Strong similarities were found between overall Piloderma mat and non-mat bacterial communities; however, Piloderma mat and non-mat soils impose selection pressure on a small subset of bacterial taxa masked when the community is considered as a whole. This work contributes to the body of knowledge regarding complex microbial community dynamics of EmF mats. The occurrence and distinct microbial taxa of Piloderma mats in these forests suggests large-scale spatial differences in ecological function. The extent of functional differences is currently unknown, but Piloderma mats present a unique microbial system, supported by over 30 years of research, to test difficult microbial ecology questions.
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