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Fungal and bacterial contributions to hyphosphere enzyme activity Brooks, Denise Dowdy


In temperate forests, trees form symbiotic associations with fungi on their roots; the majority being an ectomycorrhizal alliance. Ectomycorrhizal fungi (EMF) secrete phosphatase enzymes which mobilize phosphorus from soil organic matter to their host. Soil bacteria also contribute to phosphorus mobilization through phosphatase release. I characterized bacterial and EMF contributions to phosphatase activity in forest soils regenerating after stand-replacing wildfire or from clearcut logging followed by broadcast burning. Fire can cause ecosystem phosphorus loss and can change microbial community structure, but it is unclear what effects these changes have on phosphorus availability. To link EMF hyphae with phosphorus mobilization in forest soil, I developed a novel method for visualizing fine-scale soil enzyme activity in-situ. Visualization of phosphatase activity across a chronosequence forest stands demonstrated a change in the pattern of in-situ soil phosphatase activity; areas of phosphatase activity were smaller in stands less than 61 years-old (stand initiation to canopy closure) and became larger in stands 61 to 103 years-old (stem exclusion to post stem exclusion). To link EMF with areas of high and low phosphatase activity I also developed a new soil sampling method where enzyme activity was first visualized and then used to guide small, targeted, soil samples from the soil profile for molecular (T-RFLP) analysis. The number of EMF molecular signatures was not different between the high and low-phosphatase soil microsites in stands less than 61 years-old, but in older stands, there were typically more EMF signatures in areas of low phosphatase activity. Bacteria also contribute to soil phosphatase activity; therefore, I investigated the effect of EMF hyphae on the enzyme activities of nearby soil bacteria by trapping bacteria and EMF hyphae in-situ using sand-filled mesh bags. Bacteria from the bags with hyphal ingrowth had lower phosphatase activities than bacteria from bags without hyphae. Given the higher number of EMF signatures present in low compared to high phosphatase microsites and the lower phosphatase activities of bacteria near hyphae, it is possible that EMF species may be excluded from organic phosphorus or may compete for phosphorus by excluding other EMF and selecting for less competitive soil bacteria.

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