Graduate Thesis Or Dissertation

Microbial analysis of coniferous forest and nursery soils : effects of ectomycorrhizae, volatiles, and humic-rich organic substrates

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  • Fusarium spp. frequent conifer nursery soils yet are rarely recovered from coniferous forest soils. Experiments were performed to determine what influence humic-rich organic amendments and volatiles from soil, ectomycorrhizal fungi and ectomycorrhizae have on the populations of several physiologic and taxonomic microbial groups potentially related to this phenomenon. A comparative microbial population analysis showed that coniferous forest soils contained lower numbers of bacteria, fusaria, facultative anaerobes, and putative nitrogen-fixing bacteria than nursery soils, while the populations of actinomycetes, extracellular chitinase producers, fluorescent pseudomonads, and phosphate solubilizers did not differ. Higher concentrations of NH₄-N which in the gaseous ammonia form can deleteriously affect Fusarium, were found in forest soils. The effects of volatiles purged from forest soils differed from that of nursery soils on the populations of several microbial groups in a nursery soil. Volatiles from a forest soil planted with a Douglas-fir seedling had a different effect on soil microbial populations than volatiles from the soil without a seedling. The importance of volatile-mediated microbial group population changes in relation to Fusarium exclusion from coniferous forest soils was not readily determinable. Volatiles from the soil surrounding ectomycorrhizal Douglas-fir seedlings significantly increased the bacterial population of a soil, suggesting that volatiles from ectomycorrhizae are involved in maintaining microbial populations in the ectomycorrhizosphere. Volatiles from ectomycorrhizal fungi frequently stimulated Fusarium oxysporum colony expansion in vitro. Volatiles from forest or nursery soils had no differential effect on F. oxysporum colony expansion. Amending three coniferous nursery soils with three different organic materials with a high content of humic substances frequently increased, but sometimes decreased, seedling growth. The degree of seedling growth stimulation or inhibition varied with the amendment, soil, and amount of amendment. Composted grape pomace stimulated tree growth in all soils and also increased the numbers of several soil microbial groups potentially deleterious to Fusarium. All amendments reduced Fusarium-induced damping-off in one of the soils. Ectomycorrhizae were increased by only one amendment and then in only one soil.
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