http://andrewsforest.oregonstate.edu/pubs/pdf/pub1374.pdf

Stabilization and destabilization of soil organic matter: mechanisms and controls

Publisher Summary This chapter discusses the ecological implications of mycorrhizal associations in natural ecosystems and the role of soil or environmental factors, mycorrhizal fungus characteristics or host plant properties. Mycorrhizal associations are regulated by features of the host plant and mycorrhizal fungus, as well as by soil conditions and environmental factors. Factors which can influence the occurrence and effectiveness of mycorrhizal associations include (1) root properties (2) edaphic or climatic factors (3) soil organisms, (4) soil disturbance, and (5) host-fungus compatibility. Several other complex ecological topics also discussed in the chapter include (1) mycorrhizal phenology , (2) factors responsible for varying degrees of mycorrhizal dependency in host plants, (3) the role of mycorrhizal hyphae in soil (4) nutrient competition involving mycorrhizal and non-mycorrhizal plants, and (5) mycorrhizal interactions involving pollution and other stresses, the rhizosphere, soil properties and allelopathy. Finally, the chapter also discusses the population ecology of mycorrhizal fungi and the influence of their associations on plant population ecology.

Mycorrhizas in Natural Ecosystems

Ectomycorrhizal short roots, mycelia, rhizomorphs and mats from conifer soil were examined in relation to their hydrophobic properties. In some cases connected fruit bodies were included in the study. Mycorrhizal soils gathered from the forest and/or colonized in a laboratory rhizoscope were studied, as were mycelia in pure culture. Most forest-derived species were hydrophobic. The drought-resistant Cenococcum geophilum and the more ruderal and moisture-dependent Thelephora terrestris were both strongly hydrophilic. The hydrophobic mycelium seemed solely responsible for the water repellence properties, and adjacent soil and plant debris remained unaffected and hydrophilic. In hydrophobic fungi, mat formation was induced in the rhizoscope by hyphal contact with alder litter leaves. This stimulating effect was not found when the leaves were covered by water or when fresh, green alder leaves were used. Thelephora terrestris did not form such mats in vitro and spread sparsely in air pockets as well as in the adjacent water film. The possibility is discussed that many mycorrhizal fungi in the forest may partly control their soil environment via aeration created by their hydrophobia.

Water repellency, mat formation, and leaf-stimulated growth of some ectomycorrhizal fungi

Studies were conducted on an 15N-enriched Illinois Mollisol to determine the extractability and distribution of newly immobilized N in the classical humic fractions. Only about 50% of the added 15N was recovered by sequential extraction with 150 mM Na4P2O7 and 100 mM KOH; another 25 to 30% N was solubilized by further extractions with a variety of inorganic and organic solvents in sequence. In comparison to the native soil N, higher proportions of the applied 15N were accounted for in the humic acid (24 vs 16%) and fulvic acid (25 vs 22%) fractions. For the fulvic acid fraction, a higher proportion of the applied 15 (30%) was recovered as low molecular weight compounds, as compared to the native soil N (20%). The findings support the concept that a significant fraction of the recently immobilized N in soil occurs as insoluble components of microbial tissues, such as fungal melanins.

Incorporation of newly immobilized 15N into stable organic forms in soil.

The interactions of the collembolan insect Proisotoma minuta with ectomycorrhizal and/or pathogenic fungi was examined in three experiments: (1) in vitro analysis of feeding patterns, (2) in vitro food preference test, and (3) in situ analysis of ectomycorrhizal colonization in relation to population density. The ectomycorrhizal fungi Laccaria laccata, Pisolithus tinctorius, Suillus luteus, Thelephora terrestris and the pathogenic fungi Rhizoctonia solani were employed in all experiments. In vitro and in situ experiments revealed that Pr. minuta consumed all the ectomycorrhizal fungi tested but the feeding pattern and consumption varied with each isolate. In a comparative in vitro feeding preference test, where Pr. minuta was given a choice, R. solani was grazed more heavily than the ectomycorrhizal fungi. Among the ectomycorrhizal fungi examined, Pi. tinctorius was consumed significantly less than L. laccata, S. luteus or T. terrestris in the presence of R. solani. A 10-week in situ analysis of loblolly pine (Pinus taeda L.) seedling root systems inoculated with Pr. minuta revealed that ectomycorrhizal colonization was significantly less than that of control plants (without Pr. minuta). Collectively, these data suggest that mycophagous Collembola may play a major role in the distribution and biomass of ectomycorrhizal fungi in the rhizosphere of tree seedlings.

The feeding preference of mycophagous Collembola varies with the ectomycorrhizal symbiont

Formation of organic compounds by the ectomycorrhizal fungus, Pisolithus tinctorius, was studied to establish the relationship of these microbial products to humic and fulvic acids. The fungus was grown in a Melins-Norkrans liquid culture with either sucrose or a mixture of L-malic and L-succinic acid as the C and energy source. The biologically synthesized substances were characterized by chemical and infrared analyses. The fungus grown on the malic-succinic acid medium produced a black substance, whereas the organism grown on sucrose yielded only small amounts of a greenish-brown colored compound. The greenish-brown substance behaved similarly to fulvic and humic acids, when subjected to fractionation procedures using NaOH and HCl. The fraction soluble in base and acid had an infrared spectrum resembling that of fulvic acids. The fraction soluble in NaOH but insoluble in HCl had infrared features of humic acid. The black substance was insoluble in NaOH and HCl, and identification of infrared spectra revealed the black colored product to be uronic acids.

Formation of Humic Acid Like Compounds by the Ectomycorrhizal Fungus, Pisolithus tinctorius1

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Formation of Humic Acid Like Compounds by the Ectomycorrhizal Fungus, Pisolithus tinctorius1