Abstract

The extraradical mycelium (ERM) of mycorrhizal fungi constitutes an important pathway for the translocation of energy-rich photoassimilates from plant to soil. Because of the large surface of the mycelium, and its provision of carbon, the ERM potentially constitutes an important site for interactions with other microorganisms in soils. The focus of this thesis is on the ERM of arbuscular mycorrhizal (AM) fungi and its interactions with soil bacteria, and the possible ecological significance of these interactions. In an in vitro study, soil bacteria differed in their ability to attach to AM fungal hyphae. These physical associations between bacteria and fungi were influenced by hyphal vitality and fungal strain. The degree of attachment of bacteria to living or dead hyphae may reflect the ecological niche differentiation of these bacteria, and consequently their trophic relationship (e.g. mutualistic or saprotrophic) to the mycorrhizal fungi. Carbohydrates of mycelial origin, mainly in the form of glucose, acetate and formiate, were identified in exudates from an AM fungus. Mycelial exudates promoted the occurrence of certain bacterial taxa in vitro, further supporting the view that specific interactions may occur between some bacteria and AM fungi. In a field study with long-term applications of different organic and inorganic fertilisers, amendments with sewage sludge or ammonium sulphate changed the bacterial community composition in root-associated soil aggregates. In addition, ammonium sulphate reduced the taxon richness and the occurrence of specific taxa of both AM fungi and bacteria in the mycorrhizosphere. These changes were primarily explained by the low pH of soils amended with sewage sludge or ammonium sulphate. However, taxon richness of both AM fungi and bacteria also appeared to be promoted by high soil organic content. In summary, interactions between bacteria and the ERM of AM fungi may be specific and physical associations and mycelial exudates may influence these interactions. Microbial interactions are important in determining soil fertility and plant health. In agricultural systems some microorganisms may be either positively or negatively affected by different types of fertilisation, therefore it is possible soil management practices could be customised to promote microorganisms that are beneficial to farming systems.