The external mycorrhizal mycelium - growth and interactions with saprophytic microorganisms

The interactions between external mycorrhizal mycelia and saprophytic microorganisms were studied in experiments with growth chambers specially designed to enable the mycelia to develop in root-free soil compartments. The growth of the mycorrhizal mycelia was estimated by use of biochemical signature compounds. It was shown possible to estimate the biomass of the external mycelium of different arbuscular mycorrhizal fungi by use of the signature fatty acid 16:1w5 in soil and sand. The amount of this fatty acid was estimated as indicator both of phospholipids (PLFAs) and neutral lipids (NLFAs). The results indicated that PLFAs reflect mycelium on the AM fungi and NLFAs storage structures.No effects of the AM mycelium on the bacterial community were recorded in an agricultural soil, even though the AM mycelium constituted a large part of the biomass. In a study involving calcareous dune sand, the growth on the external AM mycelium decreased the amount of PLFA 18:2w6,9, indicating negative effects on growth of saprophytic fungi. Ectomycorrhizal mycelia of six different species reduced bacterial activity, estimated as thymidine incorporation, in experiments with a sandy soil. In these experiments no negative effect on bacterial biomass was observed. In a study on pines growing for eight months in a peat/sand mixture, both bacterial activity and biomass were lowered due to mycorrhizal inoculation. However, in the soils to which primary minerals as apatite and biotite had been added, the growth of Suillus variegatus increased bacterial activity. This demonstrated that, under certain, circumstances, ectomycorrhizal fungi may contribute to a significant carbon input to the soil which stimulates bacterial growth