The stimulating effect of apatite on ectomycorrhizal growth diminishes after PK fertilization

The objective of this study was to test if apatite amendment stimulates growth of ectomycorrhizal (EM) fungi and if this is influenced by the phosphorus status of the forest. The production of EM mycelia was estimated in four Norway spruce forests in south-western Sweden by incubating sand-filled mesh bags (with or without apatite amendment) in the soil for 8 or 20 months. Fungal biomass in the mesh bags was estimated by phospholipid fatty acid (PLFA) analysis. Subplots of the forests were fertilized with phosphorus and potassium to alleviate possible deficiencies of these elements. The addition of apatite had a positive influence on the growth of EM mycelia but this effect diminished when the forests were fertilized with PK. The extent of the EM growth response to apatite was strongly correlated to the needle P status which supports the view that the forest trees regulate growth of EM fungi in response to P source in the soil. Rare earth elements originating from the apatite accumulated in ectomycorrhizal root tips surrounding the mesh bags but no differences in concentration were found between root tips from control and PK fertilized plots. The potential of EM fungi to reduce phosphorus deficiency in phosphorus poor forests by increasing weathering rates is discussed.