Biotite and microcline as potassium sources in ectomycorrhizal and non-mycorrhizal Pinus sylvestris seedlings

The aim of this study was to investigate the role of plants colonised by tw o ectomycorrhizal fungi, Paxillus involutus and Suillus variegatus, in mobi lising potassium (K) from biotite and microcline, two minerals common in ac id to medium-acid bedrock. This was carried out in a 33-week pot study with seedlings of Pinus sylvestris growing in symbiosis with the fungi, where n o K was added or where K was added in the form of biotite or microcline. Th e mineral additions were similar to those found in natural soils. All seedl ings, including non-mycorrhizal, were able to access the K in biotite, lead ing to stimulated growth and K uptake relative to controls. Microcline addi tion induced growth depression in all seedlings except those colonised by P . involutus, which were stimulated. The soil solution from S. variegatus-co lonised seedlings grown with biotite had higher concentrations of citric an d oxalic acid. Citric acid concentration was positively correlated to the f ungal biomass (ergosterol) in the soil, as well as to the foliar K in S. va riegatus-colonised seedlings. Seedlings growing without K addition had low K concentrations in the shoot. Magnesium (Mg) concentrations were enhanced in seedlings with severe K shortage, indicating that Mg can substitute for K, while calcium concentrations did not vary significantly.