Estimation of the biomass and seasonal growth of external mycelium of ectomycorrhizal fungi in the field.

•  In-growth mesh bags were used to quantify the production of external mycelium of ectomycorrhizal (EM) fungi in the field. •  Colonization of the mesh bags was followed by visual estimation of the amount of mycelium, and by measuring fungal biomarkers (the phospholipid fatty acid (PLFA) 18 : 2ω6,9 and ergosterol). Mesh bags were placed inside and outside plots that were root isolated in order to estimate the amount of saprotrophic mycelium in relation to EM mycelium. The majority of mycelium in the mesh bags were EM, and this was confirmed by analysis of the δ13 C value in mycelia. •  Fungal colonization of mesh bags peaked during autumn. The total amount of EM mycelium produced in the mesh bags during a year was calculated to be between 125 and 200 kg ha-1 . The total amount of EM mycelium (including EM mantles) in the humus was estimated to be 700-900 kg ha-1 . •  The biomass of EM mycelium in the soil was in the same range as the biomass of fine roots and peaks of mycelial growth coincided with periods of maximum growth of fine-roots.