Response of ectomycorrhizal extramatrical mycelium production and isotopic composition to in-growth bag size and soil fauna

•Extramatrical ectomycorrhizal mycelia (EMM) production was estimated in the field.•The effect of sizes of in-growth mesh bags and soil fauna on EMM production was tested.•Nitrogen (N) accumulated over time in EMM suggesting N immobilization.•δ13C and δ15N abundances in EMM changed over time, possibly due to plant phenology.•Small diam. mesh bags seem superior to large diam. ones.

In-growth bags are increasingly used to study extramatrical mycelium (EMM) of ectomycorrhizal fungi in forest soils. In this paper we tested whether bag size and presence of soil fauna in bags influence the production, isotopic composition, carbon (C) and nitrogen (N) content of the EMM. Cylindrical in-growth mesh bags (2- or 5-cm-diameter; with or without openings – (1 or 2 mm), allowing faunal colonization or not) were harvested 37, 48, 81 and 283 days after installation in July and the EMM biomass was determined from elemental analyses of the extractable amount of mycelia. The occurrence of openings allowed animals to invade the bags but this did not affect the amount of EMM. We suggest further studies in this matter since the number of animals was low and variable. In the first harvest, mycelial biomass C was three times greater in 2-cm than in 5-cm-bags. After 81 days, mycelial biomass C was 54% greater in the 2-cm (54 kg ha−1) than in the 5-cm bags (35 kg ha−1). While total mycelial C did not change over winter, N content increased suggesting a role for the EMM in the storage of N from autumn to spring. The δ13C and δ15N of the EMM changed between the first three harvests. We hypothesize these changes to be mainly driven by changes in plant C and N sinks. The relation between the isotopic composition of sporocarp exploration type, plant roots and EMM is discussed.