Measuring external mycelia production of ectomycorrhizal fungi in the field: the soil matrix matters

Assessing mycorrhizal fungi production in field settings has been hindered by the inability to measure external mycelia. Recently, external mycelia production was measured in the field using a novel in-growth core technique with acid-washed sand as the in-growth matrix. Here, we tested the assumption that external mycelia production in acid-washed sand is representative of that in native soil. External mycelia production was estimated as the difference in fungal growth between closed (allowing only saprotrophic fungal production) and open (allowing mycorrhizal and saprotrophic fungal production) cores using a factorial design of soil matrices (acid-washed sand vs native) and fertilization treatments (control vs nitrogen (N)) in a longleaf pine (Pinus palustris) plantation. In native soils, the ectomycorrhizal to saprotrophic fungal biomass signal was strong and consistent facilitating the assessment of external mycelia production, which was 300% higher than corresponding rates in acid-washed sand and inversely correlated with soil N. These results demonstrate the efficacy and importance of using native soil as the in-growth matrix to measure ectomycorrhizal fungi external mycelia production in field settings.