External Mycelia of Mycorrhizal Fungi - responses to elevated N in forest ecosystems

Most plants live in symbiosis with mycorrhizal fungi. Mycorrhizal roots constitute the interface between the plant and the soil, and almost every fine root of forest trees in nitrogen-limited boreal and temperate forests is colonised by ectomycorrhizal (EM) fungi. The mycelia of EM and ericoid mycorrhizal (ErM) fungi are very important for plant N uptake and N cycling in forest soils. Earlier laboratory studies have shown that elevated N levels have a negative influence on the growth of EM mycelia. I have developed methods to quantify the biomass and the production of external mycorrhizal fungi in the field. In-growth mesh bags filled with sand were buried in forest soils to determine EM mycelial growth. I have also estimated the biomass of mycorrhizal mycelia in soil by measuring the amounts of fungal biomarkers in soil samples that degraded after depriving mycorrhizal fungi from its energy source, the C flow from tree roots, by root trenching or incubation of soil samples. The production of EM extramatrical mycelia in spruce forest soils in southern Sweden was found to correspond to 125 to 200 kg ha-1 y-1 and the biomass of EM mycelia in soil was found to constitute a large proportion of the belowground biomass. External EM mycelia extending from the fine roots contributed most (c. 80%) to the total EM fungal biomass, with considerably smaller proportions on EM root tips (15-20%) and EM fruit bodies (less than 1%). Fertilisation of spruce forests (c. 1,000 kg N ha-1 during a 10-year period) in SW Sweden was found to reduce EM mycelial production and biomass. The EM biomass also declined with higher nutrient availability along natural nutrient gradients in N Sweden. In the most nutrient-poor soils in these gradients ErM contributed significantly to the mycorrhizal biomass, and the production of EM mycelia was lowest in the most nutrient-rich soils. In oak forests in S Sweden EM mycelial production was lower in regions with higher N deposition (c. 20 kg N ha-1 y-1) than in regions with only half that deposition. The biomass of AM fungi was found to be stimulated by high soil pH, both in the natural nutrient gradients and in oak forests. EM mycelial production is thought to be controlled by belowground C allocation in the tree, which here is hypothesized to be regulated by forest tree N status rather than by soil N concentration. Low production of EM mycelia was observed to coincide with high nitrate leaching in at least two situations, indicating that EM mycelia may be important in retaining added N in forest soils