Changes in fungal communities along a boreal forest soil fertility gradient.

Boreal forests harbour diverse fungal communities with decisive roles in decomposition and plant nutrition. Although changes in boreal plant communities along gradients in soil acidity and nitrogen (N) availability are well described, less is known about how fungal taxonomic and functional groups respond to soil fertility factors. We analysed fungal communities in humus and litter from 25 Swedish old-growth forests, ranging from N-rich Picea abies stands to acidic and N-poor Pinus sylvestris stands. 454-pyrosequencing of ITS2 amplicons was used to analyse community composition, and biomass was estimated by ergosterol analysis. Fungal community composition was significantly related to soil fertility at the levels of species, genera/orders and functional groups. Ascomycetes dominated in less fertile forests, whereas basidiomycetes increased in abundance in more fertile forests, both in litter and humus. The relative abundance of mycorrhizal fungi in the humus layer remained high even in the most fertile soils. Tolerance to acidity and nitrogen deficiency seems to be of greater importance than plant carbon (C) allocation patterns in determining responses of fungal communities to soil fertility, in old-growth boreal forests.

Production of ectomycorrhizal mycelium peaks during canopy closure in Norway spruce forests.

*Here, species composition and biomass production of actively growing ectomycorrhizal (EM) mycelia were studied over the rotation period of managed Norway spruce (Picea abies) stands in south-western Sweden. *The EM mycelia were collected using ingrowth mesh bags incubated in the forest soil during one growing season. Fungal biomass was estimated by ergosterol analysis and the EM species were identified by 454 sequencing of internal transcribed spacer (ITS) amplicons. Nutrient availability and the fungal biomass in soil samples were also estimated. *Biomass production peaked in young stands (10-30 yr old) before the first thinning phase. Tylospora fibrillosa dominated the EM community, especially in these young stands, where it constituted 80% of the EM amplicons derived from the mesh bags. Species richness increased in older stands. *The establishment of EM mycelial networks in young Norway spruce stands requires large amounts of carbon, while much less is needed to sustain the EM community in older stands. The variation in EM biomass production over the rotation period has implications for carbon sequestration rates in forest soils.

Contrasting effects of ectomycorrhizal fungi on early and late stage decomposition in a boreal forest.

Symbiotic ectomycorrhizal fungi have received increasing attention as regulators of below-ground organic matter storage. They are proposed to promote organic matter accumulation by suppressing saprotrophs, but have also been suggested to play an active role in decomposition themselves. Here we show that exclusion of tree roots and associated ectomycorrhizal fungi in a boreal forest increased decomposition of surface litter by 11% by alleviating nitrogen limitation of saprotrophs-a "Gadgil effect". At the same time, root exclusion decreased Mn-peroxidase activity in the deeper mor layer by 91%. Our results show that ectomycorrhizal fungi may hamper short-term litter decomposition, but also support a crucial role of ectomycorrhizal fungi in driving long-term organic matter oxidation. These observations stress the importance of ectomycorrhizal fungi in regulation of below-ground organic matter accumulation. By different mechanisms they may either hamper or stimulate decomposition, depending upon stage of decomposition and location in the soil profile.