Summary• 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 : 2 ω 6,9 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 δ 13 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.

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Summary• A field study was carried out to evaluate the influence of N fertilization on the growth of the external mycelium of ectomycorrhizal (EM) fungi in a Norway spruce forest in SW Sweden.• Nylon mesh bags filled with sand were buried in the soil for 6 -18 months and the ingrowth of mycelium was used as an estimate of EM mycelial growth. Root-isolated, trenched plots were used to estimate background growth of saprotrophic fungi.• Mycelial growth of EM fungi in N-treated plots was reduced to c. 50% of that in nonfertilized plots. Local addition of apatite stimulated the EM mycelial growth in N-treated plots.• The negative influence of N on the growth of external EM mycelium observed earlier in laboratory studies was confirmed in the present field study. The growth of EM mycelia was not directly related to N concentration in the soil but rather to the N status of the trees, although other factors induced by the N treatment may also have influenced EM mycelial growth.

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Summary• Total fungal biomass, the biomass of ectomycorrhizal and ericoid mycorrhizal (EM + ErM), and arbuscular mycorrhizal (AM) fungi, as well as the production of EM and AM fungi, were estimated in coniferous forest soils along four natural nutrient gradients. Plant community changes, forest productivity, soil pH and N availability increase over relatively short distances (< 100 m) along the gradients.• The amounts of the phospholipid fatty acid (PLFA) 18 : 2 ω 6,9 were used to estimate total fungi (not including AM), and the PLFA 16 : 1 ω 5 to estimate AM fungi in soil samples. The decrease in the PLFA 18 : 2 ω 6,9 during incubation of soils was used to estimate EM + ErM biomass. Production of AM and EM mycorrhiza was estimated using ingrowth mesh bags.• Total fungal biomass was highest in soils with the lowest nutrient availability and tree productivity. Biomass of ErM + EM was also highest in these soils. We found tendencies that EM mycelial production was lowest in the soils with the highest nutrient availability and tree productivity. Production of AM fungi was highest in nutrient-rich soils with high pH.• Our results suggest that mycorrhizal communities change from being ErM-, to EM-and finally to AM-dominated along these gradients. The observed changes in mycorrhizal type in the short nutrient gradients follow similar patterns to those suggested for altitudinal or latitudinal gradients over longer distances.

Summary• Nitrogen (N) availability is known to influence ectomycorrhizal fungal components, such as fungal community composition, biomass of root tips and production of mycelia, but effects have never been demonstrated within the same forest.• We measured concurrently the abundance of ectomycorrhizal root tips and the production of external mycelia, and explored the changes in the ectomycorrhizal community composition, across a stand-scale N deposition gradient (from 27 to 43 kg N ha) at the edge of a spruce forest. The N status was affected along the gradient as shown by a range of N availability indices.• Ectomycorrhizal root tip abundance and mycelial production decreased five and 10-fold, respectively, with increasing N deposition. In addition, the ectomycorrhizal fungal community changed and the species richness decreased. The changes were correlated with the measured indices of N status, in particular N deposition and N leaching.• The relationship between the altered ectomycorrhizal community, root tip abundance and mycelial production is discussed in the context of the N parameters. We suggest that increased N deposition to forests will cause large changes in ectomycorrhizal fungal community structure and functioning, which, in turn, may result in reduced N uptake by roots and fungi, and increased losses of N by leaching.

Deciduous forests may respond differently from coniferous forests to the anthropogenic deposition of nitrogen (N). Since fungi, especially ectomycorrhizal (EM) fungi, are known to be negatively affected by N deposition, the effects of N deposition on the soil microbial community, total fungal biomass and mycelial growth of EM fungi were studied in oak-dominated deciduous forests along a nitrogen deposition gradient in southern Sweden. In-growth mesh bags were used to estimate the production of mycelia by EM fungi in 19 oak stands in the N deposition gradient, and the results were compared with nitrate leaching data obtained previously. Soil samples from 154 oak forest sites were analysed regarding the content of phospholipid fatty acids (PLFAs). Thirty PLFAs associated with microbes were analysed and the PLFA 18:2omega6,9 was used as an indicator to estimate the total fungal biomass. Higher N deposition (20 kg N ha(-1)y(-1) compared with 10 kg N ha(-1)y(-1)) tended to reduce EM mycelial growth. The total soil fungal biomass was not affected by N deposition or soil pH, while the PLFA 16:1omega5, a biomarker for arbuscular mycorrhizal (AM) fungi, was negatively affected by N deposition, but also positively correlated to soil pH. Other PLFAs positively affected by soil pH were, e.g., i14:0, a15:0, 16:1omega9, a17:0 and 18:1omega7, while some were negatively affected by pH, such as i15:0, 16:1omega7t, 10Me17:0 and cy19:0. In addition, N deposition had an effect on the PLFAs 16:1omega7c and 16:1omega9 (negatively) and cy19:0 (positively). The production of EM mycelia is probably more sensitive to N deposition than total fungal biomass according to the fungal biomarker PLFA 18:2omega6,9. Low amounts of EM mycelia covaried with increased nitrate leaching, suggesting that EM mycelia possibly play an important role in forest soil N retention at increased N input.

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