Production of external mycelium by ectomycorrhizal fungi in a norway spruce forest was reduced in response to nitrogen fertilization

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Abstract
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.