Ectomycorrhizal C ortinarius species participate in enzymatic oxidation of humus in northern forest ecosystems

Summary In northern forests, belowground sequestration of nitrogen ( N ) in complex organic pools restricts nutrient availability to plants. Oxidative extracellular enzymes produced by ectomycorrhizal fungi may aid plant N acquisition by providing access to N in macromolecular complexes. We test the hypotheses that ectomycorrhizal C ortinarius species produce M n‐dependent peroxidases, and that the activity of these enzymes declines at elevated concentrations of inorganic N . In a boreal pine forest and a sub‐arctic birch forest, C ortinarius DNA was assessed by 454‐sequencing of ITS amplicons and related to M n‐peroxidase activity in humus samples with‐ and without previous N amendment. Transcription of C ortinarius M n‐peroxidase genes was investigated in field samples. Phylogenetic analyses of C ortinarius peroxidase amplicons and genome sequences were performed. We found a significant co‐localization of high peroxidase activity and DNA from C ortinariu s species. Peroxidase activity was reduced by high ammonium concentrations. Amplification of m RNA sequences indicated transcription of C ortinarius M n‐peroxidase genes under field conditions. The C ortinarius glaucopus genome encodes 11 peroxidases – a number comparable to many white‐rot wood decomposers. These results support the hypothesis that some ectomycorrhizal fungi – C ortinarius species in particular – may play an important role in decomposition of complex organic matter, linked to their mobilization of organically bound N .