Recovery of the ectomycorrhizal community after termination of long-term nitrogen fertilisation of a boreal Norway spruce forest

Abstract Ectomycorrhizal fungi (ECM) are a fundamental component of boreal forests promoting tree growth and participating in soil nutrient cycling. Increased nitrogen (N) input is known to largely influence ECM communities but their potential recovery is not well understood. Therefore, we studied the effects of long-term N-fertilisation on ECM communities, and their recovery after termination of N treatment. Fungal ITS sequencing data indicated that N-fertilisation (34 kg N ha −1 y −1 ) for 46 y decreased the relative abundance of ECM species in the fungal community and suppressed originally dominating medium-distance fringe exploration types adapted to N-limited conditions, while the ECM diversity remained unaffected. In other plots, 23 y after termination of fertilisation at 73 kg N ha −1 y −1 for 23 y, the relative abundance of ECM species shifted closer to, but did not reach, control levels. These observations indicate only slow recovery of ECM community, likely due to a high soil N retention capacity.