Six years later: Ectomycorrhizal fungal communities recovering after the end of long-term nitrogen and sulfur addition in a mixed-species temperate North American forest

We examined how ectomycorrhizal (ECM) fungal communities of a North American mixed species temperate forest recovered from 26 years of experimental nitrogen and sulfur addition. We used Illumina-based metabarcoding of the ribosomal DNA ITS region to compare the ECM fungal communities in soil samples collected at the end (in 2016) and six years after (in 2022) the conclusion of the Bear Brook Watershed Experiment, Maine, USA, a 26 year ammonium-sulfate addition study conducted in adjacent ∼10 ha treated and reference watersheds. Six years after the experiment ended, ECM fungal species richness in the treated watershed returned to reference watershed levels with a significant increase by Cortinarius, a nitrophobic genus. In addition, the ECM fungal community composition of the treated watershed was significantly different six years after the experiment ended. Because this forest is in a region of historically low nitrogen deposition, this study establishes a baseline for the study of recovery in other regions where nitrogen deposition is higher and more widespread.