Recovery of ectomycorrhiza after ‘nitrogen saturation’ of a conifer forest

Abstract

P>Trees reduce their carbon (C) allocation to roots and mycorrhizal fungi in response to high nitrogen (N) additions, which should reduce the N retention capacity of forests. The time needed for recovery of mycorrhizas after termination of N loading remains unknown. Here, we report the long-term impact of N loading and the recovery of ectomycorrhiza after high N loading on a Pinus sylvestris forest. We analysed the N% and abundance of the stable isotope 15N in tree needles and soil, soil microbial fatty acid biomarkers and fungal DNA. Needles in N-loaded plots became enriched in 15N, reflecting decreased N retention by mycorrhizal fungi and isotopic discrimination against 15N during loss of N. Meanwhile, needles in N-limited (control) plots became depleted in 15N, reflecting high retention of 15N by mycorrhizal fungi. N loading was terminated after 20 yr. The delta 15N and N% of the needles decreased 6 yr after N loading had been terminated, and approached values in control plots after 15 yr. This decrease, and the larger contributions compared with N-loaded plots of a fungal fatty acid biomarker and ectomycorrhizal sequences, suggest recovery of ectomycorrhiza. High N loading rapidly decreased the functional role of ectomycorrhiza in the forest N cycle, but significant recovery occurred within 6-15 yr after termination of N loading.