In a long-term experiment in northern hardwood forests, annual woody growth increment was enhanced by experimental N deposition of 3 g N m-2 y-1, added from 1994 to 2017. This increase in tree growth was driven not by enhanced photosynthesis, but by a reduction in belowground C allocation to mycorrhizal fungi. During August of 2018, we examined sugar maple fine roots and soil to determine if indicators of arbuscular mycorrhizal fungi (AMF) abundance continued to be suppressed during the first year after N additions had ceased. Phospholipid fatty acid (PLFA) and neutral lipid fatty acid (NLFA) biomarkers specific to AMF were quantified in fine root samples and PLFA specific to AMF and those indicative of general fungal and bacterial biomass were quantified in soil
Results/Conclusions
The abundance of AMF was still greatly suppressed, as indicated by 38 and 72% reductions in PLFA and NLFA indicators of AMF in roots. This response may persist for some time, even if available soil N gradually declines, as tree biomass in the N deposition treatment has accrued large amounts of stored N, especially in coarse roots. This continued high N availability of could eliminate the need to share C with mycorrhizae to obtain N, which could in turn help sustain the increase in aboveground woody biomass production, despite reduced N inputs. Similar reductions occurred in soil for indicators of AMF as well as those for overall fungal and bacterial biomass. Thus long-term reductions in litter and soil organic matter decomposition associated with elevated N deposition in the experiment may persist for the foreseeable future.