After long-term nitrogen fertilization stops, arbuscular mycorrhizal stands recover faster than ectomycorrhizal stands

Background/Question/Methods

Nitrogen (N) deposition has increased the storage of carbon (C) in the soils of temperate forests in the Eastern US. Recent research suggests that this increase is partly driven by reductions in the strength of plant-microbial interactions. Specifically, under elevated N, trees reduce their C investment in N acquisition which leads to declines in root stimulation of microbial decomposition. However, N deposition is fading across the region due to the success of the Clean Air Act. As such, there is a critical need to quantify the extent to which these enhanced soil C stores will persist as N inputs decline. To address this need, we followed the initial response of plant-microbial interactions to the cessation of a long-term, whole watershed, N fertilization at the Fernow Experimental Forest in West Virginia, USA. Prior to the cessation of fertilization, previous work at the site has shown declines in belowground C investment by trees that were directly linked to reduced microbial decomposition. To follow recovery, we sampled soils from long-term plots located in the recovering and control watersheds and assayed the response of microbial and root processes including enzyme activity, mycorrhizal colonization, and root biomass and morphology.

Results/Conclusions

On the root side, we found that root biomass remained lower in the recovering watershed compared to the control watershed. However, arbuscular (AM) mycorrhizal colonization showed some signs of recovery with colonization levels in the formerly fertilized watershed nearing those of the control. Ectomycorrhizal (ECM) colonization, by contrast, remained lower in the recovering watershed. On the microbial side, the recovery of colonization was directly mirrored by changes in enzyme activity. That is, enzyme activity in AM dominated plots in the recovering watershed were similar to the control watershed, although enzyme activities in ECM dominated plots remained at N-fertilized levels. While these results are only indicative of the short-term recovery of forests from excess N inputs, they suggest that species composition and mycorrhizal association are an important control on the pace of recovery. Moving forward, we plan to continue monitoring recovery at the Fernow to identify the extent to which the recovering watershed returns to the control state or enters a new stable state.