COS 97-4 - The recovery of plant-microbial interactions from long-term nitrogen fertilization differs by mycorrhizal type in Central Appalachian forests

Abstract: In the temperate forests of the Eastern US, nitrogen (N) fertilization experiments and observations across N deposition gradients show that enhanced N availability leads to increases in soil carbon (C). At the Fernow Experimental Forest in WV, thirty years of N fertilization led to increases in soil C due to a decline in belowground C allocation by trees to rhizosphere microbes and subsequent reduction in microbial decomposition. The strength of this response varied by mycorrhizal association. Arbuscular mycorrhizal (AM) trees preferentially reduced C allocation to fine roots, whereas ectomycorrhizal (ECM) trees primarily decreased C allocation to mycorrhizae. Moreover, the declines in microbial decomposition were greater in ECM soils than AM soils. Given that N deposition is declining and AM trees are becoming more dominant in the Eastern US, examining whether these patterns persist, return to their unfertilized state, or reach a new steady state is critical to determining the fate of N-induced soil C gains.

To examine differences between the trajectories of AM and ECM plots in their recovery from N fertilization, we measured the changes to belowground C allocation and microbial decomposition in the years following the cessation of fertilization at the Fernow in 2019. We measured the recovery of fine root biomass, mycorrhizal colonization rate, and soil enzyme activities as a proxy for the shifts in belowground C allocation and the resulting changes in microbial decomposition and soil C pools. Overall, we found that AM plots were recovering to their unfertilized state, while ECM plots appeared to be approaching a new steady-state. Although AM root biomass remained suppressed, mycorrhizal colonization rate rebounded faster in AM plots than ECM plots. Similarly, in AM plots, the activity of enzymes that degrade simple and complex C, and mobilize phosphorus were nearing levels found in the unfertilized watershed. By contrast, these same enzymes in ECM plots appeared to be reaching a new steady state with levels beyond those observed in unfertilized ECM plots. However, enzymes that mobilized N from fungal necromass remained suppressed in both AM and ECM plots, likely reflecting a persistent decline in the importance of fungi to microbial community composition. Collectively, these results suggest that if these patterns hold, N-induced soil C gains may be lost faster in ECM than AM stands.