Long-term nitrogen fertilization impacts plant-microbial interactions differently in arbuscular and ectomycorrhizal trees

Enhanced nitrogen (N) availability in temperate forests has altered ecosystem carbon (C) and N cycling. Recent research has shown that these alterations lead to reductions in belowground C allocation by trees and that the consequences of these reductions on soil C and nutrient cycling may vary by mycorrhizal type. We hypothesized that trees that associate with ectomycorrhizal fungi (ECM) would reduce C allocation towards roots and mycorrhizal fungi to a greater extent than trees that associate with arbuscular mycorrhizal fungi (AM) in response to > 25 years of N fertilization. We further hypothesized that N induced decoupling of roots and microbes in ECM trees would be evidenced by greater declines in extracellular enzyme activities. We measured belowground C allocation to fine root biomass and mycorrhizal colonization in 6 AM and 6 ECM dominated plots in the N fertilized and reference watersheds at the Fernow Experimental Forest in West Virginia, USA. We compared these to measurements of simple-C, complex-C, nitrogen, and phosphorus acquiring enzyme activities in organic horizon, bulk mineral, and rhizosphere soil fractions. N fertilization reduced fine root biomass and mycorrhizal colonization in both AM and ECM stands. We found more consistent reductions in enzyme activities in ECM soils than AM soils under N fertilization which may have been driven by greater declines in root-C transfer to soil microbes. This mechanism helps to explain variability in soil C cycling responses across N gradient and fertilization experiments and may prove useful in predicting the fate of soil C stocks in response to N deposition.