Nitrogen (N) deposition has increased carbon (C) storage in temperate forests, but there is variability between forests in the magnitude of this response. Mycorrhizal association may explain this variability given recent research showing distinct differences between ectomycorrhizal (ECM) and arbuscular mycorrhizal (AM) trees in the strength of plant-microbial interactions. In particular, ECM trees send more C to rhizosphere microbes to stimulate enzyme activity and nutrient mobilization than AM trees. Given that N deposition likely reduces the strength of plant-microbial interactions, our objective was to examine the extent to which mycorrhizal association drives soil decomposition responses to elevated N. We hypothesized that N fertilization would decrease plant-microbial interactions and soil decomposition in ECM stands more than in AM stands. To test this hypothesis, we measured the response of plant-microbial interactions in ECM and AM plots in two long-term N fertilization studies, the Fernow Experimental Forest, WV and Bear Brook Watershed, ME. We measured the activity of hydrolytic and oxidative enzymes in rhizosphere, bulk, and organic horizons and linked these to assays of plant-C investment belowground including fine root responses, mycorrhizal colonization, and root exudation.
Results/Conclusions
Our results indicate that N fertilization decreased fine root biomass, root branching, root exudation, and mycorrhizal colonization in both mycorrhizal types with the greatest declines occurring in ECM stands. Similarly, N fertilization reduced oxidative and hydrolytic enzyme activities across all soil fractions with strongest reductions in ECM stands. Fine root and enzyme responses in AM and ECM stands were greater at Bear Brook Watershed than the Fernow Experimental Forest. At Bear Brook, rhizosphere stimulation of enzyme activity was higher than Fernow under ambient N conditions and sharply declined in the N fertilized stand. This result likely reflects the greater ambient N loading that has occurred historically in the central Appalachian region. Collectively, our results suggest that N fertilization decoupled root-microbial interactions in ECM stands to a greater extent than in AM stands. Thus, N deposition may enhance soil C storage capacity more in ECM than AM stands.