LB 7-60 - Nitrogen deposition influences soil microorganisms and carbon biogeochemistry in arbuscular mycorrhizal ecosystem more than in ectomycorrhizal ecosystem

Background/Question/Methods:

Globally, tree species and their symbiotic mycorrhizal fungi shape soil environments and mediate belowground carbon (C) processes. In temperate forest ecosystem, most tree species associate with arbuscular (AM) or ectomycorrhizal (ECM) fungi. Differences in the physiology and nutrient acquisition traits of AM or ECM-ecosystem are associated with distinct microbial communities, carbon biogeochemistry, and soil organic carbon stocks. AM-ecosystem are characterized by lower carbon to nitrogen (C:N) litter, more diverse saprotrophic communities, and higher rates of decomposition. Conversely, in ECM-ecosystems trees produce C-rich, N-poor, detritus and are more dependent on ECM-fungi to mine for organic-N. At the global scale, N-deposition is predicted to increased and can impact plant-microbe interactions. Since terrestrial ecosystems represent a large proportion of the C-pool, it is imperative to understand how N-deposition will influence microbial processes and soil organic matter formation in AM and ECM ecosystems. This experiment examined the response of microbial communities (activity, biomass, carbon use efficiency and composition) by utilizing soil from a long-term N-deposition watershed experiment with established AM and ECM-stands. Additionally, quantitative stable isotope probing (qSIP) with 13C-synthetic root exudates and 18O-H2O was used to explore taxon-specific carbon assimilation and growth in relation to mycorrhizal association and N deposition.

Results/Conclusions:

Under ambient N-deposition, there were differences in the microbial community composition and activity between AM and ECM soils. In general, AM-soils had higher microbial alpha diversity (prokaryotes and eukaryotes) and respiration. The effects of elevated N-deposition on microbial community composition and activity were more pronounced in AM-soils than ECM soils. Specifically, elevated N-deposition decreased microbial respiration ~25%, biomass~16%, and carbon use efficiency ~14% in AM systems. However, priming and substrate-derived respiration increased in response to N deposition in AM soils by 27% and 14% respectively. In comparison, elevated N-deposition had little effect on these microbial processes in the ECM-soils. Although there was an overall reduction in microbial biodiversity in both AM and ECM-soils under N-deposition, there was a more consistent reduction in growth and C-assimilation in the AM-soil community based on qSIP data. At the phylum level in the AM-soils, N-deposition reduced the activity of Bacteroidetes, Firmicutes, Nitrospirae, Planctomycetes, Proteobacteria, and WPS-2. Many of these bacteria groups are known to play an important role in soil N-cycling. The reduction in microbial respiration and activity suggest that N-deposition may enhance soil C by decreasing decomposition and promote more stable soil C in AM-ecosystem.