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Harvard Forest Symposium Abstract 2013

• Title: Diversity and physiology of fungi in response to nitrogen additions and soil warming
• Primary Author: Linda van Diepen (University of New Hampshire - Main Campus)
• Additional Authors: Serita Frey (University of New Hampshire - Main Campus); Eric Morrison (University of New Hampshire - Main Campus); Anne Pringle (University of Wisconsin - Madison); Jesse Sadowsky (University of New Hampshire - Main Campus); christopher sthultz (Harvard University Faculty of Arts and Sciences)
• Abstract:

  Fungi are ubiquitous in terrestrial ecosystems and play an important role in biogeochemical cycling because of their function as litter decomposers. Fungi are a highly diverse and non-static group and fungal lineages can evolve in response to climate change. This evolution could redirect fungal function and as a consequence influence nutrient cycling within an ecosystem. Our study focuses on understanding the mechanisms by which fungi persist in a changing climate and explores the potential feedbacks on ecosystem functioning. Our goal is to identify the biodiversity of saprotrophic (decomposer) and ectomycorrhizal fungi, understand their ecology and physiological response to a changing climate, and examine the relationship between fungal diversity and litter decay in response to climate change.
  
  
  
  Study Sites
  
  Our study takes place at two of the LTER sites at Harvard forest: the Chronic Nitrogen (N) Addition plots and the Barre Woods Soil Warming Study. The Chronic N Addition Experiment consists of three N treatments (ambient, 5 and 15 g N m-2 y-1) and the Soil Warming Study has two treatments (ambient and +5°C warming).
  
  
  
  Litter decomposer fungi
  
  A litterbag study was implemented in 2010 at the Chronic N and Soil Warming study sites, and replicate sets of litterbags were harvested after one and two years of decomposition. Mass loss, litter C:N, lignin/cellulose content and microbial enzyme activity were measured on litter subsamples from both sets of litterbags. Fungi associated with the decomposed plant litter were cultured, and in addition fungal community composition was analyzed using high throughput sequencing. For a subset of cultured fungal species, we measured growth rates and decomposition ability under varying N conditions in the lab (representing field N availability).
  
  In both years litter mass loss was reduced under N addition, with most of the mass loss observed in the first year compared to the second year (70% and 30% of total mass loss, respectively). Both years showed either increased activity for some hydrolytic enzymes (e.g. cellobiohydrolase) or no difference (e.g. ß-N-acetylglucosaminidase) with increased N. The oxidative enzymes (e.g. peroxidases) showed no difference in activity in the first year, but had a highly reduced activity in year 2 under elevated N conditions. Soil warming did not affect litter mass loss, and only had an effect on the activity of a few enzymes.
  
  A few fungal species cultured from litterbags showed reduced growth rate under increased N lab conditions, while other species were not affected by elevated N concentrations. The decomposition experiment with the fungal cultures showed that several fungi isolated from the high N treatment had reduced decomposition rates in vivo compared to those isolated from the control treatment. These changes in growth rates and decomposition ability indicated that our global change factors could affect the physiology of fungal species or may favor species with different traits.
  
  To assess the diversity of the fungal community associated with decomposing litter we sequenced the internal transcribed spacer (ITS) region of extracted DNA from the litter harvested in year 1 (2011) and are currently extracting both RNA and DNA from the second year litterbags. We found that N addition slightly increased fungal species richness, and both N addition and soil warming caused significant shifts in fungal community composition. Changes in mass loss and litter chemistry (lignin, cellulose, C/N) were strongly influenced by the diversity of the fungal communities.
  
  Results add to a growing body of data documenting shifts in biodiversity caused by global change and suggest that these shifts are influencing fundamental ecosystem processes.
  
  
  
  Soil fungal community
  
  To complement our litter fungal diversity analysis, an assessment of soil fungal diversity associated with the forest floor (O-horizon) was conducted at the Chronic N site. Soil samples were taken in November of 2009 at the Chronic N site with three replicate samples per plot. We performed a marker gene study of the fungal community in the organic soil horizon using 454 sequencing of three loci: ITS1, ITS2, and rDNA large subunit D2-D3 region.
  
  The dominant genus at the site was Russula, which are primarily ectomycorrhizal. The Russulas had variable responses to N treatments; the most dominant operational taxonomic unit (OTU) increased significantly in relative abundance with N treatments, and the combined relative abundance of all other members of this genus declined with N addition. Ascomycete species richness and Shannon’s diversity increased with N enrichment, and relative abundance of at least one genus of the Ascomycetes, Hypocrea, responded positively to N fertilization. In general, the N treatments caused a shift in the community composition of Basidiomycetes as described using a phylogenetic metric, Unifrac, suggesting a functional shift in this portion of the community.
  
  These results support two alternative hypotheses: that N additions may favor N-limited Ascomycete decomposers, causing competitive exclusion of Basidiomycete decomposers; or, that N addition may have a direct effect on Basidiomycete activity (e.g. decreased enzyme production), allowing proliferation of a more diverse Ascomycete community.
  
  
  
  Ectomycorrhizal diversity and enzyme activity
  
  At the Chronic N Addition site, we assessed extracellular carbohydrolase and oxidase enzyme activity on the surface of ectomycorrhizal rootlets and in bulk soil on five dates between May and December of 2012. Ectomycorrhizal fungi that were subjected to enzyme assays are currently being identified by sequencing of the ITS region.
  
  Ectomycorrhizal fungal production of carbohydrolases peaked at leaf emergence and after leaf senescence and remained at one-third of peak levels while the tree canopy was present. Ectomycorrhizal fungal production of oxidases varied by two-fold among sampling dates and did not follow the same temporal pattern as carbohydrolases. Of the enzymes studied, only cellobiohydrolase (CBH) and peroxidase were consistently correlated between ectomycorrhizal fungi and organic-horizon soil. Positive (CBH) and negative (peroxidase) correlations on four of five sampling dates were driven by higher ectomycorrhizal production in N-enriched soils.
  
  In summary, ectomycorrhizal fungal contributions to soil carbon transformations appear to be strongest under elevated N deposition and, for carbohydrolytic activity, while tree photosynthesis is inactive.
  
  
  
  Summary
  
  Using all of the above analyses we are starting to bridge the gap in understanding how the interaction between ecosystem function and fungal community dynamics are altered by anthropogenic changes in climate and nutrient availability.
  
  

• Research Category: Biodiversity Studies
  Physiological Ecology, Population Dynamics, and Species Interactions
  Soil Carbon and Nitrogen Dynamics