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Harvard Forest Symposium Abstract 2014

  • Title: Diversity and physiology of fungi, and soil biogeochemical cycling in response to nitrogen additions and soil warming
  • Primary Author: Linda van Diepen (University of New Hampshire - Main Campus)
  • Additional Authors: Mark Anthony (University of New Hampshire - Main Campus); Serita Frey (University of New Hampshire - Main Campus); Melissa Knorr (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)
  • 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 research 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, organic matter decay, and biogeochemical cycling in response to climate change.



    Study Sites

    Our research takes place at two of the LTER sites at Harvard forest: the Chronic Nitrogen (N) Addition plots (CNA), and the Soil Warming and Nitrogen Addition Study (SWaN). The CNA experiment consists of three N treatments (ambient, 5 and 15 g N m-2 y-1), and SWaN consists of four treatments (ambient, N (5 g N m-2 y-1), +5°C warming, N and warming).





    Litter decomposer fungi

    Litterbag studies were implemented in 2010 at the CNA study site using mixed litter from the control plots. In addition, we implemented a reciprocal transplant litterbag study using the most abundant litter species (Quercus spp.). Replicate sets of litterbags were harvested after one and two years of decomposition, and mass loss, litter C:N, lignin/cellulose content, and microbial extracellular enzyme activities were measured. 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 in vivo decomposition capacity under varying N conditions in the lab (representing field N availability).

    In the reciprocal litter transplant study, we found that after one year of decomposition, only the litter from the high N plots was negatively affected by N treatment, while after two years, all litter types were negatively affected by N treatment. Ligninolytic enzyme activities were also negatively affected by increased litter N concentration in the first year, and by N treatment in the second year.

    Overall, fungi isolated from the N addition plots were less able to degrade plant litter, as compared to isolates of the same species from control plots, even when the N isolates were grown in control environments. Observed changes in physiology suggest the species have undergone an evolutionary change that is not readily reversed.

    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 one (2011) and sequenced the ITS region of extracted DNA and RNA of litter harvested in year two (2012). N additions caused shifts in fungal community composition, and negatively affected richness of active fungi relative to the standing community. In particular, high levels of N addition decreased the relative abundance of active ascomycetes compared to standing ascomycetes.

    In addition, fungal community composition was the single best predictor of extracellular enzyme activities after two years of decomposition. Our 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.



    Ectomycorrhizal diversity and enzyme activity

    We assessed the effects of long-term simulated nitrogen (N) deposition on extracellular enzyme pools of common ectomycorrhizal fungal species in the Chronic Nitrogen Addition study at Harvard Forest. Samples were taken at six-week intervals over five dates in 2012. Fungal hyphae encapsulating the surface of ectomycorrhizal root tips were subjected to aminopeptidase, carbohydrolase, oxidase, and phosphatase enzyme assays and then identified by DNA sequencing. With low and high rates of N additions, the relative abundance of Russula spp. doubled while Cortinarius spp. abundance declined significantly. Ectomycorrhizae formed by Russula had larger carbohydrolase and oxidase pools than Cortinarius, which in turn had larger aminopeptidase pools than Russula. Fungal genera and N addition rates interacted to affect enzyme pools; added N increased Russula spp. carbohydrolase pools and Cortinarius spp. aminopeptidase pools. In conclusion, N additions had an opposing effect on Cortinarius and Russula abundances, but enhanced pools of enzymes that each taxon specialized in producing.



    Biogeochemical cycling

    At our Soil Warming x Nitrogen Addition experiment continued soil respiration measurements indicate that previously stimulated CO2 flux with warming is no longer occuring. After seven years of warming, the soil CO2 flux in the warming plots has returned to rates comparable to those in the ambient (non-heated) plots. Eukaryotic transcriptomic analyses of soil samples that were collected in October 2011 from all treatment plots revealed differential expression and richness of several lignocellulolytic genes involved in decomposition. Transcriptomic data is currently being analyzed for patterns in co-expression of genes to understand if particular clusters of genes can explain differences in biogeochemical cycling we are observing at our study site.



    Invasive species

    A new project at SWaN has been initiated to examine how Alliaria petiolata (garlic mustard) invasion interacts with soil warming and nitrogen addition. Currently, A. petiolata plants have been maintained in naturally observed densities in three replicate treatment plots at SWaN. As part of this project, independent plots have been set-up in areas where Alliaria has already invaded. These plots are part of a regional study to query the impacts and factors facilitating successful invasions, with a focus on the effects on fungal community composition and functioning.



    Summary

    Using the above approaches 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

  • Research Category: Biodiversity Studies, Forest-Atmosphere Exchange, Invasive Plants, Pests & Pathogens, Large Experiments and Permanent Plot Studies, Physiological Ecology, Population Dynamics, and Species Interactions, Soil Carbon and Nitrogen Dynamics