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Harvard Forest Symposium Abstract 2013

  • Title: Microbial community dynamics in a long-term soil warming chronosequence
  • Primary Author: Kristen DeAngelis (University of Massachusetts Amherst)
  • Additional Authors: Jeffrey Blanchard (University of Massachusetts - Amherst ); Serita Frey (University of New Hampshire - Main Campus); Jerry Melillo (Marine Biological Laboratory); Rebecca Varney (University of Massachusetts Amherst); Linda van Diepen (University of New Hampshire - Main Campus)
  • Abstract:

    Though soil microbes are major drivers of terrestrial C cycling, we lack an understanding of how temperature affects SOM decomposition. Climate warming is causing both biophysical and biogeochemical (e.g., carbon cycle) feedbacks to the climate system. Increased temperature alters soil organic matter (SOM) processing, and if warming accelerates SOM decomposition, then carbon (C) stored in soils will transfer to the atmosphere, resulting in a self-reinforcing (positive) feedback to climate. The mechanisms and magnitudes of terrestrial feedbacks to climate are still largely unknown. Ongoing replicated field warming studies at the Harvard Forest (Petersham, MA) have experienced 5oC above ambient soil temperatures at three sites for 6, 9 and 21 years, which now correspond to three distinct phases of CO2 emissions. This chronosequence offers a unique opportunity to understand how warming affects soil microbial community composition and activity.



    Methods. Soil samples were collected in October 2011 from each of the three sites in the chronosequence at Barre Woods, Prospect Hill and the Soil Warming x Nitrogen Addition (SWaN) Study. The experimental design includes two soil horizons (organic horizon and the upper 10 cm of mineral soil) in the heated and control plots, with four replicates totaling 48 samples. DNA was extracted in triplicate from soils using a CTAB bead beating method followed by a column clean-up. Libraries were constructed based on PCR amplification of small subunit (ssu) rRNA genes using universal primers for bacteria. The Human Microbiome Project protocol was followed for Illumina MiSeq sequencing and analysis using Qiime. Quantitative PCR was also performed to measure absolute abundances of bacteria and fungi.



    Results. Along the chronosequence, it took 20 years for warming to affect bacterial communities. Of the three sites, only the Prospect Hill site showed an effect of warming on bacterial community alpha diversity (with increased diversity and evenness, but not richness) and beta diversity (as measured using principle coordinates analysis with a phylogenetic distance measure). Bacteria showed more response with warming in the organic soils than in the mineral, though diversity changed for both. Groups most affected by warming included members of the Firmicutes, Planctomycetes, Acidobacteria and α-Proteobacteria; most taxa increased in relative abundance with warming. Quantitative PCR revealed that for the organic soils, there was an increase in bacteria but a decrease in fungi with heating; no changes were observed in the mineral soil.



    Significance. Measures of bacterial community diversity and abundance suggest that long-term warming is creating more niche space in organic layer soils, which is manifested as increased bacterial alpha diversity, shifting beta diversity, and increased abundance of bacteria. These data potentially support the recalcitrant decomposer hypothesis, which suggests that long-term warming causes a shift in the carbon available to microbes, from labile C, which has been shown to become depleted after about a decade, to more recalcitrant or previously physically protected C. The mineral soil bacterial community seems to be somewhat insulated from the effects of warming, though mineral soils were also strongly increased in diversity. Further studies, such as metagenomic and metatranscriptomic analyses, promise to illuminate the C cycling pathways in these communities that are responding to warming and associated changes in microbially available carbon.

  • Research Category: Large Experiments and Permanent Plot Studies
    Physiological Ecology, Population Dynamics, and Species Interactions
    Soil Carbon and Nitrogen Dynamics