Knorr Melissa

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  • Article
    Fungal community response to long-term soil warming with potential implications for soil carbon dynamics
    (Ecological Society of America, 2021-05-11) Pec, Gregory J. ; van Diepen, Linda T. A. ; Knorr, Melissa ; Grandy, A. Stuart ; Melillo, Jerry M. ; DeAngelis, Kristen M. ; Blanchard, Jeffrey L. ; Frey, Serita D.
    The direction and magnitude of climate warming effects on ecosystem processes such as carbon cycling remain uncertain. Soil fungi are central to these processes due to their roles as decomposers of soil organic matter, as mycorrhizal symbionts, and as determinants of plant diversity. Yet despite their importance to ecosystem functioning, we lack a clear understanding of the long-term response of soil fungal communities to warming. Toward this goal, we characterized soil fungal communities in two replicated soil warming experiments at the Harvard Forest (Petersham, Massachusetts, USA) which had experienced 5°C above ambient soil temperatures for 5 and 20 yr at the time of sampling. We assessed fungal diversity and community composition by sequencing the ITS2 region of rDNA using Illumina technology, along with soil C concentrations and chemistry. Three main findings emerged: (1) long-, but not short-term warming resulted in compositional shifts in the soil fungal community, particularly in the saprotrophic and unknown components of the community; (2) soil C concentrations and the total C stored in the organic horizon declined in response to both short- (5 yr) and long-term (20 yr) warming; and (3) following long-term warming, shifts in fungal guild relative abundances were associated with substantial changes in soil organic matter chemistry, particularly the relative abundance of lignin. Taken together, our results suggest that shifts with warming in the relative abundance of fungal functional groups and dominant fungal taxa are related to observed losses in total soil C.
  • Article
    Substrate availability and not thermal acclimation controls microbial temperature sensitivity response to long‐term warming
    (Wiley, 2022-11-30) Domeignoz‐Horta, Luiz A. ; Pold, Grace ; Erb, Hailey ; Sebag, David ; Verrecchia, Eric ; Northen, Trent ; Louie, Katherine ; Eloe‐Fadrosh, Emiley ; Pennacchio, Christa ; Knorr, Melissa A. ; Frey, Serita D. ; Melillo, Jerry M. ; DeAngelis, Kristen M.
    Microbes are responsible for cycling carbon (C) through soils, and predicted changes in soil C stocks under climate change are highly sensitive to shifts in the mechanisms assumed to control the microbial physiological response to warming. Two mechanisms have been suggested to explain the long‐term warming impact on microbial physiology: microbial thermal acclimation and changes in the quantity and quality of substrates available for microbial metabolism. Yet studies disentangling these two mechanisms are lacking. To resolve the drivers of changes in microbial physiology in response to long‐term warming, we sampled soils from 13‐ and 28‐year‐old soil warming experiments in different seasons. We performed short‐term laboratory incubations across a range of temperatures to measure the relationships between temperature sensitivity of physiology (growth, respiration, carbon use efficiency, and extracellular enzyme activity) and the chemical composition of soil organic matter. We observed apparent thermal acclimation of microbial respiration, but only in summer, when warming had exacerbated the seasonally‐induced, already small dissolved organic matter pools. Irrespective of warming, greater quantity and quality of soil carbon increased the extracellular enzymatic pool and its temperature sensitivity. We propose that fresh litter input into the system seasonally cancels apparent thermal acclimation of C‐cycling processes to decadal warming. Our findings reveal that long‐term warming has indirectly affected microbial physiology via reduced C availability in this system, implying that earth system models including these negative feedbacks may be best suited to describe long‐term warming effects on these soils.Warming can accelerate or decelerate soil microbial response to warmer temperatures. Here we provide support for the hypothesis that microbial temperature sensitivity is contingent upon substrate availability, which itself is reduced by warming. Thus we show the complex interplay between microbial activity and changes in soil carbon stocks.