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1. Limitations of the Q10 Coefficient for Quantifying Temperature Sensitivity of Anaerobic Organic Matter Decomposition: A Modeling Based Assessment

   Wu, Qiong ; Ye, Rongzhong ; Bridgham, Scott D. ; Jin, Qusheng ( August 2021 , Journal of geophysical research Biogeosciences)

   The Q10 coefficient is the ratio of reaction rates at two temperatures 10°C apart, and has been widely applied to quantify the temperature sensitivity of organic matter decomposition. However, biogeochemists and ecologists have long recognized that a constant Q10 coefficient does not describe the temperature sensitivity of organic matter decomposition accurately. To examine the consequences of the constant Q10 assumption, we built a biogeochemical reaction model to simulate anaerobic organic matter decomposition in peatlands in the Upper Peninsula of Michigan, USA, and compared the simulation results to the predictions with Q10 coefficients. By accounting for the reactions of extracellular enzymes, mesophilic fermenting and methanogenic microbes, and their temperature responses, the biogeochemical reaction model reproduces the observations of previous laboratory incubation experiments, including the temporal variations in the concentrations of dissolved organic carbon, acetate, dihydrogen, carbon dioxide, and methane, and confirms that fermentation limits the progress of anaerobic organic matter decomposition. The modeling results illustrate the oversimplification inherent in the constant Q10 assumption and how the assumption undermines the kinetic prediction of anaerobic organic matter decomposition. In particular, the model predicts that between 5°C and 30°C, the decomposition rate increases almost linearly with increasing temperature, which stands in sharp contrast to the exponential relationship given by the Q10 coefficient. As a result, the constant Q10 approach tends to underestimate the rates of organic matter decomposition within the temperature ranges where Q10 values are determined, and overestimate the rates outside the temperature ranges. The results also show how biogeochemical reaction modeling, combined with laboratory experiments, can help uncover the temperature sensitivity of organic matter decomposition arising from underlying catalytic mechanisms. 

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2. Soil incubation methods lead to large differences in inferred methane production temperature sensitivity

   https://doi.org/10.1088/1748-9326/ad3565  

   Li, Zhen ; Grant, Robert F. ; Chang, Kuang-Yu ; Hodgkins, Suzanne B. ; Tang, Jinyun ; Cory, Alexandra ; Mekonnen, Zelalem A. ; Saleska, Scott R. ; Brodie, Eoin L. ; Varner, Ruth K. ; et al ( April 2024 , Environmental Research Letters)

   Quantifying the temperature sensitivity of methane (CH₄) production is crucial for predicting how wetland ecosystems will respond to climate warming. Typically, the temperature sensitivity (often quantified as a Q₁₀value) is derived from laboratory incubation studies and then used in biogeochemical models. However, studies report wide variation in incubation-inferred Q₁₀values, with a large portion of this variation remaining unexplained. Here we applied observations in a thawing permafrost peatland (Stordalen Mire) and a well-tested process-rich model (ecosys) to interpret incubation observations and investigate controls on inferred CH₄production temperature sensitivity. We developed a field-storage-incubation modeling approach to mimic the full incubation sequence, including field sampling at a particular time in the growing season, refrigerated storage, and laboratory incubation, followed by model evaluation. We found that CH₄production rates during incubation are regulated by substrate availability and active microbial biomass of key microbial functional groups, which are affected by soil storage duration and temperature. Seasonal variation in substrate availability and active microbial biomass of key microbial functional groups led to strong time-of-sampling impacts on CH₄production. CH₄production is higher with less perturbation post-sampling, i.e. shorter storage duration and lower storage temperature. We found a wide range of inferred Q₁₀values (1.2–3.5), which we attribute to incubation temperatures, incubation duration, storage duration, and sampling time. We also show that Q₁₀values of CH₄production are controlled by interacting biological, biochemical, and physical processes, which cause the inferred Q₁₀values to differ substantially from those of the component processes. Terrestrial ecosystem models that use a constant Q₁₀value to represent temperature responses may therefore predict biased soil carbon cycling under future climate scenarios.

    

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3. Temperature sensitivity of biomass‐specific microbial exo‐enzyme activities and CO 2 efflux is resistant to change across short‐ and long‐term timescales

   https://doi.org/10.1111/gcb.14605  

   Min, Kyungjin ; Buckeridge, Kate ; Ziegler, Susan E. ; Edwards, Kate A. ; Bagchi, Samik ; Billings, Sharon A. ( March 2019 , Global Change Biology)

   Accurate representation of temperature sensitivity (Q₁₀) of soil microbial activity across time is critical for projecting soil CO₂efflux. As microorganisms mediate soil carbon (C) loss via exo‐enzyme activity and respiration, we explore temperature sensitivities of microbial exo‐enzyme activity and respiratory CO₂loss across time and assess mechanisms associated with these potential changes in microbial temperature responses. We collected soils along a latitudinal boreal forest transect with different temperature regimes (long‐term timescale) and exposed these soils to laboratory temperature manipulations at 5, 15, and 25°C for 84 days (short‐term timescale). We quantified temperature sensitivity of microbial activity per g soil and per g microbial biomass at days 9, 34, 55, and 84, and determined bacterial and fungal community structure before the incubation and at days 9 and 84. All biomass‐specific rates exhibited temperature sensitivities resistant to change across short‐ and long‐term timescales (meanQ₁₀ = 2.77 ± 0.25, 2.63 ± 0.26, 1.78 ± 0.26, 2.27 ± 0.25, 3.28 ± 0.44, 2.89 ± 0.55 for β‐glucosidase,N‐acetyl‐β‐d‐glucosaminidase, leucine amino peptidase, acid phosphatase, cellobiohydrolase, and CO₂efflux, respectively). In contrast, temperature sensitivity of soil mass‐specific rates exhibited either resilience (theQ₁₀value changed and returned to the original value over time) or resistance to change. Regardless of the microbial flux responses, bacterial and fungal community structure was susceptible to change with temperature, significantly differing with short‐ and long‐term exposure to different temperature regimes. Our results highlight that temperature responses of microbial resource allocation to exo‐enzyme production and associated respiratory CO₂loss per unit biomass can remain invariant across time, and thus, that vulnerability of soil organic C stocks to rising temperatures may persist in the long term. Furthermore, resistant temperature sensitivities of biomass‐specific rates in spite of different community structures imply decoupling of community constituents and the temperature responses of soil microbial activities.

    

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4. Temperature sensitivities of extracellular enzyme V max and K m across thermal environments

   https://doi.org/10.1111/gcb.14045  

   Allison, Steven D. ; Romero‐Olivares, Adriana L. ; Lu, Ying ; Taylor, John W. ; Treseder, Kathleen K. ( February 2018 , Global Change Biology)

   The magnitude and direction of carbon cycle feedbacks under climate warming remain uncertain due to insufficient knowledge about the temperature sensitivities of soil microbial processes. Enzymatic rates could increase at higher temperatures, but this response could change over time if soil microbes adapt to warming. We used the Arrhenius relationship, biochemical transition state theory, and thermal physiology theory to predict the responses of extracellular enzymeV_maxandK_mto temperature. Based on these concepts, we hypothesized thatV_maxandK_mwould correlate positively with each other and show positive temperature sensitivities. For enzymes from warmer environments, we expected to find lowerV_max,K_m, andK_mtemperature sensitivity but higherV_maxtemperature sensitivity. We tested these hypotheses with isolates of the filamentous fungusNeurospora discretacollected from around the globe and with decomposing leaf litter from a warming experiment in Alaskan boreal forest. ForNeurosporaextracellular enzymes,V_maxQ₁₀ranged from 1.48 to 2.25, andK_mQ₁₀ranged from 0.71 to 2.80. In agreement with theory,V_maxandK_mwere positively correlated for some enzymes, andV_maxdeclined under experimental warming in Alaskan litter. However, the temperature sensitivities ofV_maxandK_mdid not vary as expected with warming. We also found no relationship between temperature sensitivity ofV_maxorK_mand mean annual temperature of the isolation site forNeurosporastrains. DecliningV_maxin the Alaskan warming treatment implies a short‐term negative feedback to climate change, but theNeurosporaresults suggest that climate‐driven changes in plant inputs and soil properties are important controls on enzyme kinetics in the long term. Our empirical data on enzymeV_max,K_m, and temperature sensitivities should be useful for parameterizing existing biogeochemical models, but they reveal a need to develop new theory on thermal adaptation mechanisms.

    

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5. Where old meets new: An ecosystem study of methanogenesis in a reflooded agricultural peatland

   https://doi.org/10.1111/gcb.14916  

   McNicol, Gavin ; Knox, Sara H. ; Guilderson, Thomas P. ; Baldocchi, Dennis D. ; Silver, Whendee L. ( December 2019 , Global Change Biology)

   Reflooding formerly drained peatlands has been proposed as a means to reduce losses of organic matter and sequester soil carbon for climate change mitigation, but a renewal of high methane emissions has been reported for these ecosystems, offsetting mitigation potential. Our ability to interpret observed methane fluxes in reflooded peatlands and make predictions about future flux trends is limited due to a lack of detailed studies of methanogenic processes. In this study we investigate methanogenesis in a reflooded agricultural peatland in the Sacramento Delta, California. We use the stable‐and radio‐carbon isotopic signatures of wetland sediment methane, ecosystem‐scale eddy covariance flux observations, and laboratory incubation experiments, to identify which carbon sources and methanogenic production pathways fuel methanogenesis and how these processes are affected by vegetation and seasonality. We found that the old peat contribution to annual methane emissions was large (~30%) compared to intact wetlands, indicating a biogeochemical legacy of drainage. However, fresh carbon and the acetoclastic pathway still accounted for the majority of methanogenesis throughout the year. Although temperature sensitivities for bulk peat methanogenesis were similar between open‐water (Q₁₀ = 2.1) and vegetated (Q₁₀ = 2.3) soils, methane production from both fresh and old carbon sources showed pronounced seasonality in vegetated zones. We conclude that high methane emissions in restored wetlands constitute a biogeochemical trade‐off with contemporary carbon uptake, given that methane efflux is fueled primarily by fresh carbon inputs.

    

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