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1. 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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2. More Snow Accelerates Legacy Carbon Emissions From Arctic Permafrost

   https://doi.org/10.1029/2023AV000942  

   Pedron, S. A. ; Jespersen, R. G. ; Xu, X. ; Khazindar, Y. ; Welker, J. M. ; Czimczik, C. I. ( August 2023 , AGU Advances)

   Snow is critically important to the energy budget, biogeochemistry, ecology, and people of the Arctic. While climate change continues to shorten the duration of the snow cover period, snow mass (the depth of the snow pack) has been increasing in many parts of the Arctic. Previous work has shown that deeper snow can rapidly thaw permafrost and expose the large amounts of ancient (legacy) organic matter contained within it to microbial decomposition. This process releases carbonaceous greenhouse gases but also nutrients, which promote plant growth and carbon sequestration. The net effect of increased snow depth on greenhouse gas emissions from Arctic ecosystems remains uncertain. Here we show that 25 years of snow addition turned tussock tundra, one of the most spatially extensive Arctic ecosystems, into a year‐round source of ancient carbon dioxide. More snow quadrupled the amount of organic matter available to microbial decomposition, much of it previously preserved in permafrost, due to deeper seasonal thaw, soil compaction and subsidence as well as the proliferation of deciduous shrubs that lead to 10% greater carbon uptake during the growing season. However, more snow also sustained warmer soil temperatures, causing greater carbon loss during winter (+200% from October to May) and year‐round. We find that increasing snow mass will accelerate the ongoing transformation of Arctic ecosystems and cause earlier‐than‐expected losses of climate‐warming legacy carbon from permafrost.

    

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3. Belowground impacts of alpine woody encroachment are determined by plant traits, local climate, and soil conditions

   https://doi.org/10.1111/gcb.15340  

   Collins, Courtney G. ; Spasojevic, Marko J. ; Alados, Concepción L. ; Aronson, Emma L. ; Benavides, Juan C. ; Cannone, Nicoletta ; Caviezel, Chatrina ; Grau, Oriol ; Guo, Hui ; Kudo, Gaku ; et al ( October 2020 , Global Change Biology)

   Global climate and land use change are causing woody plant encroachment in arctic, alpine, and arid/semi‐arid ecosystems around the world, yet our understanding of the belowground impacts of this phenomenon is limited. We conducted a globally distributed field study of 13 alpine sites across four continents undergoing woody plant encroachment and sampled soils from both woody encroached and nearby herbaceous plant community types. We found that woody plant encroachment influenced soil microbial richness and community composition across sites based on multiple factors including woody plant traits, site level climate, and abiotic soil conditions. In particular, root symbiont type was a key determinant of belowground effects, as Nitrogen‐fixing woody plants had higher soil fungal richness, while Ecto/Ericoid mycorrhizal species had higher soil bacterial richness and symbiont types had distinct soil microbial community composition. Woody plant leaf traits indirectly influenced soil microbes through their impact on soil abiotic conditions, primarily soil pH and C:N ratios. Finally, site‐level climate affected the overall magnitude and direction of woody plant influence, as soil fungal and bacterial richness were either higher or lower in woody encroached versus herbaceous soils depending on mean annual temperature and precipitation. All together, these results document global impacts of woody plant encroachment on soil microbial communities, but highlight that multiple biotic and abiotic pathways must be considered to scale up globally from site‐ and species‐level patterns. Considering both the aboveground and belowground effects of woody encroachment will be critical to predict future changes in alpine ecosystem structure and function and subsequent feedbacks to the global climate system.

    

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4. Decomposing decomposition: isolating direct effects of temperature from other drivers of detrital processing

   https://doi.org/10.1002/ecy.3467  

   Wilmot, Oliver J. ; Hood, James M. ; Huryn, Alexander D. ; Benstead, Jonathan P. ( August 2021 , Ecology)

   Understanding the observed temperature dependence of decomposition (i.e., its apparent activation energy) requires separation of direct effects of temperature on consumer metabolism (i.e., the inherent activation energy) from those driven by indirect seasonal patterns in phenology and biomass, and by longer‐term, climate‐driven shifts in acclimation, adaptation, and community assembly. Such parsing is important because studies that relate temperature to decomposition usually involve multi‐season data and/or spatial proxies for long‐term shifts, and so incorporate these indirect factors. The various effects of such factors can obscure the inherent temperature dependence of detrital processing. Separating the inherent temperature dependence of decomposition from other drivers is important for accurate prediction of the contribution of detritus‐sourced greenhouse gases to climate warming and requires novel approaches to data collection and analysis. Here, we present breakdown rates of red maple litter incubated in coarse‐ and fine‐mesh litterbags (the latter excluding macroinvertebrates) for serial approximately one‐month increments over one year in nine streams along a natural temperature gradient (mean annual: 12.8°–16.4°C) from north Georgia to central Alabama, USA. We analyzed these data using distance‐based redundancy analysis and generalized additive mixed models to parse the dependence of decomposition rates on temperature, seasonality, and shredding macroinvertebrate biomass. Microbial decomposition in fine‐mesh bags was significantly influenced by both temperature and seasonality. Accounting for seasonality corrected the temperature dependence of decomposition rate from 0.25 to 0.08 eV. Shredder assemblage structure in coarse‐mesh bags was related to temperature across both sites and seasons, shifting from “cold” stonefly‐dominated communities to “warm” communities dominated by snails or crayfish. Shredder biomass was not a significant predictor of either coarse‐mesh or macroinvertebrate‐mediated (i.e., coarse‐ minus fine‐mesh) breakdown rates, which were also jointly influenced by temperature and seasonality. Unlike fine‐mesh bags, however, temperature dependence of litter breakdown did not differ between models with and without seasonality for either coarse‐mesh (0.36 eV) or macroinvertebrate‐mediated (0.13 eV) rates. We conclude that indirect (non‐thermal) seasonal and site‐level effects play a variable and potentially strong role in shaping the apparent temperature dependence of detrital breakdown. Such effects should be incorporated into studies designed to estimate inherent temperature dependence of slow ecological processes.

    

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5. Belowground community turnover accelerates the decomposition of standing dead wood

   https://doi.org/10.1002/ecy.3484  

   Bradford, Mark A. ; Maynard, Daniel S. ; Crowther, Thomas W. ; Frankson, Paul T. ; Mohan, Jacqueline E. ; Steinrueck, Corinna ; Veen, G. F. ; King, Joshua R. ; Warren, II, Robert J. ( October 2021 , Ecology)

   Standing dead trees (snags) decompose more slowly than downed dead wood and provide critical habitat for many species. The rate at which snags fall therefore influences forest carbon dynamics and biodiversity. Fall rates correlate strongly with mean annual temperature, presumably because warmer climates facilitate faster wood decomposition and hence degradation of the structural stability of standing wood. These faster decomposition rates coincide with turnover from fungal‐dominated wood decomposer communities in cooler forests to codomination by fungi and termites in warmer regions. A key question for projecting forest dynamics is therefore whether temperature effects on wood decomposition arise primarily because warmer conditions facilitate faster decomposer metabolism, or are also influenced indirectly by belowground community turnover (e.g., termites exert additional influence beyond fungal‐plus‐bacterial mediated decomposition). To test between these possibilities, we simulate standing dead trees with untreated wooden posts and follow them in the field across 5 yr at 12 sites, before measuring buried, soil–air interface and aerial post sections to quantify wood decomposition and organism activities. High termite activities at the warmer sites are associated with rates of postfall that are three times higher than at the cooler sites. Termites primarily consume buried wood, with decomposition rates greatest where termite activities are highest. However, where higher microbial and termite activities co‐occur, they appear to compensate for one another first, and then to slow decomposition rates at their highest activities, suggestive of interference competition. If the range of microbial and termite codomination of wood decomposer communities expands under climate warming, our data suggest that expansion will accelerate snag fall with consequent effects on forest carbon cycling and biodiversity in forests previously dominated by microbial decomposers.

    

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