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1. A decade of boreal rich fen greenhouse gas fluxes in response to natural and experimental water table variability

   https://doi.org/10.1111/gcb.13612  

   Olefeldt, David ; Euskirchen, Eugénie S. ; Harden, Jennifer ; Kane, Evan ; McGuire, A. David ; Waldrop, Mark P. ; Turetsky, Merritt R. ( January 2017 , Global Change Biology)

   Rich fens are common boreal ecosystems with distinct hydrology, biogeochemistry and ecology that influence their carbon (C) balance. We present growing season soil chamber methane emission (F_CH4), ecosystem respiration (ER), net ecosystem exchange (NEE) and gross primary production (GPP) fluxes from a 9‐years water table manipulation experiment in an Alaskan rich fen. The study included major flood and drought years, where wetting and drying treatments further modified the severity of droughts. Results support previous findings from peatlands that drought causes reduced magnitude of growing seasonF_CH4,GPPandNEE, thus reducing or reversing their C sink function. Experimentally exacerbated droughts further reduced the capacity for the fen to act as a C sink by causing shifts in vegetation and thus reducing magnitude of maximum growing seasonGPPin subsequent flood years by ~15% compared to control plots. Conversely, water table position had only a weak influence onER, but dominant contribution toERswitched from autotrophic respiration in wet years to heterotrophic in dry years. Droughts did not cause inter‐annual lag effects onERin this rich fen, as has been observed in several nutrient‐poor peatlands. WhileERwas dependent on soil temperatures at 2 cm depth,F_CH4was linked to soil temperatures at 25 cm. Inter‐annual variability of deep soil temperatures was in turn dependent on wetness rather than air temperature, and higherF_CH4in flooded years was thus equally due to increased methane production at depth and decreased methane oxidation near the surface. Short‐term fluctuations in wetness caused significant lag effects onF_CH4, but droughts caused no inter‐annual lag effects onF_CH4. Our results show that frequency and severity of droughts and floods can have characteristic effects on the exchange of greenhouse gases, and emphasize the need to project future hydrological regimes in rich fens.

    

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2. Evidence for older carbon loss with lowered water tables and changing plant functional groups in peatlands

   https://doi.org/10.1111/gcb.16508  

   Stuart, Julia E. M. ; Tucker, Colin L. ; Lilleskov, Erik A. ; Kolka, Randall K. ; Chimner, Rodney A. ; Heckman, Katherine A. ; Kane, Evan S. ( November 2022 , Global Change Biology)

   A small imbalance in plant productivity and decomposition accounts for the carbon (C) accumulation capacity of peatlands. As climate changes, the continuity of peatland net C storage relies on rising primary production to offset increasing ecosystem respiration (ER) along with the persistence of older C in waterlogged peat. A lowering in the water table position in peatlands often increases decomposition rates, but concurrent plant community shifts can interactively alter ER and plant productivity responses. The combined effects of water table variation and plant communities on older peat C loss are unknown. We used a full‐factorial 1‐m³mesocosm array with vascular plant functional group manipulations (Unmanipulated Control, Sedge only, and Ericaceous only) and water table depth (natural and lowered) treatments to test the effects of plants and water depth on CO₂fluxes, decomposition, and older C loss. We used Δ¹⁴C and δ¹³C of ecosystem CO₂respiration, bulk peat, plants, and porewater dissolved inorganic C to construct mixing models partitioning ER among potential sources. We found that the lowered water table treatments were respiring C fixed before the bomb spike (1955) from deep waterlogged peat. Lowered water table Sedge treatments had the oldest dissolved inorganic¹⁴C signature and the highest proportional peat contribution to ER. Decomposition assays corroborated sustained high rates of decomposition with lowered water tables down to 40 cm below the peat surface. Heterotrophic respiration exceeded plant respiration at the height of the growing season in lowered water table treatments. Rates of gross primary production were only impacted by vegetation, whereas ER was affected by vegetation and water table depth treatments. The decoupling of respiration and primary production with lowered water tables combined with older C losses suggests that climate and land‐use‐induced changes in peatland hydrology can increase the vulnerability of peatland C stores.

    

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3. Fresh Air for the Mire-Breathing Hypothesis: Sphagnum Moss and Peat Structure Regulate the Response of CO2 Exchange to Altered Hydrology in a Northern Peatland Ecosystem

   https://doi.org/10.3390/w14203239  

   O’Neill, Ally ; Tucker, Colin ; Kane, Evan S. ( October 2022 , Water)

   Sphagnum-dominated peatlands store more carbon than all of Earth’s forests, playing a large role in the balance of carbon dioxide. However, these carbon sinks face an uncertain future as the changing climate is likely to cause water stress, potentially reducing Sphagnum productivity and transitioning peatlands to carbon sources. A mesocosm experiment was performed on thirty-two peat cores collected from two peatland landforms: elevated mounds (hummocks) and lower, flat areas of the peatland (hollows). Both rainfall treatments and water tables were manipulated, and CO2 fluxes were measured. Other studies have observed peat subsiding and tracking the water table downward when experiencing water stress, thought to be a self-preservation technique termed ‘Mire-breathing’. However, we found that hummocks tended to compress inwards, rather than subsiding towards the lowered water table as significantly as hollows. Lower peat height was linearly associated with reduced gross primary production (GPP) in response to lowered water tables, indicating that peat subsidence did not significantly enhance the resistance of GPP to drought. Conversely, Sphagnum peat compression was found to stabilize GPP, indicating that this mechanism of resilience to drought may transmit across the landscape depending on which Sphagnum landform types are dominant. This study draws direct connections between Sphagnum traits and peatland hydrology and carbon cycling. 

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4. CO 2 emissions from an undrained tropical peatland: Interacting influences of temperature, shading and water table depth

   https://doi.org/10.1111/gcb.14702  

   Hoyt, Alison M. ; Gandois, Laure ; Eri, Jangarun ; Kai, Fuu Ming ; Harvey, Charles F. ; Cobb, Alexander R. ( July 2019 , Global Change Biology)

   Emission of CO₂from tropical peatlands is an important component of the global carbon budget. Over days to months, these fluxes are largely controlled by water table depth. However, the diurnal cycle is less well understood, in part, because most measurements have been collected daily at midday. We used an automated chamber system to make hourly measurements of peat surface CO₂emissions from chambers root‐cut to 30 cm. We then used these data to disentangle the relationship between temperature, water table and heterotrophic respiration (R_het). We made two central observations. First, we found strong diurnal cycles in CO₂flux and near‐surface peat temperature (<10 cm depth), both peaking at midday. The magnitude of diurnal oscillations was strongly influenced by shading and water table depth, highlighting the limitations of relying on daytime measurements and/or a single correction factor to remove daytime bias in flux measurements. Second, we found mean daily R_hethad a strong linear relationship to the depth of the water table, and under flooded conditions, R_hetwas small and constant. We used this relationship between R_hetand water table depth to estimate carbon export from both R_hetand dissolved organic carbon over the course of a year based on water table records. R_hetdominates annual carbon export, demonstrating the potential for peatland drainage to increase regional CO₂emissions. Finally, we discuss an apparent incompatibility between hourly and daily average observations of CO₂flux, water table and temperature: water table and daily average flux data suggest that CO₂is produced across the entire unsaturated peat profile, whereas temperature and hourly flux data appear to suggest that CO₂fluxes are controlled by very near surface peat. We explore how temperature‐, moisture‐ and gas transport‐related mechanisms could cause mean CO₂emissions to increase linearly with water table depth and also have a large diurnal cycle.

    

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5. Peatland microbial community responses to plant functional group and drought are depth‐dependent

   https://doi.org/10.1111/mec.16125  

   Lamit, Louis J. ; Romanowicz, Karl J. ; Potvin, Lynette R. ; Lennon, Jay T. ; Tringe, Susannah G. ; Chimner, Rodney A. ; Kolka, Randall K. ; Kane, Evan S. ; Lilleskov, Erik A. ( September 2021 , Molecular Ecology)

   Peatlands store one‐third of Earth's soil carbon, the stability of which is uncertain due to climate change‐driven shifts in hydrology and vegetation, and consequent impacts on microbial communities that mediate decomposition. Peatland carbon cycling varies over steep physicochemical gradients characterizing vertical peat profiles. However, it is unclear how drought‐mediated changes in plant functional groups (PFGs) and water table (WT) levels affect microbial communities at different depths. We combined a multiyear mesocosm experiment with community sequencing across a 70‐cm depth gradient, to test the hypotheses that vascular PFGs (Ericaceae vs. sedges) and WT (high vs. low) structure peatland microbial communities in depth‐dependent ways. Several key results emerged. (i) Both fungal and prokaryote (bacteria and archaea) community structure shifted with WT and PFG manipulation, but fungi were much more sensitive to PFG whereas prokaryotes were much more sensitive to WT. (ii) PFG effects were largely driven by Ericaceae, although sedge effects were evident in specific cases (e.g., methanotrophs). (iii) Treatment effects varied with depth: the influence of PFG was strongest in shallow peat (0–10, 10–20 cm), whereas WT effects were strongest at the surface and middle depths (0–10, 30–40 cm), and all treatment effects waned in the deepest peat (60–70 cm). Our results underline the depth‐dependent and taxon‐specific ways that plant communities and hydrologic variability shape peatland microbial communities, pointing to the importance of understanding how these factors integrate across soil profiles when examining peatland responses to climate change.

    

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