While a stimulating effect of plant primary productivity on soil carbon dioxide (CO2) emissions has been well documented, links between gross primary productivity (GPP) and wetland methane (CH4) ...emissions are less well investigated. Determination of the influence of primary productivity on wetland CH4 emissions (FCH4) is complicated by confounding influences of water table level and temperature on CH4 production, which also vary seasonally. Here, we evaluate the link between preceding GPP and subsequent FCH4 at two fens in Wisconsin using eddy covariance flux towers, Lost Creek (US‐Los) and Allequash Creek (US‐ALQ). Both wetlands are mosaics of forested and shrub wetlands, with US‐Los being larger in scale and having a more open canopy. Co‐located sites with multi‐year observations of flux, hydrology, and meteorology provide an opportunity to measure and compare lag effects on FCH4 without interference due to differing climate. Daily average FCH4 from US‐Los reached a maximum of 47.7 ηmol CH4 m−2 s−1 during the study period, while US‐ALQ was more than double at 117.9 ηmol CH4 m−2 s−1. The lagged influence of GPP on temperature‐normalized FCH4 (Tair‐FCH4) was weaker and more delayed in a year with anomalously high precipitation than a following drier year at both sites. FCH4 at US‐ALQ was lower coincident with higher stream discharge in the wet year (2019), potentially due to soil gas flushing during high precipitation events and lower water temperatures. Better understanding of the lagged influence of GPP on FCH4 due to this study has implications for climate modeling and more accurate carbon budgeting.
Plain Language Summary
Research on what controls wetland methane emissions is continually advancing, and while this is beneficial for predicting future climate scenarios, there is still a need to understand how changes in plant productivity will influence wetland methane emissions. In this study, we investigated the strength and lag time of the relationship between gross primary productivity due to photosynthesizing plants and wetland methane flux in two closely situated sites. We also looked at how hydrology might change that relationship. We found the total amount of methane emitted in an extremely wet year was less than what was emitted in the following drier year at both wetlands potentially because of less carbon provided to the soil by photosynthesizing plants. The difference in methane emissions from one year to the next could be influenced by wetland hydrology, water temperature, or other conditions that impact methane‐producing bacteria. Results from this study will help scientists better predict methane emissions following high precipitation years which may become more common in a changing climate.
Key Points
Analyzed lagged response of methane flux to different driver variables at two closely located fen wetlands in Wisconsin
Air‐temperature normalization of methane flux was crucial for interpretation of lagged responses, especially in wet year
Lagged response of methane flux to gross primary productivity surpassed 60 days and had weaker correlation during wet year at both sites
Trough ponds are ubiquitous features of Arctic landscapes and an important component of freshwater aquatic ecosystems. Permafrost thaw causes ground subsidence, creating depressions that gather ...water, creating ponds. Permafrost thaw also releases solutes and nutrients, which may fertilize these newly formed ponds. We measured water budget elements and chloride, ammonium, and dissolved organic nitrogen (DON) across a chronosequence of trough ponds representing different stages of ice wedge degradation and stabilization. We developed a coupled hydrologic and biogeochemical model to explore how ice wedge degradation affects hydrology and nutrient availability in trough ponds in the advanced degradation stages (DAs), which are characterized by deep troughs with warmer temperatures relative to the other stages. DAs experienced greater evaporation than the other stages, and subsurface inflows entered the DAs from a wide area. Chloride accumulated in the ponds with time since thaw, implying that subsurface fluxes are delivering solutes from the thawing permafrost. Ammonium accumulated at high rates in the initial degradation stage and was seasonally depleted over the summer in all degradation stages. Ammonium trends in the DAs were consistent with high concentration inflows and in‐pond assimilation at rates between 0.37 and 2.0 mg N m−2 day−1. Seasonal DON trends indicated that the accumulation of recalcitrant organic matter may eventually limit aquatic ecosystem production and foster pond infilling. These results provide direct evidence of nutrient release from thawing permafrost and the utilization of these nutrients by Arctic trough pond ecosystems and highlight infilling as a mechanism by which Arctic surface waters may be lost.
Key Points
Ice wedge degradation promotes pond formation and releases solutes and nutrients
Subsurface flow delivers water, solutes, and nutrients to deep trough ponds at advanced stages of ice wedge degradation
Ammonium rapidly decreases in deep trough ponds due to biogeochemical cycling
Carbon dioxide (CO2) and methane (CH4) exchange between the atmosphere and a subalpine wetland located in Rocky Mountain National Park, Colorado, at 3200 m elevation were measured during 1996–1998. ...Respiration, net CO2 flux, and CH4 flux were measured using the closed chamber method during snow‐free periods and using gas diffusion calculations during snow‐covered periods. The ranges of measured flux were 1.2‐526 mmol CO2 m−2 d−1 (respiration), −1056−100 mmol CO2m−2 d−1 (net CO2exchange), and 0.1–36.8 mmol CH4m−2 d−1 (a positive value represents efflux to the atmosphere). Respiration and CH4 emission were significantly correlated with 5 cm soil temperature. Annual respiration and CH4 emission were modeled by applying the flux‐temperature relationships to a continuous soil temperature record during 1996–1998. Gross photosynthesis was modeled using a hyperbolic equation relating gross photosynthesis, photon flux density, and soil temperature. Modeled annual flux estimates indicate that the wetland was a net source of carbon gas to the atmosphere each of the three years: 8.9 mol C m−2 yr−1 in 1996, 9.5 mol C m−2 yr−1 in 1997, and 9.6 mol C m−2 yr−1 in 1998. This contrasts with the long‐term carbon accumulation of ∼0.7 mol m−2 yr−1 determined from 14C analyses of a peat core collected from the wetland.
Fluxes of CO2 and CH4 through a seasonal snowpack were measured in and adjacent to a subalpine wetland in Rocky Mountain National Park, Colorado. Gas diffusion through the snow was controlled by gas ...production or consumption in the soil and by physical snowpack properties. The snowpack insulated soils from cold midwinter air temperatures allowing microbial activity to continue through the winter. All soil types studied were net sources of CO2 to the atmosphere through the winter, whereas saturated soils in the wetland center were net emitters of CH4 and soils adjacent to the wetland were net CH4 consumers. Most sites showed similar temporal patterns in winter gas fluxes; the lowest fluxes occurred in early winter, and maximum fluxes occurred at the onset of snowmelt. Temporal changes in fluxes probably were related to changes in soil‐moisture conditions and hydrology because soil temperatures were relatively constant under the snowpack. Average winter CO2 fluxes were 42.3, 31.2, and 14.6 mmol m−2 d−1 over dry, moist, and saturated soils, respectively, which accounted for 8 to 23% of the gross annual CO2 emissions from these soils. Average winter CH4 fluxes were −0.016, 0.274, and 2.87 mmol m−2 d−1 over dry, moist, and saturated soils, respectively. Microbial activity under snow cover accounted for 12% of the annual CH4 consumption in dry soils and 58 and 12% of the annual CH4 emitted from moist and saturated soils, respectively. The observed ranges in CO2 and CH4 flux through snow indicated that winter fluxes are an important part of the annual carbon budget in seasonally snow‐covered terrains.
Abstract
While a stimulating effect of plant primary productivity on soil carbon dioxide (CO
2
) emissions has been well documented, links between gross primary productivity (GPP) and wetland methane ...(CH
4
) emissions are less well investigated. Determination of the influence of primary productivity on wetland CH
4
emissions (FCH
4
) is complicated by confounding influences of water table level and temperature on CH
4
production, which also vary seasonally. Here, we evaluate the link between preceding GPP and subsequent FCH
4
at two fens in Wisconsin using eddy covariance flux towers, Lost Creek (US‐Los) and Allequash Creek (US‐ALQ). Both wetlands are mosaics of forested and shrub wetlands, with US‐Los being larger in scale and having a more open canopy. Co‐located sites with multi‐year observations of flux, hydrology, and meteorology provide an opportunity to measure and compare lag effects on FCH
4
without interference due to differing climate. Daily average FCH
4
from US‐Los reached a maximum of 47.7 ηmol CH
4
m
−2
s
−1
during the study period, while US‐ALQ was more than double at 117.9 ηmol CH
4
m
−2
s
−1
. The lagged influence of GPP on temperature‐normalized FCH
4
(
T
air
‐FCH
4
) was weaker and more delayed in a year with anomalously high precipitation than a following drier year at both sites. FCH
4
at US‐ALQ was lower coincident with higher stream discharge in the wet year (2019), potentially due to soil gas flushing during high precipitation events and lower water temperatures. Better understanding of the lagged influence of GPP on FCH
4
due to this study has implications for climate modeling and more accurate carbon budgeting.
Plain Language Summary
Research on what controls wetland methane emissions is continually advancing, and while this is beneficial for predicting future climate scenarios, there is still a need to understand how changes in plant productivity will influence wetland methane emissions. In this study, we investigated the strength and lag time of the relationship between gross primary productivity due to photosynthesizing plants and wetland methane flux in two closely situated sites. We also looked at how hydrology might change that relationship. We found the total amount of methane emitted in an extremely wet year was less than what was emitted in the following drier year at both wetlands potentially because of less carbon provided to the soil by photosynthesizing plants. The difference in methane emissions from one year to the next could be influenced by wetland hydrology, water temperature, or other conditions that impact methane‐producing bacteria. Results from this study will help scientists better predict methane emissions following high precipitation years which may become more common in a changing climate.
Key Points
Analyzed lagged response of methane flux to different driver variables at two closely located fen wetlands in Wisconsin
Air‐temperature normalization of methane flux was crucial for interpretation of lagged responses, especially in wet year
Lagged response of methane flux to gross primary productivity surpassed 60 days and had weaker correlation during wet year at both sites
Methane exchange between the atmosphere and subalpine wetland and unsaturated soils was evaluated over a 15‐month period during 1995–1996. Four vegetation community types along a moisture gradient ...(wetland, moist‐grassy, moist‐mossy, and dry) were included in a 100 m sampling transect situated at 3200 m elevation in Rocky Mountain National Park, Colorado. Methane fluxes and soil temperature were measured during snow‐free and snow‐covered periods, and soil moisture content was measured during snow‐free periods. The range of mean measured fluxes through all seasons (a positive value represents CH4 efflux to the atmosphere) were: 0.3 to 29.2 mmol CH4 m−2 d−1 wetland area; 0.1 to 1.8 mmol CH4 m−2 d−1, moist‐grassy area; −0.04 to 0.7 mmol CH4 m−2 d−1, moist‐mossy area; and −0.6 to 0 mmol CH4m−2 d−1, dry area. Methane efflux was significantly correlated with soil temperature (5 cm) at the continuously saturated wetland area during snow‐free periods. Consumption of atmospheric methane was significantly correlated with moisture content in the upper 5 cm of soil at the dry area. A model based on the wetland flux‐temperature relationship estimated an annual methane emission of 2.53 mol CH4 m−2 from the wetland. Estimates of annual methane flux based on field measurements at the other sites were 0.12 mol CH4 m−2, moist‐grassy area; 0.03 mol CH4 m−2, moist‐mossy area; and −0.04 mol CH4 m−2, dry area. Methane fluxes during snow‐covered periods were responsible for 25, 73, 23, and 43% of the annual fluxes at the wetland, moist‐grassy, moist‐mossy, and dry sites, respectively.
Permafrost soils are large reservoirs of potentially labile carbon (C). Understanding the dynamics of C release from these soils requires us to account for the impact of wildfires, which are ...increasing in frequency as the climate changes. Boreal wildfires contribute to global emission of greenhouse gases (GHG-CO2, CH4 and N2O) and indirectly result in the thawing of near-surface permafrost. In this study, we aimed to define the impact of fire on soil microbial communities and metabolic potential for GHG fluxes in samples collected up to 1 m depth from an upland black spruce forest near Nome Creek, Alaska. We measured geochemistry, GHG fluxes, potential soil enzyme activities and microbial community structure via 16SrRNA gene and metagenome sequencing. We found that soil moisture, C content and the potential for respiration were reduced by fire, as were microbial community diversity and metabolic potential. There were shifts in dominance of several microbial community members, including a higher abundance of candidate phylum AD3 after fire. The metagenome data showed that fire had a pervasive impact on genes involved in carbohydrate metabolism, methanogenesis and the nitrogen cycle. Although fire resulted in an immediate release of CO2 from surface soils, our results suggest that the potential for emission of GHG was ultimately reduced at all soil depths over the longer term. Because of the size of the permafrost C reservoir, these results are crucial for understanding whether fire produces a positive or negative feedback loop contributing to the global C cycle.
Quantification of the components of ecosystem respiration is essential to understanding carbon (C) cycling of natural and disturbed landscapes. Soil respiration, which includes autotrophic and ...heterotrophic respiration from throughout the soil profile, is the second largest flux in the global carbon cycle. We measured soil respiration (soil CO2 emission) at an undisturbed mature jack pine (Pinus banksiana Lamb.) stand in Saskatchewan (old jack pine, OJP), and at a formerly continuous portion of the stand that was clear-cut during the previous winter (clear-cut, CC). Tree harvesting reduced soil CO2 emission from approximately 22.5 to approximately 9.1 mol CO2.m-2 for the 1994 growing season. OJP was a small net sink of atmospheric CO2, while CC was a net source of CO2. Winter emissions were similar at both sites. Reduction of soil respiration was attributed to disruption of the soil surface and to the death of tree roots. Flux simulations for CC and OJP identify 40% of CO2 emission at the undisturbed OJP site as near-surface respiration, 25% as deep-soil respiration, and 35% as tree-root respiration. The near-surface component was larger than the estimated annual C input to soil, suggesting fast C turnover and no net C accumulation in these boreal uplands in 1994.
Quantification of the components of ecosystem respiration is essential to understanding carbon (C) cycling of natural and disturbed landscapes. Soil respiration, which includes autotrophic and ...heterotrophic respiration from throughout the soil profile, is the second largest flux in the global carbon cycle. We measured soil respiration (soil CO sub(2) emission) at an undisturbed mature jack pine (Pinus banksiana Lamb.) stand in Saskatchewan (old jack pine, OJP), and at a formerly continuous portion of the stand that was clear-cut during the previous winter (clear-cut, CC). Tree harvesting reduced soil CO sub(2) emission from similar to 22.5 to similar to 9.1 mol CO sub(2) times m super(-2) for the 1994 growing season. OJP was a small net sink of atmospheric CO sub(2), while CC was a net source of CO sub(2). Winter emissions were similar at both sites. Reduction of soil respiration was attributed to disruption of the soil surface and to the death of tree roots. Flux simulations for CC and OJP identify 40% of CO sub(2) emission at the undisturbed OJP site as near-surface respiration, 25% as deep-soil respiration, and 35% as tree-root respiration. The near-surface component was larger than the estimated annual C input to soil, suggesting fast C turnover and no net C accumulation in these boreal uplands in 1994.
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