Carbon Cycling In Wetlands Research Articles

Impacts of fire and flooding on sediment carbon storage in a large, forested floodplain

Summary Natural disturbances influence wetland carbon cycling, and fire is a key driver of terrestrial carbon stocks. However, the influence of fire on wetland carbon cycling remains poorly understood. Here, we investigated how prescribed fire and wildfire impact soil carbon storage in a forested floodplain of south-eastern Australia. We sampled four areas within Murray Valley National Park, the world’s largest river red gum (Eucalyptus camaldulensis) stand, and compared soil carbon (C), nitrogen (N) and C:N ratios between control (unburnt in the 50 years prior to sampling), prescribed burn and wildfire-impacted floodplain areas. Mean soil C and N concentrations were 4.7% ± 0.32% and 0.36% ± 0.02%, respectively, and mean C:N ratios were 14.23 ± 0.33. Carbon concentrations and C:N were highest in control areas of the floodplain, while N concentrations were highest at wildfire-impacted areas. However, flood frequency was a stronger driver of soil C than fire disturbance. Soils at more frequently flooded areas had higher C concentrations compared to less frequently flooded areas, suggesting that resilience to C loss through fire could be enhanced through hydrological restoration. We believe this warrants further research as a potential nature-based climate measure. Mean C density data indicate soil C stocks of 9.4 Tg across Barmah-Millewa Forest, highlighting the significant carbon storage value of this ecosystem.

Carbon Stocks and Fluxes From a Boreal Conifer Swamp: Filling a Knowledge Gap for Understanding the Boreal C Cycle

AbstractThe carbon (C) dynamics of boreal coniferous swamps are a largely understudied component of wetland carbon cycling. We investigated the above‐ and below‐ground carbon stocks and growing season carbon dioxide (CO2) and methane (CH4) fluxes from a representative boreal coniferous swamp in northern Alberta, Canada in 2022. Tree inventories, understory vegetation biomass and peat cores were collected across three sub‐sites within the broader swamp, with gas flux collars placed in the dominant plant communities present. Alongside the C flux measurements, environmental variables such as water table depth, soil temperature and growing season understory green leaf phenology were measured. Our results show that these boreal coniferous swamps store large volumes of organic C in their biomass and soil (134 kg C m−2), comparable with other wetland and forest types, although 95% of the total C stock at our site was within the soil organic carbon. We also found that understory CO2 and CH4 fluxes indicated that the ground layer of the site is a source of greenhouse gases (GHGs) to the atmosphere across the growing season. However, we did not measure litterfall input, tree GHG fluxes or net primary productivity of the overstory, therefore we are not able to say whether the site is an overall source of C to the atmosphere. This study provides a much‐needed insight into the C dynamics of these under‐valued wetland ecosystems, and we highlight the need for a coordinated effort across boreal regions to try to improve inventories of C stocks and fluxes.

Changes in inundation drive carbon dioxide and methane fluxes in a temperate wetland

Wetlands cycle carbon by being net sinks for carbon dioxide (CO2) and net sources of methane (CH4). Daily and seasonal temporal patterns, dissolved oxygen (DO) availability, inundation status (flooded or dry/partially flooded), water depth, and vegetation can affect the magnitude of carbon uptake or emissions, but the extent and interactive effects of these variables on carbon gas fluxes are poorly understood. We characterized the linkages between carbon fluxes and these environmental and temporal drivers at the Old Woman Creek National Estuarine Research Reserve (OWC), OH. We measured diurnal gas flux patterns in an upstream side channel (called the cove) using chamber measurements at six sites (three vegetated and three non-vegetated). We sampled hourly from 7 AM to 7 PM and monthly from July to October 2022. DO concentrations and water levels were measured monthly. Water inundation status had the most influential effect on carbon fluxes with flooded conditions supporting higher CH4 fluxes (0.39 μmol CH4 m−2 s−1; −1.23 μmol CO2 m−2 s−1) and drier conditions supporting higher CO2 fluxes (0.03 μmol CH4 m−2 s−1; 0.86 μmol CO2 m−2 s−1). When flooded, the wetland was a net CO2 sink; however, it became a source for both CH4 and CO2 when water levels were low. We compared chamber-based gas fluxes from the cove in flooded (July) and dry (August) months to fluxes measured with an eddy covariance tower whose footprint covers flooded portions of the wetland. The diurnal pattern of carbon fluxes at the tower did not vary with changing water levels but remained a CO2 sink and a CH4 source even when the cove where we performed the chamber measurements dried out. These results emphasize the role of inundation status on wetland carbon cycling and highlight the importance of fluctuating hydrologic patterns, especially hydrologic drawdowns, under changing climatic conditions.

Wetlands harbor lactic acid-driven chain elongators.

Wetlands are globally significant carbon cycling hotspots that both sequester large amounts of CO2 as soil carbon as well as emit a third of all CH4 globally. Their outsized role in the global carbon cycle makes it critical to understand microbial processes contributing to carbon breakdown and storage in these ecosystems. Here, we confirm the presence of chain-elongating organisms in freshwater wetland soils. These organisms take small carbon compounds formed during the breakdown of biomass and turn them into larger compounds (six to eight carbon organic acids) that may potentially contribute to the formation of soil organic matter and long-term carbon storage. Moreover, we find that these chain-elongating organisms may be widely distributed in wetlands globally. Future work should identify these organisms' contribution to carbon cycling in wetlands and the potential role of the products they form in carbon sequestration in wetlands.

Restored forested wetland surprisingly resistant to experimental salinization.

Salinization of coastal freshwater wetlands is an increasingly common and widespread phenomenon resulting from climate change. The ecosystem consequences of added salinity are poorly constrained and highly variable across prior observational and experimental studies. We added 1.8 metric tons of marine salts to replicated 200 m2 plots within a restored forested wetland in Eastern North Carolina over the course of four years. Based on prior small-scale experiments at this site, we predicted that salinization would lead to slower tree growth and suppressed soil carbon cycling. Results from this large-scale field experiment were subtle and inconsistent over space and time. By the fourth year of the experiment, we observed the predicted suppression of soil respiration and a reduction of water extractable carbon from soils receiving salt treatments. However, we found no cumulative effects of four years of salinization on total soil carbon stocks, tree growth, or root biomass. We observed substantial variation in soil solution chemistry (notably, pH and base saturation) across replicated treatment blocks; the effective salt levels, ionic composition, and pH varied following treatment depending upon pre-existing differences in edaphic factors. Our multi-year monitoring also revealed an underlying trend of wetland acidification across the entire site, a suspected effect of ecosystem recovery following wetland restoration on former agricultural land. The overwhelming resistance to our salt treatments could be attributed to the vigor of a relatively young, healthy wetland ecosystem. The heterogeneous responses to salt that we observed over space and time merits further investigation into the environmental factors that control carbon cycling in wetlands. This work highlights the importance of multi-year, large-scale field experiments for investigating ecosystem responses to global environmental change.

Seasonal flooding provides limitations and opportunities for ecosystem carbon accounting from space

Abstract Flooding controls wetland carbon cycling and hinders accurate measurements of ecosystem structure from remotely sensed data. Single measurements over a changeable surface can yield large uncertainties, exacerbated by insufficient repeat sampling of inundation dynamics across globally distributed locations. In forested wetlands, flood frequency and duration is critical to controlling carbon cycling, but high canopy cover obscures the view of inundation stage where large fluctuations in flooding directly impact measurements of ecosystem structure.

An Integrative Model for Soil Biogeochemistry and Methane Processes. II: Warming and Elevated CO2 Effects on Peatland CH4 Emissions

AbstractPeatlands are one of the largest natural sources for atmospheric methane (CH4), a potent greenhouse gas. Climate warming and elevated atmospheric carbon dioxide (CO2) are two important environmental factors that have been confirmed to stimulate peatland CH4 emissions; however, the mechanisms underlying enhanced emissions remain elusive. A data‐model integration approach was applied to understand the CH4 processes in a northern temperate peatland under a gradient of warming and doubled atmospheric CO2 concentration. We found that warming and elevated CO2 stimulated CH4 emissions through different mechanisms. Warming initially stimulated but then suppressed vegetative productivity while stimulating soil organic matter (SOM) mineralization and dissolved organic carbon (DOC) fermentation, which led to higher acetate production and enhanced acetoclastic and hydrogenotrophic methanogenesis. Warming also enhanced surface CH4 emissions, which combined with warming‐caused decreases in CH4 solubility led to slightly lower dissolved CH4 concentrations through the soil profiles. Elevated CO2 enhanced ecosystem productivity and SOM mineralization, resulting in higher DOC and acetate concentrations. Higher DOC and acetate concentrations increased acetoclastic and hydrogenotrophic methanogenesis and led to higher dissolved CH4 concentrations and CH4 emissions. Both warming and elevated CO2 had minor impacts on CH4 oxidation. A meta‐analysis of warming and elevated CO2 impacts on carbon cycling in wetlands agreed well with a majority of the modeled mechanisms. This mechanistic understanding of the stimulating impacts of warming and elevated CO2 on peatland CH4 emissions enhances our predictability on the climate‐ecosystem feedback.

Factors controlling <i>Carex brevicuspis</i> leaf litter decomposition and its contribution to surface soil organic carbon pool at different water levels

Abstract. Litter decomposition plays a vital role in wetland carbon cycling. However, the contribution of aboveground litter decomposition to the wetland soil organic carbon (SOC) pool has not yet been quantified. Here, we conducted a Carex brevicuspis leaf litter input experiment to clarify the intrinsic factors controlling litter decomposition and quantify its contribution to the SOC pool at different water levels. The Carex genus is ubiquitous in global freshwater wetlands. We sampled this plant leaf litter at −25, 0, and +25 cm relative to the soil surface over 280 d and analysed leaf litter decomposition and its contribution to the SOC pool. The percentage litter dry weight loss and the instantaneous litter dry weight decomposition rate were the highest at +25 cm water level (61.8 %, 0.01307 d−1), followed by the 0 cm water level (49.8 %, 0.00908 d−1), and the lowest at −25 cm water level (32.4 %, 0.00527 d−1). Significant amounts of litter carbon, nitrogen, and phosphorus were released at all three water levels. Litter input significantly increased the soil microbial biomass and fungal density but had nonsignificant impacts on soil bacteria, actinomycetes, and the fungal∕bacterial concentrations at all three water levels. Compared with litter removal, litter addition increased the SOC by 16.93 %, 9.44 %, and 2.51 % at the +25, 0, and −25 cm water levels, respectively. Hence, higher water levels facilitate the release of organic carbon from leaf litter into the soil via water leaching. In this way, they increase the soil carbon pool. At lower water levels, soil carbon is lost due to the slower litter decomposition rate and active microbial (actinomycete) respiration. Our results revealed that the water level in natural wetlands influenced litter decomposition mainly by leaching and microbial activity, by extension, and affected the wetland surface carbon pool.

Snow depths\u2019 impact on soil microbial activities and carbon dioxide fluxes from a temperate wetland in Northeast China

Snow depth may have a complex influence on carbon cycling in winter. Here we set up a field experiment to investigate how different snow depths (0 cm, 60 cm, 90 cm) influenced carbon dioxide (CO2) in a wetland. The mean ± standard error of CO2 emissions under snow addition treatments (60 cm and 90 cm snow depths) were 0.92 ± 0.16 g·cm−2·s−1 and 0.53 ± 0.16 g·cm−2·s−1, respectively, compared with snow removal treatment (0 cm snow depth), 0.11 ± 0.05 g·cm−2·s−1. In general, snow addition increased CO2 fluxes significantly. As snow depths increased, microbial biomass carbon (MBC) and bacterial diversities increased drastically. More important, the community of bacteria differed under different treatments. Firmicutes, which can resist dehydration and extremely low temperatures, was widely distributed in the snow removal treatment, where it sustained soil biochemical processes. Overall, our study indicates that snow cover counteracts the negative effects on soil microbial activities caused by low temperatures and could play a critical role in winter carbon cycling in wetlands.

Controls on soil carbon dioxide and methane fluxes from a peat swamp vary by hydrogeomorphic setting

AbstractCarbon dynamics of temperate peat swamps are a largely understudied component of wetland carbon cycling. Under a changing climate, hydrometeorological conditions may change, and understanding how peat swamps may be impacted is important. We investigated the importance of hydrogeomorphic setting on controlling soil carbon dioxide (CO2) and methane (CH4) fluxes from a temperate peat swamp in southern Ontario, Canada over a 3‐year period. We chose three different hydrogeomorphic settings: (a) a site with strong wetland‐stream interactions (i.e., an unconfined stream channel; unconfined), (b) a site with limited wetland‐stream interactions (confined), and (c) an interior site (no wetland–stream interaction). The differing hydrogeomorphic conditions between the sites resulted in differences in carbon fluxes. The unconfined site maintained a higher water table across all three study years, providing conditions that are not favourable to CO2 production. The confined and interior sites sustained a much lower water table, with conditions more conducive to CO2 efflux. The unconfined site also had the highest CH4 emissions due to the increased anoxic conditions favourable for CH4 production as a result of the higher water table position. Hydrogeomorphic setting was found to be important for understanding within site variation, suggesting the sites may respond differently to longer‐term shifts in environmental conditions. This may change relationships between sites if the responses are strong enough to significantly alter carbon fluxes, decomposition, and potential peat accumulation rates. It is important to understand the locally specific responses to environmental conditions within peat swamp ecosystems, in order to make future predictions about whole ecosystem function under changing conditions.

High rates of anaerobic methane oxidation in freshwater wetlands reduce potential atmospheric methane emissions

The role of anaerobic oxidation of methane (AOM) in wetlands, the largest natural source of atmospheric methane, is poorly constrained. Here we report rates of microbially mediated AOM (average rate=20 nmol cm(-3) per day) in three freshwater wetlands that span multiple biogeographical provinces. The observed AOM rates rival those in marine environments. Most AOM activity may have been coupled to sulphate reduction, but other electron acceptors remain feasible. Lipid biomarkers typically associated with anaerobic methane-oxidizing archaea were more enriched in (13)C than those characteristic of marine systems, potentially due to distinct microbial metabolic pathways or dilution with heterotrophic isotope signals. On the basis of this extensive data set, AOM in freshwater wetlands may consume 200 Tg methane per year, reducing their potential methane emissions by over 50%. These findings challenge precepts surrounding wetland carbon cycling and demonstrate the environmental relevance of an anaerobic methane sink in ecosystems traditionally considered strong methane sources.

Effect of Disturbance on Carbon Cycling in Wetland Ecosystem

Wetland is a special terraqueous ecosystem. Wetland ecosystem is an important carbon pool in the world and the carbon stored in which accounts for 15% of the total terrestrial carbon storage. Carbon cycling in wetlands largely affected global carbon cycling and also possibly global climatic change. Carbon cycle model of wetland has been changed by human disturbance is increased which exert a profound and lasting influence upon global change. Some researches show that wetland restoration and reconstruction are helpful to carbon accumulation and GHG reduction. So, through analyze the effect of human disturbance on carbon cycling in wetland, a necessary scientific basis is provided for the study of the effect of wetland on global change.

Seasonal ice and hydrologic controls on dissolved organic carbon and nitrogen concentrations in a boreal‐rich fen

Boreal wetland carbon cycling is vulnerable to climate change in part because hydrology and the extent of frozen ground have strong influences on plant and microbial functions. We examined the response of dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) across an experimental manipulation of water table position (both raised and lowered water table treatments) in a boreal‐rich fen in interior Alaska. DOC and TDN responses to water table manipulation exhibited an interaction with seasonal ice dynamics. We observed consistently higher DOC and TDN concentrations in the lowered water table treatment (71.7 ± 6.5 and 3.0 ± 0.3 mg−L) than in both the control (55.6 ± 5.1 and 2.3 ± 0.2 mg−L) and raised (49.1 ± 4.3 and 1.9 ± 0.1 mg L−1, respectively) water table treatments. Across all plots, pore water DOC concentrations at 20 cm increased as the depth to water table increased (R2 = 0.43, p < 0.001). DOC concentrations also increased as the seasonal thaw depth increased, with solutes increasing most rapidly in the drained plot (R2 = 0.62, p < 0.001). About half of the TDN pool was composed of dissolved organic N (DON). Inorganic N and DON were both highly correlated with changes in DOC, and their respective constraints to mineralization are discussed. These results demonstrate that a declining water table position and dryer conditions affect thaw depth and peat temperatures, and interactions among these ecosystem properties will likely increase DOC and TDN loading and potential for export in these systems.

The importance of nutrient co‐limitation in regulating algal community composition, productivity and algal‐derived DOC in an oligotrophic marsh in interior Alaska

Summary1. Compared to lakes and streams, we know relatively little about the factors that regulate algae in freshwater wetlands. This discrepancy is particularly acute in boreal regions, where wetlands are abundant and processes related to climate change (i.e. increased permafrost collapse and soil weathering) are expected to increase nutrient inputs into aquatic systems. To investigate how accelerated nutrient inputs might affect algal structure and function in northern boreal wetlands, we added nitrogen, phosphorus and silica to mesocosms in an oligotrophic marsh in interior Alaska.2. We conducted two in situ mesocosm enrichment experiments during consecutive summer growing seasons, each lasting 24 days. In 2007, we investigated the effects of +N, +P, +Si and +N+P+Si enrichment on benthic algal biomass (chlorophyll‐a, ash‐free dry mass, biovolume), chemistry (N : P ratio) and community composition. In 2008, we expanded our first experiment to investigate the effects +N+P, +N+Si, +P+Si and +N+P+Si on the same algal parameters as well as productivity (mg C m−2 h−1).3. In both experiments, we measured water‐column dissolved organic carbon (DOC) inside treatment enclosures and related changes in DOC to standing algal biomass.4. Benthic algal accrual did not increase following 24 days of enrichment with any nutrient alone or with P and Si together (+P+Si), but increased significantly with the addition of N in any combination with P and Si (+N+P, +N+Si, +N+P+Si).5. Algal productivity (20 mg C m−2 h−1) increased between three‐ and seven‐fold (57–127 mg C m−2 h−1) with the addition of N in combination with any other nutrient (+N+P, +N+Si, +N+P+Si). Water‐column DOC concentration was significantly higher inside N‐combination treatments compared to the control during each season, and DOC increased linearly with benthic algal biomass in 2007 (r2 = 0.89, P < 0.0001) and 2008 (r2 = 0.74, P < 0.0001).6. Taxonomic composition of the wetland algal community responded most strongly to N‐combination treatments in both seasons. In 2007, there was a significant shift from Euglena and Mougeotia in the control treatment to Chroococcus and Gloeocystis with +N+P+Si enrichment, and in 2008, a Mougeotia‐dominated community was replaced by Gloeocystis in the +N+P treatment and by Nitzschia in +N+Si and +N+P+Si treatments.7. Together, these data provide several lines of evidence for co‐limitation, and the central importance of N as a co‐limiting nutrient for the wetland algal community. Changes in algal dynamics with increased nutrient concentrations could have important implications for wetland food webs and suggest that algae may provide a functional link between increasing nutrient inputs and altered wetland carbon cycling in this region.

Seasonal patterns of carbohydrate translocation and synthesis of structural carbon components in Typha angustifolia

Quantitative studies of material budgets and resource allocation patterns of emergent plants are needed to fully understand nutrient and carbon cycling in wetlands. Whole-plant translocation patterns of nonstructural carbohydrates and synthesis of structural carbon were documented using two (shallow and deepwater) populations of Typha angustifolia in floodplain habitats of the Arakawa River, Japan. Monthly and bimonthly measurements of the concentrations of total carbon, non-structural carbohydrates, and water-soluble carbohydrates, as well as, estimates of standing stocks of aboveground (AG) and belowground (BG) biomass for both populations from 2002 to 2004 are described here. Annual patterns of carbon gain, rates of carbohydrate translocation between AG and BG organs, and rates of synthesis of structural carbon were estimated. Upward translocation supported all AG production for approximately 30 days. Afterward, the fraction of AG production supported by upward translocation decreased linearly with time, and completely diminished by Day 80 (counted from first day of growth; March 15). At Day 80, material translocation was directed downward and the percentage of downward translocation relative to AG net production increased until Day 170; there was a vigorous increase in downward translocation prior to senescence. In early summer (Day 80–110), more than half of the translocated materials were transformed into structural components, but by Day 125 only 30% was synthesized into total structural carbohydrates. Prior to senescence, there was a greater percentage of nonstructural carbohydrates in the rhizomes. Across the growing season (Day 60–153), the total amount of downward translocation was found to be proportional to AG production and the quantitative evaluation of the carbon budget shed further insight into the translocation process of this rhizomatous aquatic plants. In addition, insights on the differences between shallow and deepwater populations were gained. Especially, the deepwater population supported a higher fraction of vertical rhizomes compared to individuals sampled from the shallow water population.

SEASONAL PATTERNS AND PLANT-MEDIATED CONTROLS OF SUBSURFACE WETLAND BIOGEOCHEMISTRY

In tidal marshes, spatial and temporal variability in the importance of mi- crobial metabolic pathways influences ecosystem-level processes such as soil carbon stor- age, the regeneration of inorganic nutrients, and the production of atmospherically important trace gases. We measured seasonal changes in rates of microbial Fe(III) reduction, sulfate reduction, and methanogenesis in tidal freshwater and brackish marshes on the Patuxent River, Maryland, USA, and assessed the ability of plant roots to influence these processes by regenerating electron acceptors and supplying electron donors. In both marshes, the importance of microbial Fe(III) reduction was greatest early in the summer and decreased through the study period. Coincident with the seasonal decline in Fe(III) reduction, me- thanogenesis (freshwater marsh) or sulfate reduction (brackish site) increased in importance. At the brackish marsh, the partitioning of anaerobic carbon metabolism between Fe(III) reduction and sulfate reduction was similar within and below the root zone, suggesting that rhizosphere processes did not control anaerobic metabolism at this site. Instead, seasonal biogeochemical patterns at the brackish marsh were affected by factors such as water table depth and iron-sulfur interactions. At the tidal freshwater site, our results suggest that changes in rates of Fe(III) reduction and methanogenesis were directly affected by plant- mediated processes. In midsummer, Fe(III) reduction accounted for a greater fraction of total anaerobic metabolism in rhizosphere-influenced surface soils than in soils below the root zone. High rates of Fe(III) reduction occurred at the expense of methanogenesis. This study documented strong temporal variations in the outcome of microbial competition for electron donors that ultimately affected the balance between Fe(III) reduction and me- thanogenesis within tidal freshwater marsh soils. Our data suggested that variations in microbial metabolic pathways were regulated by physiochemical factors at the brackish site and plant activity at the freshwater site. Plant regulation of Fe(III) reduction is a largely unstudied mechanism by which plants influence wetland carbon cycling and greenhouse gas production.