Salinity affects microbial activity and soil organic matter content in tidal wetlands

Climate change‐associated sea level rise is expected to cause saltwater intrusion into many historically freshwater ecosystems. Of particular concern are tidal freshwater wetlands, which perform several important ecological functions including carbon sequestration. To predict the impact of saltwater intrusion in these environments, we must first gain a better understanding of how salinity regulates decomposition in natural systems. This study sampled eight tidal wetlands ranging from freshwater to oligohaline (0–2 ppt) in four rivers near the Chesapeake Bay (Virginia). To help isolate salinity effects, sites were selected to be highly similar in terms of plant community composition and tidal influence. Overall, salinity was found to be strongly negatively correlated with soil organic matter content (OM%) and C : N, but unrelated to the other studied environmental parameters (pH, redox, and above‐ and below‐ground plant biomass). Partial correlation analysis, controlling for these environmental covariates, supported direct effects of salinity on the activity of carbon‐degrading extracellular enzymes (β‐1, 4‐glucosidase, 1, 4‐β‐cellobiosidase, β‐D‐xylosidase, and phenol oxidase) as well as alkaline phosphatase, using a per unit OM basis. As enzyme activity is the putative rate‐limiting step in decomposition, enhanced activity due to salinity increases could dramatically affect soil OM accumulation. Salinity was also found to be positively related to bacterial abundance (qPCR of the 16S rRNA gene) and tightly linked with community composition (T‐RFLP). Furthermore, strong relationships were found between bacterial abundance and/or composition with the activity of specific enzymes (1, 4‐β‐cellobiosidase, arylsulfatase, alkaline phosphatase, and phenol oxidase) suggesting salinity's impact on decomposition could be due, at least in part, to its effect on the bacterial community. Together, these results indicate that salinity increases microbial decomposition rates in low salinity wetlands, and suggests that these ecosystems may experience decreased soil OM accumulation, accretion, and carbon sequestration rates even with modest levels of saltwater intrusion.     

盐分对河口淡水、微咸水沼泽湿地土壤甲烷产生潜力的影响

海平面上升导致河口区盐水入侵现象日益明显,深刻影响着河口潮汐淡水、微咸水湿地生物地球化学循环。采集闽江河口区淡水、微咸水短叶茳芏潮汐沼泽湿地表层土样,室内添加盐度为5,10,15,21 g/L的人造海水、Na Cl溶液及盐度为0的去离子水,通过室内泥浆厌氧培养试验,对比研究海水和Na Cl溶液对淡水、微咸水沼泽湿地土壤甲烷产生潜力的影响。与对照相比,1—12 d培养期内4个盐度的海水处理均显著抑制河口淡水、微咸水沼泽湿地甲烷产生潜力,抑制率在93%以上,盐度10—21 g/L的3个海水处理对于河口淡水、微咸水沼泽湿地甲烷产生潜力的抑制效应无显著差异。Na Cl溶液只有在盐度达到15和21 g/L才显著抑制淡水、微咸水沼泽湿地甲烷产生潜力,且抑制率最多为80.9%,盐度为5、10 g/L的Na Cl溶液对淡水、微咸水沼泽湿地甲烷产生潜力的抑制作用不显著,抑制率多小于30%。伴随着盐水入侵而发生的硫酸盐还原作用及离子胁迫作用对河口淡水、微咸水沼泽湿地甲烷产生具有显著的抑制效应。     

Increased methane concentration alters soil prokaryotic community structure along an artificial pH gradient

Global climate change may have a large impact on increased emission rates of carbon dioxide and methane to total greenhouse gas emissions from terrestrial wetlands. Methane consumption by soil microbiota in alpine wet meadows serves as a biofilter for the methane produced in the waterlogged soil below. Altered pH regimes change microbial community composition and structure by exerting selection pressure on soil microorganisms with different ecological strategies and thus affect greenhouse gas emissions resulting from the metabolic activity of soil microorganisms. However, responses of prokaryotic communities to artificial pH shift under elevated methane concentration remain unclear. In this study, we assessed diversity and relative abundance of soil prokaryotes in an alpine meadow under elevated methane concentration along an artificial pH gradient using laboratory incubation experiments. We established an incubation experiment treated with artificial pH gradient (pH 4.5–8.5). After 3 months of incubation, 300 ml of methane at a concentration of 20,000 ppm was added to stimulate potential methanothrophs in topsoil. Sequencing of 16S rRNA gene indicated increasing of relative abundances of Crenarchaeota, Chloroflexi, Bacteroidetes, and Planctomycetes in soil after addition of methane, while the relative abundances of Actinobacteria and Gemmatimonadetes did not significant change before and after methane treatment. Results of phylogenetic relatedness of soil prokaryotes showed that microbial community is mostly shaped by deterministic factors. Species indicator analysis revealed distinct OTUs among various pH and methane treatments. Network analysis revealed distinct co-occurrence patterns of soil prokaryotic community before and after methane addition, and different correlation patterns among various prokaryotic taxa. Linear regression model revealed significant decrease of methane oxidation along elevated pH gradient. Soil pH constituted a strong environmental filter in species assembly of soil prokaryotic community. Methane oxidation rates decreased significantly with elevated pH. The interactive effects of elevated methane concentration and pH are therefore promising topic for future research.     

Effect of salinity-altering pulsing events on soil organic carbon loss along an intertidal wetland gradient: a laboratory experiment

Salinity changes resulting from storm surge, tides, precipitation, and stormwater run-off are common in coastal wetlands. Soil microbial communities respond quickly to salinity changes, altering the rate of soil organic carbon (SOC) loss and associated biogeochemical processes. This study quantified the impact of salinity-altering pulses on SOC loss, defined as microbial respiration (CO₂ flux) at high and low tide, CH₄ flux, and dissolved OC (DOC) release, in 3 intertidal wetlands (Jacksonville, FL, USA). Intact soil cores from a freshwater tidal, brackish, and salt marsh were exposed to simulated tides and 3 salinity pulsing events during a 53-day laboratory experiment. Soil and water physio-chemical properties, nutrient release, and microbial indicators were measured. Microbial respiration was the dominate pathway of SOC loss (>97 %). Soil hydraulic conductivity was greater in brackish and salt marshes and was critical to overall soil respiration. High tide CO₂ flux was greatest in the freshwater marsh (58 % of SOC loss) and positively correlated with DOC concentration; low tide CO₂ flux was greatest in brackish and salt marshes (62 and 70 % of SOC loss, respectively) and correlated with NH₄ ⁺ and microbial biomass. The freshwater marsh was sensitive to brackish pulses, causing a 112 % increase in respiration, presumably from accelerated sulfate reduction and N-cycling. SOC loss increased in the salt marsh pulsed with freshwater, suggesting freshwater run-off may reduce a salt marsh’s ability to keep-pace with sea level rise. Increased inundation from storm surges could accelerate SOC loss in freshwater marshes, while decreasing SOC loss in brackish and salt marshes.     

Net ecosystem carbon exchange and the greenhouse gas balance of tidal marshes along an estuarine salinity gradient

Tidal wetlands are productive ecosystems with the capacity to sequester large amounts of carbon (C), but we know relatively little about the impact of climate change on wetland C cycling in lower salinity (oligohaline and tidal freshwater) coastal marshes. In this study we assessed plant production, C cycling and sequestration, and microbial organic matter mineralization at tidal freshwater, oligohaline, and salt marsh sites along the salinity gradient in the Delaware River Estuary over four years. We measured aboveground plant biomass, carbon dioxide (CO₂) and methane (CH₄) exchange between the marsh and atmosphere, microbial sulfate reduction and methanogenesis in marsh soils, soil biogeochemistry, and C sequestration with radiodating of soils. A simple model was constructed to estimate monthly and annually integrated rates of gross ecosystem production (GEP), ecosystem respiration (ER) to carbon dioxide ( \( {\text{ER}}_{{{\text{CO}}_{2} }} \) ) or methane ( \( {\text{ER}}_{{{\text{CH}}_{4} }} \) ), net ecosystem production (NEP), the contribution of sulfate reduction and methanogenesis to ER, and the greenhouse gas (GHG) source or sink status of the wetland for 2 years (2007 and 2008). All three marsh types were highly productive but evidenced different patterns of C sequestration and GHG source/sink status. The contribution of sulfate reduction to total ER increased along the salinity gradient from tidal freshwater to salt marsh. The Spartina alterniflora dominated salt marsh was a C sink as indicated by both NEP (~140 g C m^?2 year^?1) and ²¹⁰Pb radiodating (336 g C m^?2 year^?1), a minor sink for atmospheric CH₄, and a GHG sink (~620 g CO_2-eq m^?2 year^?1). The tidal freshwater marsh was a source of CH₄ to the atmosphere (~22 g C–CH₄ m^?2 year^?1). There were large interannual differences in plant production and therefore C and GHG source/sink status at the tidal freshwater marsh, though ²¹⁰Pb radiodating indicated modest C accretion (110 g C m^?2 year^?1). The oligohaline marsh site experienced seasonal saltwater intrusion in the late summer and fall (up to 10 mS cm^?1) and the Zizania aquatica monoculture at this site responded with sharp declines in biomass and GEP in late summer. Salinity intrusion was also linked to large effluxes of CH₄ at the oligohaline site (>80 g C–CH₄ m^?2 year^?1), making this site a significant GHG source (>2,000 g CO_2-eq m^?2 year^?1). The oligohaline site did not accumulate C over the 2 year study period, though ²¹⁰Pb dating indicated long term C accumulation (250 g C m^?2 year^?1), suggesting seasonal salt-water intrusion can significantly alter C cycling and GHG exchange dynamics in tidal marsh ecosystems.     

Microbial Community Response to Seawater Amendment in Low-Salinity Tidal Sediments

Rising sea levels and excessive water withdrawals upstream are making previously freshwater coastal ecosystems saline. Plant and animal responses to variation in the freshwater–saline interface have been well studied in the coastal zone; however, microbial community structure and functional response to seawater intrusion remains relatively unexplored. Here, we used molecular approaches to evaluate the response of the prokaryotic community to controlled changes in porewater salinity levels in freshwater sediments from the Altamaha River, Georgia, USA. This work is a companion to a previously published study describing results from an experiment using laboratory flow-through sediment core bioreactors to document biogeochemical changes as porewater salinity was increased from 0 to 10 over 35 days. As reported in Weston et al. (Biogeochemistry, 77:375–408, 62), porewater chemistry was monitored, and cores were sacrificed at 0, 9, 15, and 35 days, at which time we completed terminal restriction fragment length polymorphism and 16S rRNA clone library analyses of sediment microbial communities. The biogeochemical study documented changes in mineralization pathways in response to artificial seawater additions, with a decline in methanogenesis, a transient increase in iron reduction, and finally a dominance of sulfate reduction. Here, we report that, despite these dramatic and significant changes in microbial activity at the biogeochemical level, no significant differences were found between microbial community composition of control vs. seawater-amended treatments for either Bacterial or Archaeal members. Further, taxa in the seawater-amended treatment community did not become more “marine-like” through time. Our experiment suggests that, as seawater intrudes into freshwater sediments, observed changes in metabolic activity and carbon mineralization on the time scale of weeks are driven more by shifts in gene expression and regulation than by changes in the composition of the microbial community.     

Recovery of freshwater marsh vegetation after a saltwater intrusion event

Greenhouse mesocosms of freshwater marsh vegetation were exposed to a simulated saltwater intrusion event followed by a recovery period during which water levels and interstitial water salinity were adjusted over a range of conditions. Virtually all above-ground vegetation, including the three dominant species, Sagittaria lancifolia L., Leersia oryzoides (L.) Swartz, and Panicum hemitomon Schultes, was killed by the initial saltwater intrusion event. P. hemitomon did not recover, but S. lancifolia and L. oryzoides, as well as many of the other species initially present, exhibited some ability to recover depending on post-saltwater intrusion conditions. Increasingly harsh recovery conditions (for freshwater marsh vegetation), including more reduced soil conditions, higher interstitial salinities, and higher interstitial sulfide concentrations were associated with decreased live above-ground biomass and species richness. The effect of elevated salinity on vegetative recovery became more pronounced under flooded conditions. This experiment illustrates that the response of a freshwater marsh community to the long-term disturbance effect of a transient saltwater intrusion event will be strongly influenced by post-intrusion salinity and water levels.     

Accelerated microbial organic matter mineralization following salt-water intrusion into tidal freshwater marsh soils

The impact of salt-water intrusion on microbial organic carbon (C) mineralization in tidal freshwater marsh (TFM) soils was investigated in a year-long laboratory experiment in which intact soils were exposed to a simulated tidal cycle of freshwater or dilute salt-water. Gas fluxes [carbon dioxide (CO₂) and methane (CH₄)], rates of microbial processes (sulfate reduction and methanogenesis), and porewater and solid phase biogeochemistry were measured throughout the experiment. Flux rates of CO₂ and, surprisingly, CH₄ increased significantly following salt-water intrusion, and remained elevated relative to freshwater cores for 6 and 5 months, respectively. Following salt-water intrusion, rates of sulfate reduction increased significantly and remained higher than rates in the freshwater controls throughout the experiment. Rates of acetoclastic methanogenesis were higher than rates of hydrogenotrophic methanogenesis, but the rates did not differ by salinity treatment. Soil organic C content decreased significantly in soils experiencing salt-water intrusion. Estimates of total organic C mineralized in freshwater and salt-water amended soils over the 1-year experiment using gas flux measurements (18.2 and 24.9 mol C m^?2, respectively) were similar to estimates obtained from microbial rates (37.8 and 56.2 mol C m^?2, respectively), and to losses in soil organic C content (0 and 44.1 mol C m^?2, respectively). These findings indicate that salt-water intrusion stimulates microbial decomposition, accelerates the loss of organic C from TFM soils, and may put TFMs at risk of permanent inundation.     

Salinity thresholds for understory plants in coastal wetlands

The effects of sea level rise and coastal saltwater intrusion on wetland plants can extend well above the high-tide line due to drought, hurricanes, and groundwater intrusion. Research has examined how coastal salt marsh plant communities respond to increased flooding and salinity, but more inland coastal systems have received less attention. The aim of this study was to identify whether ground layer plants exhibit threshold responses to salinity exposure. We used two vegetation surveys throughout the Albemarle-Pamlico Peninsula (APP) of North Carolina, USA to assess vegetation in a low elevation landscape (≤?3.8 m) experiencing high rates of sea level rise (3–4 mm/year). We examined the primary drivers of community composition change using Non-metric Multidimensional Scaling (NMDS) and used Threshold Indicator Taxa Analysis (TITAN) to detect thresholds of compositional change based on indicator taxa, in response to potential indicators of exposure to saltwater (Na, and the Σ Ca?+?Mg) and elevation. Salinity and elevation explained 64% of the variation in community composition, and we found two salinity thresholds for both soil Na⁺ (265 and 3843 g Na⁺/g) and Ca⁺ +?Mg⁺ (42 and 126 µeq/g) where major changes in community composition occur on the APP. Similar sets of species showed sensitivity to these different metrics of salt exposure. Overall, our results showed that ground layer plants can be used as reliable indicators of salinity thresholds in coastal wetlands. These results can be used for monitoring salt exposure of ecosystems and for identifying areas at risk for undergoing future community shifts.

     

Using microbial communities and extracellular enzymes to link soil organic matter characteristics to greenhouse gas production in a tidal freshwater wetland

To gain a more mechanistic understanding of how soil organic matter (OM) characteristics can affect carbon mineralization in tidal freshwater wetlands, we conducted a long-term in situ field manipulation of OM type and monitored associated changes in carbon dioxide (CO₂) and methane (CH₄) production. In addition, we characterized microbial community structure and quantified the activity of several extracellular enzymes (EEA) involved in the acquisition of carbon, nitrogen, and phosphorus. Treatments included a plant litter addition, prepared using naturally-senescing vegetation from the site, and a compost amendment, designed to increase the concentration of aged, partially humified, OM. Both types of OM-amended soils had CO₂ production rates 40–50 % higher than unamended control soils, suggesting that the added OM had inherently higher quality and/or availability than the native soil OM. Rates of CO₂ production were not correlated with microbial community structure or EEA except a modest relationship with cellulose breakdown via the K_m of β-1,4-glucosidase. We interpret this lack of correlation to be a consequence of high functional redundancy of microorganisms that are capable of producing CO₂. Rates of CH₄ production were also influenced by OM quality, increasing by an order of magnitude with plant litter additions relative to compost-amended and control soils. Unlike CO₂, rates of CH₄ production were significantly correlated with the microbial community structure and with enzyme kinetic parameters (V_max and K_m) for both carbon (β-1,4-glucosidase, 1,4-β-cellobiosidase, and β-d-xylosidase) and nitrogen acquisition (leucyl aminopeptidase). The monophyletic nature of methanogenic archaea, combined with their reliance on a small select group of organic substrates produced via enzyme-mediated hydrolysis and subsequent bacterial fermentation, provides a basis for the strong links between microbial community structure, EEA, and CH₄ production. Our results suggest that incorporating microbial community structure and EEA into conceptual models of wetland OM decomposition may enhance our mechanistic understanding of, and predictive capacity for, biogeochemical process rates.     

闽江河口区淡水和半咸水潮汐沼泽湿地土壤产甲烷菌多样性

为认识盐度对河口潮汐沼泽湿地土壤产甲烷菌的影响,应用PCR-RFLP技术及测序分析对闽江河口区淡水-半咸水盐度梯度上分布的4个短叶茳芏潮汐沼泽湿地土壤产甲烷菌群落结构进行研究。闽江河口区短叶茳芏潮汐沼泽湿地土壤产甲烷菌群落结构受盐度影响明显,位于下洋洲和塔礁洲的短叶茳芏潮汐淡水沼泽湿地土壤产甲烷菌的香农-威纳多样性指数值分别为2.81和2.65,位于蝙蝠洲和鳝鱼滩的短叶茳芏潮汐半咸水沼泽湿地土壤产甲烷菌香农-威纳多样性指数值分别仅为2.33和2.27。系统发育分析表明:短叶茳芏沼泽湿地土壤产甲烷菌类群主要有甲烷杆菌目(Methanobacteriales),包括Methanobacterium、Methanobrevibacter和Methanobacteriaceae;甲烷微菌目(Methanomicrobiales),主要有Methanoregula,以及甲烷八叠球菌目(Methanosarcinales),主要有Methanosarcina和Methanococcoides。闽江河口区短叶茳芏潮汐淡水沼泽湿地土壤主要的优势产甲烷菌有Methanoregula、Methanosarcina和Methanobacterium,而短叶茳芏潮汐半咸水沼泽湿地土壤主要的优势产甲烷菌则转化为仅以Methanoregula为主。     

Salinity effects on greenhouse gas emissions from wetland soils are contingent upon hydrologic setting: a microcosm experiment

Coastal forested wetlands provide important ecosystem services such as carbon sequestration, nutrient retention, and flood protection, but they are also important sources of greenhouse gas emissions. Human appropriation of surface water and extensive ditching and draining of coastal plain landscapes are interacting with rising sea levels to increase the frequency and magnitude of saltwater incursion into formerly freshwater coastal wetlands. Both hydrologic change and saltwater incursion are expected to alter carbon and nutrient cycling in coastal forested wetlands. We performed a full factorial experiment in which we exposed intact soil cores from a coastal forested wetland to experimental marine salt treatments and two hydrologic treatments. We measured the resulting treatment effects on the emissions of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) over 112 days. Salinity effects were compared across four treatments to isolate the effects of increases in ionic strength from the impact of adding a terminal electron acceptor (SO₄^2?). We compared control treatments (DI addition), to artificial saltwater (ASW, target salinity of 5 parts per thousand) and to two treatments that added sulfate alone (SO₄^2?, at the concentration found in 5 ppt saltwater) and saltwater with the sulfate removed (ASW-SO₄^2?, with the 5 ppt target salinity maintained by adding additional NaCl). We found that all salt treatments suppressed CO₂ production, in both drought and flooded treatments. Contrary to our expectations, CH₄ fluxes from our flooded cores increased between 300 and 1200% relative to controls in the ASW and ASW-SO₄^2? treatments respectively. In the drought treatments, we saw virtually no CH₄ release from any core, while artificial seawater with sulfate increased N₂O fluxes by 160% above DI control. In contrast, salt and sulfate decreased N₂O fluxes by 72% in our flooded treatments. Our results indicate that salinization of forested wetlands of the coastal plain may have important climate feedbacks resulting from enhanced greenhouse gas emissions and that the magnitude and direction of these emissions are contingent upon wetland hydrology.     

闽江口芦苇沼泽湿地土壤产甲烷菌群落结构的垂直分布

应用PCR-RFLP技术及测序分析对闽江口芦苇湿地土壤产甲烷菌群落结构的垂直分布特征进行了研究。在构建的6个克隆文库中,每个克隆文库随机挑选100个克隆进行菌落PCR验证,共得到591个阳性克隆。PCR产物经限制性内切酶MspⅠ进行RFLP分析后得到37个不同的分类操作单元(OTUs)。对37个克隆子进行了序列测定,与GenBank数据库中的序列进行比对,最近相似性在91%—99%之间。RFLP分析和系统发育分析表明,闽江口芦苇湿地土壤中产甲烷菌群落包括3大类群:甲烷杆菌目(Methanobacteriales)、甲烷微菌目(Methanomirobiales)和甲烷八叠球菌目(Methanosarcinales)。不同土壤深度中产甲烷菌群落的分布呈现出不同的特征。土壤表层(0—10 cm)优势产甲烷菌类群为Methanoregula,约占76%;10—20 cm土层主要的产甲烷菌类群为Methanolinea和Methanoregula,分别约占23%和29%;20—30cm土层优势的产甲烷菌类群为Methanolinea,约占66%。Shannon指数(H’)和Simpson多样性指教(D)表明,10—20cm土层产甲烷菌多样性高于土壤表层(0—10 cm)和20—30 cm土层。37个测序OTUs中有26个OTUs属于不可培养的产甲烷菌序列,表明闽江口芦苇湿地土壤中存在大量不可培养的产甲烷菌。     

Impacts of Saltwater Incursion on Plant Communities,Anaerobic Microbial Metabolism,and Resulting Relationships in a Restored Freshwater Wetland

Saltwater incursion carries high concentrations of sea salts, including sulfate, which can alter anaerobic microbial processes and plant community composition of coastal freshwater marshes. We studied these phenomena in a recently restored wetland on the coastal plain of North Carolina. We measured water inundation patterns, porewater chemistry, microbial process rates, plant tissue chemistry and iron plaque on plant roots, and quantified plant community composition across a hydrologic and salinity gradient to understand the potential interactions between saltwater incursion and changes in microbial processes and plant communities. Plant communities showed no obvious response to incursion, but were structured by inundation patterns and plant growth form (for example, graminoid versus forb). Saltwater incursion increased chloride and sulfate concentrations in surface and porewater, and drove resulting spatial patterns in anaerobic microbial metabolism rates. Plots experiencing saltwater incursion had higher sulfate reduction rates and were dominated by graminoid plant species (for example, sedges, rushes, and grasses). Graminoid plant species’ roots had greater iron plaque formation than forb and submerged species, indicative that graminoid plant species are supplying more oxygen to the rhizosphere, potentially influencing microbial metabolism. Future studies should focus on how plant and microbial communities may respond to saltwater incursion at different time scales, and on parsing out the influence that plants and microbes have on each other as freshwater wetlands experience sea level rise.     

Plant community,primary productivity,and environmental conditions following wetland re-establishment in the Sacramento-San Joaquin Delta,California

Wetland restoration can mitigate aerobic decomposition of subsided organic soils, as well as re-establish conditions favorable for carbon storage. Rates of carbon storage result from the balance of inputs and losses, both of which are affected by wetland hydrology. We followed the effect of water depth (25 and 55 cm) on the plant community, primary production, and changes in two re-established wetlands in the Sacramento San-Joaquin River Delta, California for 9 years after flooding to determine how relatively small differences in water depth affect carbon storage rates over time. To estimate annual carbon inputs, plant species cover, standing above- and below-ground plant biomass, and annual biomass turnover rates were measured, and allometric biomass models for Schoenoplectus (Scirpus) acutus and Typha spp., the emergent marsh dominants, were developed. As the wetlands developed, environmental factors, including water temperature, depth, and pH were measured. Emergent marsh vegetation colonized the shallow wetland more rapidly than the deeper wetland. This is important to potential carbon storage because emergent marsh vegetation is more productive, and less labile, than submerged and floating vegetation. Primary production of emergent marsh vegetation ranged from 1.3 to 3.2 kg of carbon per square meter annually; and, mid-season standing live biomass represented about half of the annual primary production. Changes in species composition occurred in both submerged and emergent plant communities as the wetlands matured. Water depth, temperature, and pH were lower in areas with emergent marsh vegetation compared to submerged vegetation, all of which, in turn, can affect carbon cycling and storage rates.     

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

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.     

Microbial minorities modulate methane consumption through niche partitioning

Microbes catalyze all major geochemical cycles on earth. However, the role of microbial traits and community composition in biogeochemical cycles is still poorly understood mainly due to the inability to assess the community members that are actually performing biogeochemical conversions in complex environmental samples. Here we applied a polyphasic approach to assess the role of microbial community composition in modulating methane emission from a riparian floodplain. We show that the dynamics and intensity of methane consumption in riparian wetlands coincide with relative abundance and activity of specific subgroups of methane-oxidizing bacteria (MOB), which can be considered as a minor component of the microbial community in this ecosystem. Microarray-based community composition analyses demonstrated linear relationships of MOB diversity parameters and in vitro methane consumption. Incubations using intact cores in combination with stable isotope labeling of lipids and proteins corroborated the correlative evidence from in vitro incubations demonstrating γ-proteobacterial MOB subgroups to be responsible for methane oxidation. The results obtained within the riparian flooding gradient collectively demonstrate that niche partitioning of MOB within a community comprised of a very limited amount of active species modulates methane consumption and emission from this wetland. The implications of the results obtained for biodiversity–ecosystem functioning are discussed with special reference to the role of spatial and temporal heterogeneity and functional redundancy.     

Differential effects of chronic and acute simulated seawater intrusion on tidal freshwater marsh carbon cycling

Tidal freshwater ecosystems experience acute seawater intrusion associated with periodic droughts, but are expected to become chronically salinized as sea level rises. Here we report the results from an experimental manipulation in a tidal freshwater Zizaniopsis miliacea marsh on the Altamaha River, GA where diluted seawater was added to replicate marsh plots on either a press (constant) or pulse (2 months per year) basis. We measured changes in porewater chemistry (SO₄^2?, Cl^?, organic C, inorganic nitrogen and phosphorus), ecosystem CO₂ and CH₄ exchange, and microbial extracellular enzyme activity. We found that press (chronic) seawater additions increased porewater chloride and sulfate almost immediately, and ammonium and phosphate after 2–4 months. Chronic increases in salinity also decreased net ecosystem exchange, resulting in reduced CO₂ and CH₄ emissions from press plots. Our pulse treatment, designed to mimic natural salinity incursion in the Altamaha River (September and October), temporarily increased porewater ammonium concentrations but had few lasting effects on porewater chemistry or ecosystem carbon balance. Our findings suggest that long-term, chronic saltwater intrusion will lead to reduced C fixation and the potential for increased nutrient (N, P) export while acute pulses of saltwater will have temporary effects.     

Elevated CO<Subscript>2</Subscript> and nitrogen addition accelerate net carbon gain in a brackish marsh

Wetlands have an inordinate influence on the global greenhouse gas budget, but how global changes may alter wetland contribution to future greenhouse gas fluxes is poorly understood. We determined the greenhouse gas balance of a tidal marsh exposed to nine years of experimental carbon dioxide (CO₂) and nitrogen (N) manipulation. We estimated net carbon (C) gain rates by measuring changes in plant and soil C pools over nine years. In wetland soils that accrete primarily through organic matter inputs, long-term measurements of soil elevation, along with soil C density, provide a robust estimate of net soil C gain. We used net soil C gain along with methane and nitrous oxide fluxes to determine the radiative forcing of the marsh under elevated CO₂ and N addition. Nearly all plots exhibited a net gain of C over the study period (up to 203 g C m^?2 year^?1), and C gain rates were greater with N and CO₂ addition. Treatment effects on C gain and methane emissions dominated trends in radiative forcing while nitrous oxide fluxes in all treatments were negligible. Though these soils experience salinities that typically suppress methane emissions, our results suggest that elevated CO₂ can stimulate methane emissions, overcoming positive effects of elevated CO₂ on C gain, converting brackish marshes that are typically net greenhouse gas sinks into sources. Adding resources, either CO₂ or N, will likely increase “blue carbon” accumulation rates in tidal marshes, but importantly, each resource can have distinct influences on the direction of total greenhouse forcing.     

Applying population and community ecology theory to advance understanding of belowground biogeochemistry

Approaches to quantifying and predicting soil biogeochemical cycles mostly consider microbial biomass and community composition as products of the abiotic environment. Current numerical approaches then primarily emphasise the importance of microbe–environment interactions and physiology as controls on biogeochemical cycles. Decidedly less attention has been paid to understanding control exerted by community dynamics and biotic interactions. Yet a rich literature of theoretical and empirical contributions highlights the importance of considering how variation in microbial population ecology, especially biotic interactions, is related to variation in key biogeochemical processes like soil carbon formation. We demonstrate how a population and community ecology perspective can be used to (1) understand the impact of microbial communities on biogeochemical cycles and (2) reframe current theory and models to include more detailed microbial ecology. Through a series of simulations we illustrate how density dependence and key biotic interactions, such as competition and predation, can determine the degree to which microbes regulate soil biogeochemical cycles. The ecological perspective and model simulations we present lay the foundation for developing empirical research and complementary models that explore the diversity of ecological mechanisms that operate in microbial communities to regulate biogeochemical processes.