Soil carbon dioxide (CO2) emission (soil respiration), net CO2 exchange after photosynthetic uptake by ground-cover plants, and soil CO2 concentration versus depth below land surface were measured at ...four ages of jack pine (Pinus banksiana Lamb.) forest in central Saskatchewan. Soil respiration was smallest at a clear-cut site, largest in an 8-year-old stand, and decreased with stand age in 20-year-old and mature (60-75 years old) stands during May-September 1994 (12.1, 34.6, 31.5, and 24.9 mol C(.)m-2, respectively). Simulations of soil respiration at each stand based on continuously recorded soil temperature were within one standard deviation of measured flux for 48 of 52 measurement periods, but were 10%-30% less than linear interpolations of measured flux for the season. This was probably due to decreased soil respiration at night modeled by the temperature-flux relationships, but not documented by daytime chamber measurements. CO2 uptake by ground-cover plants ranged from 0 at the clear-cut site to 29, 25, and 9% of total growing season soil respiration at the 8-year, 20-year, and mature stands. CO2 concentrations were as great as 7150 ppmv in the upper 1 m of unsaturated zone and were proportional to measured soil respiration.
Celotno besedilo
Dostopno za:
BF, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
As the permafrost region warms, its large organic carbon pool will be increasingly vulnerable to decomposition, combustion, and hydrologic export. Models predict that some portion of this release ...will be offset by increased production of Arctic and boreal biomass; however, the lack of robust estimates of net carbon balance increases the risk of further overshooting international emissions targets. Precise empirical or model-based assessments of the critical factors driving carbon balance are unlikely in the near future, so to address this gap, we present estimates from 98 permafrost-region experts of the response of biomass, wildfire, and hydrologic carbon flux to climate change. Results suggest that contrary to model projections, total permafrost-region biomass could decrease due to water stress and disturbance, factors that are not adequately incorporated in current models. Assessments indicate that end-of-the-century organic carbon release from Arctic rivers and collapsing coastlines could increase by 75% while carbon loss via burning could increase four-fold. Experts identified water balance, shifts in vegetation community, and permafrost degradation as the key sources of uncertainty in predicting future system response. In combination with previous findings, results suggest the permafrost region will become a carbon source to the atmosphere by 2100 regardless of warming scenario but that 65%-85% of permafrost carbon release can still be avoided if human emissions are actively reduced.
Permafrost soils store over half of global soil carbon (C), and northern frozen peatlands store about 10% of global permafrost C. With thaw, inundation of high latitude lowland peatlands typically ...increases the surface-atmosphere flux of methane (CH4), a potent greenhouse gas. To examine the effects of lowland permafrost thaw over millennial timescales, we measured carbon dioxide (CO2) and CH4 exchange along sites that constitute a ∼1000 yr thaw chronosequence of thermokarst collapse bogs and adjacent fen locations at Innoko Flats Wildlife Refuge in western Alaska. Peak CH4 exchange in July (123 ± 71 mg CH4-C m−2 d−1) was observed in features that have been thawed for 30 to 70 (<100) yr, where soils were warmer than at more recently thawed sites (14 to 21 yr; emitting 1.37 ± 0.67 mg CH4-C m−2 d−1 in July) and had shallower water tables than at older sites (200 to 1400 yr; emitting 6.55 ± 2.23 mg CH4-C m−2 d−1 in July). Carbon lost via CH4 efflux during the growing season at these intermediate age sites was 8% of uptake by net ecosystem exchange. Our results provide evidence that CH4 emissions following lowland permafrost thaw are enhanced over decadal time scales, but limited over millennia. Over larger spatial scales, adjacent fen systems may contribute sustained CH4 emission, CO2 uptake, and DOC export. We argue that over timescales of decades to centuries, thaw features in high-latitude lowland peatlands, particularly those developed on poorly drained mineral substrates, are a key locus of elevated CH4 emission to the atmosphere that must be considered for a complete understanding of high latitude CH4 dynamics.
Redox-active functional groups in dissolved organic matter (DOM) are crucial for microbial electron transfer and methane emissions. However, the extent of aquatic DOM redox properties across northern ...high-latitude lakes and their relationships with DOM composition have not been thoroughly described. We quantified electron donating capacity (EDC) and electron accepting capacity (EAC) in lake DOM from Canada to Alaska and assessed their relationships with parameters from absorbance, fluorescence, and ultrahigh resolution mass spectrometry (FT-ICR MS) analyses. EDC and EAC are strongly tied to aromaticity and negatively related to aliphaticity and protein-like content. Redox-active formulae spanned a range of aromaticity, including highly unsaturated phenolic formulae, and correlated negatively with many aliphatic N and S-containing formulae. This distribution illustrates the compositional diversity of redox-sensitive functional groups and their sensitivity to ecosystem properties such as local hydrology and residence time. Finally, we developed a reducing index (RI) to predict EDC in aquatic DOM from FT-ICR MS spectra and assessed its robustness using riverine DOM. As the hydrology of the northern high-latitudes continues to change, we expect differences in the quantity and partitioning of EDC and EAC within these lakes, which have implications for local water quality and methane emissions.
Wetlands are integral to the global carbon cycle, serving as both a source and a sink for organic carbon. Their potential for carbon storage will likely change in the coming decades in response to ...higher temperatures and variable precipitation patterns. We characterized the dissolved organic carbon (DOC) and dissolved organic matter (DOM) composition from 12 different wetland sites across the USA spanning gradients in climate, landcover, sampling depth, and hydroperiod for comparison to DOM in other inland waters. Using absorption spectroscopy, parallel factor analysis modeling, and ultra‐high resolution mass spectroscopy, we identified differences in DOM sourcing and processing by geographic site. Wetland DOM composition was driven primarily by differences in landcover where forested sites contained greater aromatic and oxygenated DOM content compared to grassland/herbaceous sites which were more aliphatic and enriched in N and S molecular formulae. Furthermore, surface and porewater DOM was also influenced by properties such as soil type, organic matter content, and precipitation. Surface water DOM was relatively enriched in oxygenated higher molecular weight formulae representing HUPHigh O/C compounds than porewaters, whose DOM composition suggests abiotic sulfurization from dissolved inorganic sulfide. Finally, we identified a group of persistent molecular formulae (3,489) present across all sites and sampling depths (i.e., the signature of wetland DOM) that are likely important for riverine‐to‐coastal DOM transport. As anthropogenic disturbances continue to impact temperate wetlands, this study highlights drivers of DOM composition fundamental for understanding how wetland organic carbon will change, and thus its role in biogeochemical cycling.
Plain Language Summary
Dissolved organic matter (DOM) is often the most reactive form of organic carbon in wetlands, but its molecular characteristics and distribution are not well defined across different wetland types. We characterized DOM and analyzed dissolved organic carbon (DOC) concentrations across 12 temperate USA wetlands during wet and dry seasons from both surface‐ and porewaters. Wetland DOM primarily originates from the landscapes with greater DOC concentrations than similar inland waters such as lakes and rivers. DOM composition differs mostly by geographic site, suggesting that forested wetlands contain more aromatic DOM compounds from vegetation and soil than grassland/herbaceous wetlands, which contain DOM that is more processed. DOM composition between surface and porewaters is influenced by local ecological properties such as soil content and precipitation, with porewater compositions heavily impacted by mineral interactions. Finally, we identified common molecular signatures across all wetlands that have also been found in the largest arctic rivers, highlighting the role of wetlands as potential organic carbon sources to rivers and coastal systems. As precipitation and temperature patterns continue to change across temperate regions, the balance between different carbon pools will likely respond, particularly between the distribution and composition of wetland surface and porewater organic matter.
Key Points
Wetlands contain higher dissolved organic carbon (DOC) concentrations and greater aromatic dissolved organic matter (DOM) composition than other inland waters across the USA
Wetland DOM composition differs spatially between forested and grassland/herbaceous landcover as well as between surface and porewaters
Persistent molecular formulae are observed across all wetlands that may be important for riverine‐to‐coastal DOM cycling
Northern high‐latitude lakes are hotspots for cycling dissolved organic carbon (DOC) inputs from allochthonous sources to the atmosphere. However, the spatial distribution of lake dissolved organic ...matter (DOM) is largely unknown across Arctic‐boreal regions with respect to the surrounding landscape. We expand on regional studies of northern high‐latitude DOM composition by integrating DOC concentrations, optical properties, and molecular‐level characterization from lakes spanning the Canadian Taiga to the Alaskan Tundra. Lakes were sampled during the summer from July to early September to capture the growing season. DOM became more optically processed and molecular‐level aromaticity increased northward across the Canadian Shield to the southern Arctic and from interior Alaska to the Tundra, suggesting relatively greater DOM incorporation from allochthonous sources. Using water isotopes (δ18O‐H2O), we report a weak overall trend of increasing DOC and decreasing aromaticity in lakes that were hydrologically isolated from the landscape and enriched in δ18O‐H2O, while within‐region trends were stronger and varied depending on the landscape. Finally, DOC correlated weakly with chromophoric dissolved organic matter (CDOM) across the study sites, suggesting that autochthonous and photobleached DOM were a major component of the DOC in these regions; however, some of the northernmost and wetland‐dominated lakes followed pan‐Arctic riverine DOC‐CDOM relationships, indicating strong contributions from allochthonous inputs. As many lakes across the North American Arctic are experiencing changes in temperature and precipitation, we expect the proportions of allochthonous and autochthonous DOM to respond with aquatic optical browning with greater landscape connectivity and more internally produced DOM in hydrologically isolated lakes.
Plain Language Summary
As the Arctic responds to warming, permafrost thaw, and variations in precipitation, the distribution of carbon pools within northern high‐latitude lakes will also change. Specifically, the composition of dissolved organic matter (DOM) and how it is altered and moved from the landscape to the atmosphere will be highly dependent on local precipitation patterns and hydrology, but these relationships are not well constrained across large regions. We sampled over 70 individual lakes during the summer spanning various ecoregions from interior Canada to the Alaskan Tundra and characterized their dissolved organic carbon (DOC) concentrations and DOM composition using bulk and molecular‐level analysis. Overall, DOM from these lakes was highly influenced by aquatic primary production but increased in the relative proportion of terrestrially derived organic matter as lake setting transitioned from forests to shrublands above the tree line. We also report a weak relationship between increasing DOC and decreasing terrestrial DOM as lakes become more hydrologically isolated across the pan‐Arctic; however, regional trends were stronger within forested sampling areas and weaker in shrublands. With the hydrologic setting of many northern high‐latitude lakes predicted to change in the coming decades, we expect the proportions of land‐ and aquatic‐derived DOM to respond as well.
Key Points
Dissolved organic matter in high‐latitude Canadian lakes increases in aromaticity northward as forests transition to shrublands
High‐latitude lakes that are hydrologically connected to the landscape have lower dissolved organic carbon (DOC) and are more allochthonous than isolated lakes
DOC in many northern high‐latitude lakes was influenced by autochthonous production and correlated weakly with absorbance
IMPORTANCE: Factors associated with clinical heterogeneity in Alzheimer disease (AD) lay along a continuum hypothesized to associate with tangle distribution and are relevant for understanding glial ...activation considerations in therapeutic advancement. OBJECTIVES: To examine clinicopathologic and neuroimaging characteristics of disease heterogeneity in AD along a quantitative continuum using the corticolimbic index (CLix) to account for individuality of spatially distributed tangles found at autopsy. DESIGN, SETTING, AND PARTICIPANTS: This cross-sectional study was a retrospective medical record review performed on the Florida Autopsied Multiethnic (FLAME) cohort accessioned from 1991 to 2020. Data were analyzed from December 2022 to December 2023. Structural magnetic resonance imaging (MRI) and tau positron emission tomography (PET) were evaluated in an independent neuroimaging group. The FLAME cohort includes 2809 autopsied individuals; included in this study were neuropathologically diagnosed AD cases (FLAME-AD). A digital pathology subgroup of FLAME-AD cases was derived for glial activation analyses. MAIN OUTCOMES AND MEASURES: Clinicopathologic factors of heterogeneity that inform patient history and neuropathologic evaluation of AD; CLix score (lower, relative cortical predominance/hippocampal sparing vs higher, relative cortical sparing/limbic predominant cases); neuroimaging measures (ie, structural MRI and tau-PET). RESULTS: Of the 2809 autopsied individuals in the FLAME cohort, 1361 neuropathologically diagnosed AD cases were evaluated. A digital pathology subgroup included 60 FLAME-AD cases. The independent neuroimaging group included 93 cases. Among the 1361 FLAME-AD cases, 633 were male (47%; median range age at death, 81 54-96 years) and 728 were female (53%; median range age at death, 81 53-102 years). A younger symptomatic onset (Spearman ρ = 0.39, P < .001) and faster decline on the Mini-Mental State Examination (Spearman ρ = 0.27; P < .001) correlated with a lower CLix score in FLAME-AD series. Cases with a nonamnestic syndrome had lower CLix scores (median IQR, 13 9-18) vs not (median IQR, 21 15-27; P < .001). Hippocampal MRI volume (Spearman ρ = −0.45; P < .001) and flortaucipir tau-PET uptake in posterior cingulate and precuneus cortex (Spearman ρ = −0.74; P < .001) inversely correlated with CLix score. Although AD cases with a CLix score less than 10 had higher cortical tangle count, we found lower percentage of CD68-activated microglia/macrophage burden (median IQR, 0.46% 0.32%-0.75%) compared with cases with a CLix score of 10 to 30 (median IQR, 0.75% 0.51%-0.98%) and on par with a CLix score of 30 or greater (median IQR, 0.40% 0.32%-0.57%; P = .02). CONCLUSIONS AND RELEVANCE: Findings show that AD heterogeneity exists along a continuum of corticolimbic tangle distribution. Reduced CD68 burden may signify an underappreciated association between tau accumulation and microglia/macrophages activation that should be considered in personalized therapy for immune dysregulation.
To understand patterns in CO
2
partial pressure (P
CO2
) over time in wetlands’ surface water and porewater, we examined the relationship between P
CO2
and land–atmosphere flux of CO
2
at the ...ecosystem scale at 22 Northern Hemisphere wetland sites synthesized through an open call. Sites spanned 6 major wetland types (tidal, alpine, fen, bog, marsh, and prairie pothole/karst), 7 Köppen climates, and 16 different years. Ecosystem respiration (R
eco
) and gross primary production (GPP), components of vertical CO
2
flux, were compared to P
CO2
, a component of lateral CO
2
flux, to determine if photosynthetic rates and soil respiration consistently influence wetland surface and porewater CO
2
concentrations across wetlands. Similar to drivers of primary productivity at the ecosystem scale, P
CO2
was strongly positively correlated with air temperature (T
air
) at most sites. Monthly average P
CO2
tended to peak towards the middle of the year and was more strongly related to R
eco
than GPP. Our results suggest R
eco
may be related to biologically driven P
CO2
in wetlands, but the relationship is site-specific and could be an artifact of differently timed seasonal cycles or other factors. Higher levels of discharge do not consistently alter the relationship between R
eco
and temperature normalized P
CO2
. This work synthesizes relevant data and identifies key knowledge gaps in drivers of wetland respiration.
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