Jeffrey M. Heikoop’s research while affiliated with Los Alamos National Laboratory and other places

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Publications (85)


Location of the Teller and Kougarok field sites with respect to the municipalities of Teller, Nome, and Council. All are located on the Seward Peninsula in northwestern Alaska. RGB composite imagery from the 8-band WorldView-2 imagery obtained on 14 July 2017 at 1.5 m resolution downloaded from the DigitalGlobe website (https://www.digitalglobe.com/, last access: 14 July 2017).
Topographic map of Teller. Station areas are shown as red polygons and the topographic station transect is given as a solid blue line. The hillslope transect elevation profile is given below the map in green, with stations along the transect in blue and hillslope positions noted with red arrows and text. RGB composite imagery from the 8-band WorldView-2 imagery obtained on 27 July 2011 at 1.5 m resolution downloaded from the DigitalGlobe website (https://www.digitalglobe.com/, last access: 27 July 2011).
Topographic map of Kougarok. Station areas are shown as red polygons and the station transect is given as a solid blue line. The transect elevation profile is given below the map in green, with stations along the transect in yellow and hillslope positions noted with red arrows and text. RGB composite imagery from the 8-band WorldView-2 imagery obtained on 14 July 2017 at 1.5 m resolution downloaded from the DigitalGlobe website (https://www.digitalglobe.com/, last access: 14 July 2017).
Mean COI concentrations at Teller (blue) and Kougarok (yellow) stations. Stations are arranged (left to right) by soil moisture content determined by P-band SAR (top right). Boxplots show the first, second, and third data quartiles, with box whiskers representing either 150 % of the interquartile range (IQR), or the maximum or minimum value, when that value was less than 1.5 × IQR. Red circles represent data points outside of the 1.5 × IQR whiskers (i.e., outliers). Note that the concentration scales on the Teller and Kougarok plots often differ.
Median (50th percentile) concentrations (gray diamonds with dashed black lines) of Ca, Sr, and Mg, with distance downslope at Teller (blue) and Kougarok (yellow) along topographic transects; areas of stations are indicated by blue and yellow coloring, respectively. The elevation profiles of the hillslopes are plotted in green on separate y axes (right axes). Topographic regions of both catchments are indicated by red arrows along the elevation gradient.

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Environmental controls on observed spatial variability of soil pore water geochemistry in small headwater catchments underlain with permafrost
  • Article
  • Full-text available

September 2023

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96 Reads

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1 Citation

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Jeffrey M. Heikoop

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Emma Lathrop

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[...]

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Soil pore water (SPW) chemistry can vary substantially across multiple scales in Arctic permafrost landscapes. The magnitude of these variations and their relationship to scale are critical considerations for understanding current controls on geochemical cycling and for predicting future changes. These aspects are especially important for Arctic change modeling where accurate representation of sub-grid variability may be necessary to predict watershed-scale behaviors. Our research goal is to characterize intra- and inter-watershed soil water geochemical variations at two contrasting locations in the Seward Peninsula of Alaska, USA. We then attempt to identify the key factors controlling concentrations of important pore water solutes in these systems. The SPW geochemistry of 18 locations spanning two small Arctic catchments was examined for spatial variability and its dominant environmental controls. The primary environmental controls considered were vegetation, soil moisture and/or redox condition, water–soil interactions and hydrologic transport, and mineral solubility. The sampling locations varied in terms of vegetation type and canopy height, presence or absence of near-surface permafrost, soil moisture, and hillslope position. Vegetation was found to have a significant impact on SPW NO3- concentrations, associated with the localized presence of nitrogen-fixing alders and mineralization and nitrification of leaf litter from tall willow shrubs. The elevated NO3- concentrations were, however, frequently equipoised by increased microbial denitrification in regions with sufficient moisture to support it. Vegetation also had an observable impact on soil-moisture-sensitive constituents, but the effect was less significant. The redox conditions in both catchments were generally limited by Fe reduction, seemingly well-buffered by a cache of amorphous Fe hydroxides, with the most reducing conditions found at sampling locations with the highest soil moisture content. Non-redox-sensitive cations were affected by a wide variety of water–soil interactions that affect mineral solubility and transport. Identification of the dominant controls on current SPW hydrogeochemistry allows for qualitative prediction of future geochemical trends in small Arctic catchments that are likely to experience warming and permafrost thaw. As source areas for geochemical fluxes to the broader Arctic hydrologic system, geochemical processes occurring in these environments are particularly important to understand and predict with regards to such environmental changes.

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Representation of experimental design. Generation 0 (G0) describes the water conditioning phase where the microbiomes were either well-watered (Ww; large water drop) or water limited (WL; small water drop). The parent microbiomes in the conditioning phase originated from either agricultural (MAg; yellow) or forest (MForest; green) soil. Soil was collected from each conditioning phase pot and the microbiome was transferred to a new pot and soil for Generation 1 (G1). Half of the pots containing WW-conditioned microbiome were selected at random to receive WW watering treatment again at G1, while the remaining pots containing WW-conditioned microbiome were instead given WL watering treatment at G1. Likewise, half of the pots containing WL-conditioned microbiome received WW watering treatment and half received WL watering treatment at G1. This obtains all eight combinations of microbial conditioning treatment, watering treatment, and microbiome parent source (shown in the bottom row of the figures) specified by the experimental design. Image courtesy of LANL. Don Montoya
Plant growth trait response differences between the two factor level means for each of the three experimental factors: microbiome conditioning (G0), watering treatment (G1), and microbiome parent. Effects on plant height (a), stem diameter (b), dry root mass (c), fine root length (d), mid-diameter root length (e), coarse root length (f), root surface area (g), and specific root length (h). Bars in figures show the differences in mean response between plants treated with well-watered (WW) and water-limited (WL) conditioning (light blue); WW and WL watering treatment (dark blue); and microbiomes originating from forest (MForest) and agricultural (MAg) soil (brown). Factor level mean differences are not shown for characteristics with hypothesis test results for main effects with p > 0.05 (Table 2)
Three-factor interaction plots for response of interest (a) saturated photosynthesis (Asat) and (b) stomatal conductance (gs) (b). The left panels contain watering treatment-by-microbiome two-factor interaction plots when the third factor, conditioning, is WL. The right panels contain watering treatment-by-microbiome two-factor interaction plots when the third factor, conditioning, is WW. The x-axis indicates the two water treatment levels, WL or WW. Letters indicate significant Tukey posthoc analysis differences between means. Interaction plots were based on the significant three-way interaction between microbiome, watering treatment, and conditioning shown in Table 2 (Asatp = 0.006, gsp = 0.031)
Plant function response differences between the two-factor level means for each of the three experimental factors: microbiome conditioning (G0), watering treatment (G1), and microbiome parent. Effects on intrinsic water use efficiency (WUEi; a), photosynthetic nitrogen use efficiency (PNUE; b), time to stomatal closure (c), pore water dissolved organic carbon (DOC; d), and nitrate (NO3⁻; e). Bars in figures show the differences in mean response between plants treated with well-watered (WW) and water-limited (WL) conditioning (light blue); WW and WL watering treatment (dark blue); and microbiomes originating from forest (MForest) and agricultural (MAg) soil (brown). Factor level mean differences are not shown for characteristics with hypothesis test results for main effects with p > 0.05 (Table 1)
Volumetric water content (VWC) decline during the terminal drought. Normalized to highest content per pot, the rate that VWC declined until the plant reached the wilting point where stomatal conductance = 0 for plants that received the well-water conditioning (WW; black) or water limited conditioning (WL; gray) microbiomes. Symbols represent the current watering treatment: WL (circle) or WW (triangle)
Drought conditioning of rhizosphere microbiome influences maize water use traits

August 2023

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172 Reads

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9 Citations

Plant and Soil

Background and Aims Beneficial plant–microbe interactions can improve plant performance under drought; however, we know less about how drought-induced shifts in microbial communities affect plant traits. Methods We cultivated Zea mays in fritted clay with soil microbiomes originating from contrasting environments (agriculture or forest) under two irrigation treatments (well-watered or water limited). Using this design, we investigated whether water conditioning was carried forward through the microbiome to affect a subsequent plant cohort that was subjected to either a well-watered or water limited treatment. Results Regardless of the microbiome-origin, plants inoculated with a microbiome from a water limited legacy had traits that allowed them to avoid stress but conserve water. They produced longer roots to explore soil, generated greater soil dissolved organic carbon, potentially stimulating the microbiome, and slower soil water content loss during drought. A well-watered legacy resulted in plants that delayed permanent stomatal closure and higher photosynthetic nitrogen use efficiency. In plants with a forest-originated microbiome, a well-watered legacy and water treatment also resulted in higher rates of photosynthesis and stomatal conductance. Conclusion These results demonstrate that soil microbiomes can be developed to influence plant drought performance, impacting crop resilience, using short-term microbial conditioning.


Vadose Zone Transport of Tritium and Nitrate under Ponded Water Conditions

July 2022

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35 Reads

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1 Citation

Geosciences

Vadose zone transport of tritium and nitrate can be important considerations at radioactive waste sites, landfills, or areas with industrial impacts. These contaminants are of particular concern because they typically have a relatively higher mobility in the subsurface compared to other compounds. Here, we describe a semiarid site with tritium and nitrate contamination involving a manmade ponded water source above a thick unsaturated zone at Los Alamos National Laboratory in New Mexico, USA. This study demonstrates the value of vadose zone flow and transport modeling for the development of field investigation plans (i.e., identifying optimal borehole locations and depths for contaminant characterization), and how a combination of modeling with isotope and geochemical measurements can provide insight into how tritium and nitrate transport in the vadose zone in semiarid environments. Modeling results suggest that at this location, tritium transport is well predicted by classical multiphase theory. Our work expands the demonstrated usefulness of a standard tritium conceptual model to sites with ponded surface conditions and agrees with previous results where a standard model was able to explain the evolution of a tritium plume at an arid waste disposal site. In addition, depth-based analyses of δ18O and δ2H of pore waters helped confirm the extent of pond infiltration into the vadose zone, and the δ15N of nitrate showed that the contaminant release history of the site was different than originally assumed.


Figure 3. Ranges of moisture content (A) and NO 3 − concentration (B) distributed by unsaturated polygonal microtopographic features (high-centers, flat-centers, rims). The data used for the ranges displayed in (A,B) were not co-located but were distributions from each unsaturated feature for all NO 3 − and moisture data collected. The whisker extent and lines within the grey boxes indicate the
Increased Arctic NO3− Availability as a Hydrogeomorphic Consequence of Permafrost Degradation and Landscape Drying

May 2022

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57 Reads

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2 Citations

Nitrogen

Climate-driven permafrost thaw alters the strongly coupled carbon and nitrogen cycles within the Arctic tundra, influencing the availability of limiting nutrients including nitrate (NO3−). Researchers have identified two primary mechanisms that increase nitrogen and NO3− availability within permafrost soils: (1) the ‘frozen feast’, where previously frozen organic material becomes available as it thaws, and (2) ‘shrubification’, where expansion of nitrogen-fixing shrubs promotes increased soil nitrogen. Through the synthesis of original and previously published observational data, and the application of multiple geospatial approaches, this study investigates and highlights a third mechanism that increases NO3− availability: the hydrogeomorphic evolution of polygonal permafrost landscapes. Permafrost thaw drives changes in microtopography, increasing the drainage of topographic highs, thus increasing oxic conditions that promote NO3− production and accumulation. We extrapolate relationships between NO3− and soil moisture in elevated topographic features within our study area and the broader Alaskan Coastal Plain and investigate potential changes in NO3− availability in response to possible hydrogeomorphic evolution scenarios of permafrost landscapes. These approximations indicate that such changes could increase Arctic tundra NO3− availability by ~250–1000%. Thus, hydrogeomorphic changes that accompany continued permafrost degradation in polygonal permafrost landscapes will substantially increase soil pore water NO3− availability and boost future fertilization and productivity in the Arctic.


The Kougarok field site and sampling locations. Upper left inset map indicates the location of the Kougarok field site (denoted by a black circle) approximately 80 km inland from the town of Nome on the Seward Peninsula, AK. (a) Alder patch and transect locations at the Kougarok hillslope. Solid white lines represent middle (M) and road (R) sampling transects. Dashed white line represents the boundary between the upland area and the lowland area. (b) Higher-resolution map (corresponding to the red box in Fig. 1a) of the spatial sampling locations within the A1 and A4 transects. Phase 1 (July 2017) locations are denoted by purple dots. Green dots indicate additional locations sampled during Phase 2 (September 2017, July 2018, and September 2018). P1, P2, and P3 indicate pit locations dug in July 2018. Dashed white line represents the boundary between the upland area and the lowland area. Aerial imagery in this figure are sourced from Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community.
Map of mean NO3-N concentrations from the Phase 1 and 2 sampling locations, where yellow indicates low concentrations and red indicates high concentrations (see the key for ranges; scales differ slightly). (a) Circles represent NO3-N concentrations at locations along the A1–A5, middle, and road transects in July 2017. (b) Dots represent average NO3-N concentrations along the A1 and A4 transects and between the transects over all sampling campaigns. The green ellipses indicate samples collected within the alder shrubland, as opposed to outside the alder shrubland. Aerial imagery in this figure is sourced from Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community.
Soil pore water NO3-N time-series plots from the A1 transect. “Within” and “Downslope” denote sample locations within and downslope of shrublands along the transect; “Seep” denotes a seep in the ground located on the A1 transect at the transition between upland and lowland. Standard deviations are displayed as black brackets on each bar. (a) July 2017 daily NO3-N concentrations. (b) September 2017 daily NO3-N concentrations. (c) July 2018 daily NO3-N concentrations. Days marked with an asterisk (*) indicate precipitation events.
Elevation profiles and chemical concentrations along the (a) A1 and (b) A4 transects extending from the upland area to the lowland area during July (2018) sampling. Note that the elevation scale is different for the A1 and A4 transects. The horizontal axis and shrubs are not to scale. Depth to frozen soil or bedrock is depicted by the dashed blue line, and shrubs indicate sample sites located within the alder shrubland. Redox species – nitrate (NO3-N), manganese (Mn), iron (Fe2+), and sulfate (SO42-) – are plotted along the secondary y axis. See Tables S5–S6 in the associated Supplement for corresponding concentrations, n values, and statistics.
High nitrate variability on an Alaskan permafrost hillslope dominated by alder shrubs

May 2022

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39 Reads

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6 Citations

In Arctic ecosystems, increasing temperatures are driving the expansion of nitrogen (N) fixing shrubs across tundra landscapes. The implications of this expansion to the biogeochemistry of Arctic ecosystems are of critical importance and more work is needed to better understand the form, availability, and transportation potential of N from these shrubs across a variety of Arctic landscapes. To gain insights into the processes controlling N within a permafrost hillslope system, the spatiotemporal variability of nitrate (NO3-) and its environmental controls were investigated at an alder (Alnus viridis spp. fruticosa) dominated permafrost tundra landscape in the Seward Peninsula, Alaska, USA. Soil pore water was collected from locations within alder shrubland growing along a well-drained hillslope and was compared to soil pore water collected from locations outside (upslope, downslope, and between) the alder shrubland. Soil pore water collected within alder shrubland had an average NO3-N (nitrogen from nitrate) concentration of 4.27±8.02 mg L-1 and differed significantly from locations outside alder shrubland (0.23±0.83 mg L-1; p<0.05). Temporal variation in NO3-N within and downslope of alder shrubland co-occurred with precipitation events where NO3- that accumulated in the soil was likely flushed downslope during rainfall. These findings have important implications for nutrient availability and mobility in N-limited permafrost systems that are experiencing shrub expansion in response to a warming Arctic.


Environmental Controls on Observed Spatial Variability of Soil Pore Water Geochemistry in Small Headwater Catchments Underlain with Permafrost

May 2022

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12 Reads

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1 Citation

Soil pore water (SPW) chemistry can vary substantially across multiple scales in Arctic permafrost landscapes. The magnitude of these variations and their relationship to scale are critical considerations for understanding current controls on geochemical cycling and for predicting future changes. These aspects are especially important for Arctic change modelling where accurate representation of sub-grid variability may be necessary to predict watershed scale behaviours. Our research goal was to characterize intra- and inter-watershed soil water geochemical variations at two contrasting locations in the Seward Peninsula of Alaska, USA. We then attempt to establish which environmental factors were important for controlling concentrations of important pore water solutes in these systems. The SPW geochemistry of 18 locations spanning two small Arctic catchments were examined for spatial variability and its dominant environmental controls. The primary environmental controls considered were vegetation, soil moisture/redox condition, water/soil interactions and hydrologic transport, and mineral solubility. The sampling locations varied in terms of vegetation type and canopy height, presence or absence of near-surface permafrost, soil moisture, and hillslope position. Vegetation was found to have a significant impact on SPW NO3 concentrations, associated with the localized presence of nitrogen-fixing alders and mineralization and nitrification of leaf litter from tall willow shrubs. The elevated NO3 concentrations were however, frequently equipoised by increased microbial denitrification in regions with sufficient moisture to support it. Vegetation also had an observable impact on soil moisture sensitive constituents, but the effect was less significant. The redox conditions in both catchments were generally limited by Fe reduction, seemingly well-buffered by a cache of amorphous Fe hydroxides, with the most reducing conditions found at sampling locations with the highest soil moisture content. Non-redox-sensitive cations were affected by a wide variety of water-soil interactions that affect mineral solubility and transport. Identification of the dominant controls on current SPW hydrogeochemistry allows for qualitative prediction of future geochemical trends in small Arctic catchments that are likely to experience warming and permafrost thaw. As source areas for geochemical fluxes to the broader Arctic hydrologic system, geochemical processes occurring in these environments are particularly important to understand and predict with regards to such environmental changes.


Chemostatic Concentration‐Discharge Behavior Observed in a Headwater Catchment Underlain with Discontinuous Permafrost

May 2022

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19 Reads

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6 Citations

Hydrological Processes

Concentration‐discharge dynamics were evaluated in a small (~ 2.25 km2) headwater catchment underlain with discontinuous permafrost on the Seward Peninsula of western Alaska. A large storm, during which 48 mm of rain fell over a 24 hour period, enabled the evaluation of solute concentration‐discharge response to a sizeable hydrological event, while water stable isotopes enabled an appraisal of the contributions of event water. Under normal catchment conditions, chemostatic behavior was observed for solutes typically derived from mineral weathering (e.g. calcium, magnesium, sodium, and silica). The chemostatic behavior observed for most solutes under normal catchment conditions indicated that catchment storage and residence times are sufficiently long for many solute generating reactions to approach equilibrium. Following the storm however, most solutes exhibited dilutive and highly variable behavior. This likely indicated the exceedance of a discharge threshold where chemostatic behavior could no longer be maintained for most solutes. Dissolved organic carbon and silica were the only solutes monitored to exhibit chemostatic behavior during all time periods. This article is protected by copyright. All rights reserved.


Age and chemistry of dissolved organic carbon reveal enhanced leaching of ancient labile carbon at the permafrost thaw zone

February 2022

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141 Reads

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6 Citations

Climate change will alter the balance between frozen and thawed conditions in Arctic systems. Increased temperatures will make the extensive northern permafrost carbon stock vulnerable to decomposition and translocation. Production, cycling, and transport of dissolved organic carbon (DOC) are crucial processes for high-latitude ecosystem carbon loss that result in considerable export off the Arctic landscape. To identify where and under what conditions permafrost DOC is mobilized in an Arctic headwater catchment, we measured radiocarbon (14C) of DOC and assessed DOC composition with ultraviolet–visible spectroscopy (UV–Vis) of surface waters and shallow and deep subsurface porewaters from 17 drainages in the Barrow Environmental Observatory in Alaska. Samples were collected in July and September 2013 to assess changes in age and chemistry of DOC over time. DOC age was highly variable ranging from modern (19 ‰ Δ14C) to approximately 7000 BP (-583 ‰ Δ14C). DOC age increased with depth, over the summer as the active layer deepened, and with increasing drainage size. DOC quality indicators reflected a DOC source rich in high molecular-weight and aromatic compounds, characteristics consistent with vegetation-derived organic matter that had undergone little microbial processing, throughout the summer and a weak relationship with DOC age. In deep porewaters, DOC age was also correlated with several biogeochemical indicators (including dissolved methane concentration, δ13C, and the apparent fractionation factor), suggesting a coupling between carbon and redox biogeochemistry influencing methane production. In the drained thawed lake basins included in this study, DOC concentrations and contributions of vegetation-derived organic matter declined with increasing basin age. The weak relationship between DOC age and chemistry and consistency in DOC chemical indicators over the summer suggest a high lability of old DOC released by thawing permafrost.




Citations (61)


... However, the composition and structure of microbial communities are affected by environmental conditions and edaphic properties (Pascual et al. 2018) and the response of the plantmicrobiome complex to extreme events such as floods and droughts is still not clearly understood (Francioli et al. 2021). In respect to drought, some authors showed that soils afflicted by a history of droughts exhibit consistently lower microbial respiration rates, altered microbial community composition and functional response distributions (Veach and Zeglin 2020) but, on the other hand, it is possible to develop the soil microbiome through soil management to positively influence plant drought performance (Carter et al. 2023) and the use of microbes to improve ecosystem functions for specific functions has merited increased attention (de Vries et al. 2020). A combination of drought and heat stress might have a stronger negative influence on plant production (Cohen et al. 2021). ...

Reference:

Editorial: Belowground adaptation of plants to climate change
Drought conditioning of rhizosphere microbiome influences maize water use traits

Plant and Soil

... Unfortunately, due to saturation variability both techniques could not be used at all stations and conditions at some Kougarok stations were sometimes too dry to collect meaningful volumes of SPW using either method. Additional SPW data from Kougarok were supplemented from a separate study focused on alder-related nutrient dynamics (McCaully et al., 2022). These data were collected by MacroRhizons and are captured as Kougarok stations 10-13, which were not among the original stations established by the NGEE Arctic program. ...

High nitrate variability on an Alaskan permafrost hillslope dominated by alder shrubs

... The expression of changing river chemistry at the event scale can reflect the dominant sources and pathways of material as they are transferred from terrestrial zones to the stream channel (Godsey et al., 2009Moatar et al., 2017). While this perspective has been widely applied across temperate regions, the combination of technological advancements and increasing interest in capturing processes that control the transport of material across the terrestrial-aquatic nexus have been recently applied in permafrost-underlain watersheds (Conroy et al., 2022;Khamis et al., 2021;Shogren et al., 2021;Webster et al., 2021). Namely, the application of concentration-discharge (hereafter C-Q) metrics from high-frequency time-series data has revealed the dominant landscape and seasonal controls on lateral C and N exports in Arctic and Boreal watersheds (Khamis et al., 2021;Shogren et al., 2021;Webster et al., 2021). ...

Chemostatic Concentration‐Discharge Behavior Observed in a Headwater Catchment Underlain with Discontinuous Permafrost
  • Citing Article
  • May 2022

Hydrological Processes

... Once thawed, the ancient organic carbon in permafrost soils can be respired to carbon dioxide (CO 2 , a greenhouse gas; Schuur et al., 2015;Vaughn & Torn, 2019), contributing to the arctic amplification of climate change (McGuire et al., 2018). Permafrost thaw and thaw slump disturbances (thermokarsts; Kokelj & Jorgenson, 2013;Olefeldt et al., 2016) may also increase the lateral export of old, previouslyfrozen dissolved organic carbon (DOC) from permafrost soils to surface waters (Frey & McClelland, 2009;McFarlane et al., 2022;Plaza et al., 2019). DOC draining from permafrost soils to arctic streams is labile to microbial respiration (Drake et al., 2015;Mann et al., 2015;Spencer et al., 2015), particularly after the composition of this DOC has been altered by sunlight exposure (i.e., coupled photochemical and microbial degradation of DOC; Cory et al., 2013;Ward et al., 2017). ...

Age and chemistry of dissolved organic carbon reveal enhanced leaching of ancient labile carbon at the permafrost thaw zone

... The intermediate hydrochemistry of fen channels and streams, which connect to fen ponds and lakes, respectively, indicated additional hydrologic inputs to these fluvial components of the aquatic network. For instance, streams were more strongly associated with inorganic nutrients, especially NO 3 − , perhaps associated with nitrogen fixation by riparian alders and its subsequent nitrification and hydrologic export into streams (McCaully et al., 2021). ...

High Temporal and Spatial Nitrate Variability on an Alaskan Hillslope Dominated by Alder Shrubs

... The composition of ooidal ironstones can vary significantly and depend on the age and palaeoenvironmental conditions during deposition (Kalinina et al. 2024). In general, the mineralogy of Proterozoic ironstones is dominated by hematite but the Phanerozoic record of iron-rich sediments is dominated by goethite (Galili et al. 2019). The XRD and EDS analyses of both ironstone units discussed here reveal that the coated grains are mostly composed of goethite with some of siderite/ ankerite. ...

The geologic history of seawater oxygen isotopes from marine iron oxides

Science

... Instead, Teller and Kougarok differ in many respects and are both representative of the broad range of hillslope conditions common on the Seward Peninsula. Detailed descriptions of Teller and Kougarok have been published previously (Jafarov et al., 2018;Léger et al., 2019;Philben et al., 2019Philben et al., , 2020Salmon et al., 2019;Yang et al., 2020); therefore, only the catchment characteristics that are probable sources of variability in SPW chemistry will be highlighted here. Teller is a discrete catchment with a well-defined central drainage, a vertical declivity of approximately 200 m, and a catchment area of approximately 2.25 km 2 . ...

Stimulation of anaerobic organic matter decomposition by subsurface organic N addition in tundra soils
  • Citing Article
  • December 2018

Soil Biology and Biochemistry

... The conversion of bicarbonate C into algal biomass, even under non-optimized cultivation conditions, is much higher than that achieved with the use of CO 2 , whose values can range between 5.45 and 14.5% in optimized cultures of Nannochloropsis salina (Zidenga et al. 2018). Studies on the efficiency of bicarbonate as a C source in algal mass cultures are still in their early stage of development and further work is expected. ...

Carbon use efficiency diagnostics in Nannochloropsis salina
  • Citing Article
  • April 2018

Algal Research

... Halotolerant microalgae, particularly, have gained attention for their ability to adapt and proliferate in saline environments, making them prime candidates for the treatment of PW, which is often characterized by elevated salinity levels (TDS: 7744 to 38,000 mg/L) (Lutzu et al., 2024;Sullivan Graham et al., 2017;Zafar et al., 2021). In recent years, several studies have demonstrated the efficacy of various halotolerant microalgae in removing organic pollutants in PW. ...

Oil and gas produced water as a growth medium for microalgae cultivation: A review and feasibility analysis
  • Citing Article
  • February 2017

Algal Research