Micron-scale distribution controls metal(loid) release during simulated weathering of a Pennsylvanian coal shale
Abstract not provided
Singer, David M.↗
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Changing chemistry of particulate manganese in the near- and far-field hydrothermal plumes from 15°S East Pacific Rise and its influence on metal scavenging
Dissolved Mn(II) in the hydrothermal plume is known to be microbially oxidized to form Mn(III/IV) oxides, and the Mn oxides scavenge other trace elements in seawater. In the GEOTRACES GP16 cruise, dissolved Mn (dMn) and particulate Mn (pMn) were found to be transported over 4000 km westwards from the Southern Eastern Pacific Rise. Previous studies in this plume showed different removal rates of dMn and pMn as well as pMn size distribution between the near-field (<80 km from the ridge axis) and far-field (>80 km) plumes. In order to understand Mn cycling in these plumes, spatial distribution, oxidation states, and mineral structures of Mn in small size fraction (SSF; 0.8–51 μm) and large size fraction (LSF; >51 μm) particles from the near-field and far-field plumes were examined using micro X-ray fluorescence spectrometry (μ-XRF), X-ray absorption near-edge structure spectroscopy (XANES), chemical species mapping, and extended X-ray absorption fine-structure spectroscopy (EXAFS). In the near-field plume, pMn in the SSF is dominated by oxidized Mn with Mn(III) fractions of ~30%. They are a mixture of δ-MnO 2 and triclinic birnessites that is known to be formed as a result of autocatalytic Mn(II) oxidation at the surface of freshly-formed δ-MnO2, suggesting that both microbial and autocatalytic Mn oxidation occur in the near-field plume. The LSF pMn in the near-field plume is also oxidized and often found in large aggregates several hundreds of μm in size. These aggregates settle out in the near-field and during transport, and are not found in the far-field plume. In the far-field plume where Mn oxides are not newly formed, pMn in the SSF is oxidized, but their Mn(III) fractions are smaller than in the near-field pMn. Unlike the SSF, the far-field plume LSF pMn is dominated by reduced Mn, implying very slow aggregation of pMn in the far-field plume. The different characteristics of pMn between the near-field and far-field plumes affect its scavenging of other trace elements. In the near-field plume, Co, Mo, 231 Pa are associated with pMn, but not in the far-field plume. 231 Pa is adsorbed to pFe rather than pMn in the far-field plume, and Pb is adsorbed to pFe in the entire plume. The result shows that freshly-formed Mn oxides in the near-field plume have higher scavenging capacity than the far-field plume pMn. Our findings suggest that the mineralogical age of Mn oxides may be an important parameter that controls the scavenging of many other trace elements and isotopes.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Evaluating the Rheo-electric Performance of Aqueous Suspensions of Oxidized Carbon Black
Here, the macroscopic properties of carbon black suspensions are primarily determined by the agglomerate microstructure built of primary aggregates. Conferring colloidal stability in aqueous carbon black suspensions should thus have a drastic impact on their viscosity and conductivity. Carbon black was treated with strong acids following a wet oxidation procedure. An analysis of the resulting particle surface chemistry and electrophoretic mobility was performed in evaluating colloidal stability. Changes in suspension microstructure due to oxidation were observed using small-angle X-ray scattering. Utilizing rheo-electric measurements, the evolution of the viscosity and conductivity of the carbon black suspensions as a function of shear rate and carbon content was thoroughly studied. The carboxyl groups installed on the carbon black surface through oxidation increased the surface charge density and enhanced repulsive interactions. Electrostatic stability inhibited the formation of the large-scale agglomerates in favor of the stable primary aggregates in suspension. While shear thinning, suspension conductivities were found to be weakly dependent on the shear intensity regardless of the carbon content. Most importantly, aqueous carbon black suspensions formulated from electrostatically repulsive primary aggregates displayed a smaller rise in conductivity with carbon content compared to those formulated from attractive agglomerates.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Dispersion of high oxygen permeability ionomers in water-propanol solutions
There is a growing interest in high oxygen permeability ionomers (HOPI) for better-performing proton exchange membrane fuel cell (PEMFC) catalyst layers. In this study, we used small-angle x-ray scattering (SAXS) to analyze dispersions of two types of perfluorinated sulfonic acid (PFSA) ionomers and a HOPI in water and propanol solvents, at concentrations between 1 wt% and 15 wt%. HOPI dispersions exhibited the interference peak typical of ionomers; by separately fitting the peak and higher- q regions with several models, we demonstrated that HOPI aggregates have a closer average spacing than conventional PFSA ionomers, and smaller aggregate size when assuming a cylindrical geometry. By measuring conventional PFSA ionomers with two side chain lengths and several equivalent weights, we showed that these side chain parameters alone did not explain the differences in aggregate spacing or size. In addition, the pattern was present whether samples were prepared in a 50/50 water/propanol or a more propanol-rich mixture. As such, the structural differences of HOPI could be due to its different backbone monomer. A better understanding of the influence of the HOPI chemistry on dispersion properties could help explain the ink structure and drying characteristics of HOPI observed in prior work. Future study of ionomer-catalyst mixtures, such as measuring ionomer adsorption to carbon, would help elucidate the role of ionomer from ink formulation to the cast electrode.
Aquivion↗
Reduced alkali-silica reaction damage in recycled glass mortar samples with supplementary cementitious materials
Recycling waste glass aggregate into concrete can reduce environmental impacts but also may lead to serious alkali silica reaction (ASR) damage. This study aims to characterize ASR damage development in the mortar samples containing reactive glass aggregates and investigate the damage reduction effect of the waste supplementary cementitious materials (SCMs), including recycled glass powders and fly ash. The recycled glass aggregate mortar samples with/without SCMs were prepared in this study. The length change tests were first conducted with the prepared mortar samples based on the ASTM C1260 standard. The results demonstrated the added SCMs can largely decrease early-age ASR expansion rate. The optical microscope and Scanning Electron Microscope (SEM) with Energy Dispersive X-ray Spectroscopy (EDS) were further conducted to characterize the ASR damage in mortar samples and investigate the damage mitigation mechanism with added SCMs. The combined SEM imaging and chemical analysis indicated added SCMs can decrease both the alkali and calcium content of the generated ASR gel. In addition, the ASR damage development inside mortar samples were monitored with dynamic micron X-ray CT (μCT) over a reaction period of 63 hour at the temperature of 80°C. The scanning patterns demonstrated the ASR damage can be easily developed from the area with initial cracks and sharp corners. In conclusion, the images of glass power and fly ash samples showed significantly reduced ASR damage. This study showed that adding of SCMs can largely reduce ASR deterioration and thus facilitate the recycling glass particles into concrete.
32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
Molecular dynamics studies of lattice defect effects on tritium diffusion in zirconium
Tritium diffusion in α-Zr containing point defects such as vacancies or self-interstitial atoms (SIAs) is simulated using molecular dynamics. Point defects rapidly aggregate to form extended defects, such as 3D nanoclusters and Frank loops. The geometry of extended defects is affected by the presence of tritium. At low temperature and in the absence of tritium, vacancies aggregate to form stacking fault pyramids. Addition of tritium at these temperatures promotes aggregation of vacancies to form 3D nanoclusters, within which the tritium concentration can be sufficiently high to suggest that these defects may serve as nucleation sites for hydride precipitation. Trapping of tritium in vacancy nanocluster reduces the calculated bulk diffusivity by an amount proportional to the vacancy concentration. At high temperature, vacancy clusters change shape to form planar basal dislocation loops, which bind tritium less strongly, leading to a sharp reduction in the fraction of trapped tritium and a corresponding increase in tritium diffusivity at high temperature. In contrast, SIAs increase tritium diffusion through α-Zr. Analysis of atomic trajectories shows that tritium does not interact directly with SIAs. In conclusion, diffusion enhancement is instead related to expansion of the lattice.
36 MATERIALS SCIENCE↗
Occupant-driven end use load models for demand response and flexibility service participation of residential grid-interactive buildings
As demand response becomes increasingly used as a tool to support improved grid flexibility, it is important to consider that there are many potential types of energy end uses that may be used to support such flexibility. Residential appliances, often accounting for 30 % or more of residential energy use, are a currently untapped source of demand flexibility, particularly when aggregated together across homes. To date there has been very limited analysis of residential appliances for use as grid-interactive loads. As such, this research uses disaggregated energy end use data for 564 households, to model the electricity demand flexibility potential of the use of residential dishwashers, clothes washers, clothes dryers, ovens, and ranges (oven + stovetop) on both weekdays and weekends. This includes both at the building level, as well as aggregated to the grid level, specifically the Midcontinent Independent System Operator (MISO) region. This study was divided into two parts. Part 1 focuses on determining appliance-level loads, and Part 2, which involves aggregation to the grid. Findings suggest that among the studied appliances, clothes dryers provide the greatest demand reduction potential for most times of the day, followed by dishwashers and clothes washers. The maximum potential reduction for clothes dryers is found to be approximately at 11:00 a.m. and this potential sustains throughout most of the daytime period. When considering the willingness of households to participate, based on a survey of households in the Midwest region, clothes dryers still have the most potential for demand reduction. The availability of appliances for load modulation on weekdays and weekends indicates similar load reduction potential for all appliances. Overall, the results of this study suggest that there is an opportunity for shifting appliance usage to optimize grid efficiency and enhance demand response strategies.
32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
Effects of ultrasound-related variables on sonochemically synthesized SAPO-34 nanoparticles
The sonochemical method was developed to synthesize uniform SAPO-34 (silicoaluminophosphate molecular sieve) nanoparticles with high crystallinity using TEAOH as a structure-directing agent (SDA). The physicochemical characteristics of SAPO-34 products, i.e. crystallinity, particle size and shape can be controlled by varying the ultrasonic-related variable such as ultrasound power intensity, ultrasonic irradiation time, sonication temperature and geometrical characteristics of the ultrasonic device (e.g., sonotrode size). The products were characterized by XRD, SEM, TEM and BET. It is found that each of the parameters can play a significant role in acoustic cavitation, number of nuclei and the crystal growth. The experimental data establish that the crystallinity is related to ultrasonic intensity and diameter of the sonotrode, as well as sonication temperature. By increasing the ultrasonic power, duration and the sonication temperature, the mean sizes of particles decrease and the morphology of the products efficiently alters from spherical aggregates of cube type SAPO-34 particles to uniform spherical nanoparticles. - Graphical abstract: Increasing US power by increasing either US power intensity or the sonotrode diameter leads to smaller particle size and the morphology changes from spherical aggregates of cubic particles to uniform nanospheres. Highlights: • Effects of ultrasonic parameters on sonochemical synthesis of SAPO-34 nanoparticles. • The higher crystallinity by increasing ultrasonic power, duration and sonication temperature. • The morphology changes from spherical aggregates of cubic particles to uniform nanospheres. • Decreasing the particle size by increasing ultrasonic power, duration and sonication temperature.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Core–shell TiO₂ microsphere with enhanced photocatalytic activity and improved lithium storage
Inorganic hollow core–shell spheres have attracted considerable interest due to their singular properties and wide range of potential applications. Herein a novel facile generic strategy of combining template assisted and solvothermal alcoholysis is employed to prepare core–void–shell anatase TiO₂ nanoparticle aggregates with an excellent photocatalytic activity, and enhanced lithium storage in large quantities. Amorphous carbon can be loaded on the TiO₂ nanoparticles uniformly under a suitably formulated ethanol/water system in the solvothermal alcoholysis process, and the subsequent calcination results of the formation of core–shell–shell anatase TiO₂ nanoparticle aggregates. The intrinsic core–void–shell nature as well as high porosity of the unique nanostructures contributes greatly to the superior photocatalytic activity and improved performance as anode materials for lithium ion batteries. - Graphical abstract: A novel strategy of combining template assisted and solvothermal alcoholysis is employed to prepare unique core–void–shell anatase TiO₂ nanoparticle aggregates with the superior photocatalytic activity and improved lithium storage. Highlights: • TiO₂ mesospheres are synthesized by solvothermal alcoholysis. • It is core–void–shell structure and the thickness of shell is estimated to 80 nm. • It exhibits a remarkable photocatalytic activity and improved lithium storage.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Tailoring surface properties and structure of layered double hydroxides using silanes with different number of functional groups
Four silanes, trimethylchlorosilane (TMCS), dimethyldiethoxylsilane (DMDES), 3-aminopropyltriethoxysilane (APTES) and tetraethoxysilane (TEOS), were adopted to graft layered double hydroxides (LDH) via an induced hydrolysis silylation method (IHS). Fourier transform infrared spectra (FTIR) and {sup 29}Si MAS nuclear magnetic resonance spectra ({sup 29}Si MAS NMR) indicated that APTES and TEOS can be grafted onto LDH surfaces via condensation with hydroxyl groups of LDH, while TMCS and DMDES could only be adsorbed on the LDH surface with a small quantity. A combination of X-ray diffraction patterns (XRD) and {sup 29}Si MAS NMR spectra showed that silanes were exclusively present in the external surface and had little influence on the long range order of LDH. The surfactant intercalation experiment indicated that the adsorbed and/or grafted silane could not fix the interlamellar spacing of the LDH. However, they will form crosslink between the particles and affect the further surfactant intercalation in the silylated samples. The replacement of water by ethanol in the tactoids and/or aggregations and the polysiloxane oligomers formed during silylation procedure can dramatically increase the value of BET surface area (S{sub BET}) and total pore volumes (V{sub p}) of the products. - Graphical abstract: The replacement of water by ethanol in the tactoids and aggregations of LDHs, and the polysiloxane oligomers formed during silylation process can dramatically increase the BET surface area (S{sub BET}) and the total pore volume (V{sub p}) of the silylated products. - Highlights: • Silanes with multifunctional groups were grafted onto LDH surface in C{sub 2}H{sub 5}OH medium. • The number of hydrolysable groups in silanes affects the structure of grafted LDH. • Replacement of H{sub 2}O by C{sub 2}H{sub 5}OH in aggregations increases S{sub BET} and V{sub p} of grafted LDH. • Polysiloxane oligomers contribute to the increase of S{sub BET} and V{sub p} of grafted LDH.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Effects of Mo on microstructure of as-cast 28 wt.% Cr–2.6 wt.% C–(0–10) wt.% Mo irons
Microstructures of as-cast 28 wt.% Cr–2.6 wt.% C irons containing (0–10) wt.% Mo with the Cr/C ratio of about 10 were studied and related to hardness. The experimental irons were cast into dry sand molds. Microstructural investigation was performed by light microscopy, X-ray diffractometry, scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectrometry. It was found that the iron with about 10 wt.% Mo was eutectic/peritectic, whereas the others with less Mo content were hypoeutectic. The matrix in all irons was austenite, partly transformed to martensite during cooling. Mo addition promoted the formation of M{sub 23}C{sub 6} and M{sub 6}C. At 1 wt.% Mo, multiple eutectic carbides including M{sub 7}C{sub 3}, M{sub 23}C{sub 6} and M{sub 6}C were observed. M{sub 23}C{sub 6} existed as a transition zone between eutectic M{sub 7}C{sub 3} and M{sub 6}C, indicating a carbide transition as M{sub 7}C{sub 3}(M{sub 2.3}C) → M{sub 23}C{sub 6}(M{sub 3.8}C) → M{sub 6}C. At 6 wt.% Mo, multiple eutectic carbides including M{sub 7}C{sub 3} and M{sub 23}C{sub 6} were observed together with fine cellular/lamellar M{sub 6}C aggregates. In the iron with 10 wt.% Mo, only eutectic/peritectic M{sub 23}C{sub 6} and M{sub 6}C were found without M{sub 7}C{sub 3}. Mo distribution to all carbides has been determined to be increased from ca. 0.4 to 0.7 in mass fraction as the Mo content in the irons was increased. On the other hand, Cr distribution to all carbides is quite constant as ca. 0.6 in mass fraction. Mo addition increased Vickers macro-hardness of the irons from 495 up to 674 HV{sub 30}. High Mo content as solid-solution in the matrix and the formation of M{sub 6}C or M{sub 23}C{sub 6} aggregates were the main reasons for hardness increase, indicating potentially improved wear performance of the irons with Mo addition. - Highlights: • Mo promoted the formation of M{sub 23}C{sub 6} and M{sub 6}C in the irons with Cr/C ratio of about 10. • M{sub 23}C{sub 6} has higher Fe/Cr and Mo/Cr atm% ratios than M{sub 7}C{sub 3}. • Si content and high Mo content over about 9 atm% are characteristics of M{sub 6}C. • No M{sub 7}C{sub 3} was found at 10 wt.% Mo addition, only M{sub 23}C{sub 6} and M{sub 6}C presented. • High Mo matrices and M{sub 23}C{sub 6}/M{sub 6}C aggregates increased iron macro-hardness.
36 MATERIALS SCIENCE↗
Effect of silver ions and clusters on the luminescence properties of Eu-doped borate glasses
Highlights: • Ag{sup +} and Ag clusters are investigated in the borate glasses via ion exchange method. • The aggregation of silver ions to the clusters was controlled by the ion exchange concentration. • Eu{sup 3+}/Eu{sup 2+} ions emission was enhanced with the sensitization of the silver species. • Energy transfer process from Ag ions and Ag clusters to Eu ions is identified by the lifetime measurements. - Abstract: Silver ions and clusters were applied to Eu{sup 3+}-doped borate glasses via the Ag{sup +}–Na{sup +} ion exchange method. Eu{sup 3+}/Eu{sup 2+} ion luminescence enhancement was achieved after silver ion exchange. Absorption spectra showed no band at 420 nm, which indicates that silver nanoparticles can be excluded as a silver state in the glass. Silver ion aggregation into clusters during the ion exchange process may be inferred. The effect of silver ions and clusters on rare earth emissions was investigated using spectral information and lifetime measurements. Significant luminescence enhancements were observed from the energy transfer of Ag{sup +} ions and clusters to Eu{sup 3+}/Eu{sup 2+} ions, companied with the silver ions aggregated into the clusters state. The results of this research may extend the current understanding of interactions between rare-earth ions and Ag species.
36 MATERIALS SCIENCE↗
Room-temperature synthesis and characterization of carbon-encapsulated molybdenum nanoparticles
Highlights: • Carbon-encapsulated molybdenum nanoparticles were synthesized at room temperature. • Different hydrocarbons were used as carbon source for shell formation. • Increase of hydrocarbon mole ratio resulted in decrease of Mo-based nanoparticle cores. • Increase of laser pulses number resulted in increase of Mo content in CEMNPs. • Carbon shells prevented Mo nanoparticles from aggregation. - Abstract: Carbon-encapsulated molybdenum nanoparticles were synthesized by laser-assisted photolysis of gaseous precursors. In the proposed method, gas mixtures containing various hydrocarbons and molybdenum hexacarbonyl vapor diluted in argon were irradiated with a nanosecond pulsed UV laser at room temperature. Particle samples were analyzed by transmission electron microscopy (TEM), electron microdiffraction (EMD), energy-dispersive X-ray spectroscopy (EDS), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) techniques to observe the morphology and composition of the Mo/C nanoparticles. The influence of the type of hydrocarbon, its mole fraction, and the number of UV laser pulses on the resulting nanoparticles was investigated. The nanoparticles were heavily aggregated and consisted of Mo-based cores surrounded by carbon. The type of hydrocarbon did not have any visible effects on the nanoparticle structure and the size of the Mo-based cores or aggregates. Increasing the hydrocarbon mole fraction in the precursor mixture led to an increase in the carbon content and decrease of the Mo-based core size. Increasing the number of UV laser pulses led to larger Mo cores.
77 NANOSCIENCE AND NANOTECHNOLOGY↗
Polyamide-crosslinked graphene oxide membrane for forward osmosis
We report a novel polyamide-graphene oxide (PA-GO) membrane was synthesized on a polyethersulfone support by first intra-crosslinking GO aggregates via m-xylylenediamine (MXDA) and then inter-crosslinking GO aggregates via trimethyle chloride (TMC). This method allows for the use of hydrophilic, more porous supports in forward osmosis (FO) membranes, thereby lending much flexibility to membrane synthesis and also potentially reducing internal concentration polarization in FO. The elemental composition, morphology, and hydrophilicity of the synthesized PA-GO membrane were characterized to confirm intra- and inter-crosslinking reactions and understand membrane properties. It was found that the mixing temperature of MXDA and GO should be controlled below 20 °C to avoid the formation of large GO aggregates and hence reduce membrane heterogeneity and defects. The performance of the PA-GO membrane was compared with that of a commercial FO membrane using trisodium citrate (TSC), Na 2 SO 4 , and MgCl 2 , respectively, as draw solutes. Under the same osmotic pressure, the water flux of the PA-GO membrane was the highest with TSC as draw solute, moderate with Na 2 SO 4 , and almost zero with MgCl 2 , while the ranking of solute fluxes followed a reverse order, revealing that an effective solute barrier plays an important role in making FO membranes with high water flux and low solute flux.
42 ENGINEERING↗
A case study on parallel HDF5 dataset concatenation for high energy physics data analysis
In High Energy Physics (HEP), experimentalists generate large volumes of data that, when analyzed, helps us better understand the fundamental particles and their interactions. This data is often captured in many files of small size, creating a data management challenge for scientists. In order to better facilitate data management, transfer, and analysis on large scale platforms, it is advantageous to aggregate data further into a smaller number of larger files. However, this translation process can consume significant time and resources, and if performed incorrectly the resulting aggregated files can be inefficient for highly parallel access during analysis on large scale platforms. In this paper, we present our case study on parallel I/O strategies and HDF5 features for reducing data aggregation time, making effective use of compression, and ensuring efficient access to the resulting data during analysis at scale. We focus on NOvA detector data in this case study, a large-scale HEP experiment generating many terabytes of data. Here, the lessons learned from our case study inform the handling of similar datasets, thus expanding community knowledge related to this common data management task.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
A review of the potential impacts of climate change on bulk power system planning and operations in the United States
In this paper, climate change might impact various components of the bulk electric power system, including electricity demand; transmission; and thermal, hydropower, wind, and solar generators. Most research in this area quantifies impacts on one or a few components and does not link these impacts to effects on power system planning and operations. Here, we advance the understanding of how climate change might impact the bulk U.S. power system in three ways. First, we synthesize recent research to capture likely component-level impacts of climate change in the United States. Second, given the interconnected nature of the electric power system, we assess how aggregated component-level impacts might affect power system planning and operations. Third, we outline an agenda for future research on climate change impacts on power system planning and operations. Although component-level impacts vary in their magnitude, collectively they might significantly affect planning and operations. Most notably, increased demand plus reduced firm capacity across generation types might require systems to procure significant additional capacity to maintain planning reserve margins, and regional declines in renewable resources might need to be offset by increasing zero-carbon investment to meet decarbonization targets. Aggregated impacts might also affect operations, e.g., through shifts in dispatching and increased operational reserve requirements. Future research should aggregate component-level impacts at operational timescales, quantify impacts on wind and solar variability, and contextualize climate change impacts within ongoing shifts in the electric power system.
29 ENERGY PLANNING, POLICY, AND ECONOMY↗
Endogeic earthworm densities increase in response to higher fine-root production in a forest exposed to elevated CO 2
Net primary productivity (NPP) influences soil food webs and ultimately the amount of carbon (C) inputs in ecosystems. Earthworms can physically protect organic matter from rapid mineralization through the formation of soil aggregates. Previous studies at the Oak Ridge National Laboratory (ORNL) Free Air CO 2 Enrichment (FACE) experiment showed that elevated [CO 2 ] (e[CO 2 ]) increased fine-root production and increased soil C through soil aggregation compared to ambient [CO 2 ] (a[CO 2 ]) conditions. Our first objective was to study the response of earthworms to increased leaf and root-litter inputs caused by increased atmospheric [CO 2 ] exposure. Here, we also took advantage of the CO 2 shutdown at the ORNL FACE site to track the shift of the δ 13 C signal in leaf-litter, fine roots, earthworms, earthworm casts, and bulk soil. Densities of the most abundant endogeic earthworm, Diplocardia spp., were positively correlated with the previous-year production of leaf litter (r=0.66, P=0.02) and fine roots (r=0.62, P=0.03); and with the leaf-litter production (r=0.63, P=0.03) and fine-root production (r=0.59, P=0.05) two years before earthworms were sampled. Within two years after the CO 2 fumigation ceased, the 13 C/ 12 C ratio increased in leaf litter (P=0.01) and in fine roots (P=0.05), showing an ecosystem legacy effect on soil C inputs. However, the C isotopic composition of soil, endogeic earthworms and casts had not changed the two years after the CO 2 fumigation ended. The positive response of earthworms to increased root NPP, caused by elevated [CO 2 ], is consistent with the increased soil aggregate formation and increased soil C at the ORNL FACE in the e[CO 2 ] treatment.
54 ENVIRONMENTAL SCIENCES↗
Isolating organic carbon fractions with varying turnover rates in temperate agricultural soils – A comprehensive method comparison
Fractionation of soil organic carbon (SOC) is crucial for mechanistic understanding and modeling of soil organic matter decomposition and stabilization processes. It is often aimed at separating the bulk SOC into fractions with varying turnover rates, but a comprehensive comparison of methods to achieve this is lacking. In this study, a total of 20 different SOC fractionation methods were tested by participating laboratories for their suitability to isolate fractions with varying turnover rates, using agricultural soils from three experimental sites with vegetation change from C3 to C4 22–36 years ago. Enrichment of C4-derived carbon was traced and used as a proxy for turnover rates in the fractions. Methods that apply a combination of physical (density, size) and chemical (oxidation, extraction) fractionation were identified as most effective in separating SOC into fractions with distinct turnover rates. Coarse light SOC separated by density fractionation was the most C4-carbon enriched fraction, while oxidation-resistant SOC left after extraction with NaOCl was the least C4-carbon enriched fraction. Surprisingly, even after 36 years of C4 crop cultivation in a temperate climate, no method was able to isolate a fraction with more than 76% turnover, which challenges the link to the most active plant-derived carbon pools in models. Particles with density >2.8 g cm -3 showed similar C4-carbon enrichment as oxidation-resistant SOC, highlighting the importance of sesquioxides for SOC stabilization. The importance of clay and silt-sized particles (<50 μm) for SOC stabilization was also confirmed. Particle size fractionation significantly outperformed aggregate size fractionation, due to the fact that larger aggregates contain smaller aggregates and organic matter particles of various sizes with different turnover rates. An evaluation scheme comprising different criteria was used to identify the most suitable methods for isolating fractions with distinct turnover rates, and potential benefits and trade-offs associated with a specific choice. The study's findings can be of great help to select the appropriate method(s) for fractionation of agricultural soils.
59 BASIC BIOLOGICAL SCIENCES↗