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At least 1,063 records · Page 59
All sourcesNASA NTRSDOE OSTI
NASA NTRSDate not supplied

rHealth One Demonstration Aboard ISS: A Microfluidic Bioanalyzer Based on Sheath/Hydrodynamic Focusing Flow Cytometry

The Exploration Medical Capability (ExMC) element aims to provide astronauts with the means for their own health monitoring, diagnosis, and treatment during exploration missions. As space flight ventures further from earth, the need for autonomous medical care increases under greater constraints on size, mass, and resources. One pillar for diagnosis that crew would be separated from is laboratory analysis. Even now on ISS, blood samples must be collected and returned to earth for testing. In response, ExMC is assessing how assays for hematology, bone health, radiation exposure etc. could be addressed through commercial-off-the-shelf (COTS) and Small Business Innovation Research (SBIR) funded bioanalyzers that are miniaturizing lab technology. Since missions will reach distances where there are no timely replacements, validation on the International Space Station (ISS) is a necessary part of that assessment. In partnership with NASA Johnson Space Center (JSC) Immunology Lab and the Research Operations and Integration (ROI) element, ExMC conducted a technology demonstration on ISS of the rHEALTH ONE, a flow cytometry based bioanalyzer, to assess future devices based on this design. In flow cytometry, there are predominantly three ways to focus the cells (or particles) into a single file stream: hydrodynamic focusing, microcapillary, and acoustic focusing. The rHEALTH ONE utilizes the commercial standard, sheath-based hydrodynamic focusing. rHEALTH itself represents both a company and a suite of medical tools NASA has funded through SBIR grants towards the development of a promising diagnosis instrument for exploration missions. rHEALTH ONE is the interim version of the technology, functional as a benchtop analyzer and test bed for the next generation of rHEALTH in development. Several modifications were made to the rHEALTH ONE analyzer for operation in microgravity. For function, fluid management was key. A sheath-based analyzer uses sheath fluid to flow the sample, cleaning fluid to prevent biological contamination, and a reservoir to collect the liquid waste. The rHEALTH ONE analyzer uses bottles dependent on gravity to separate the air and liquid pathways and keep the liquids contained. It uses only 1 psig of air pressure to directly push the liquid through the device, requiring little-to-no resistance at inlet and outlet. A microtubing assembly with self-sealing luer connectors was designed – featuring 0.014 mm thick durable medical balloons to hold water inside the supply bottles – to create safe containment and easy access for the crew while maintaining the analyzer’s function. For safety, copper tape was added to the interior of the plastic housing to reduce electromagnetic interference, fluid and electrical connections were secured against vibration and leaks, and gaps were further sealed to ensure containment of the optical block and lasers. Water as the sheath and cleaning fluid and TOX 0 samples were used to reduce the biohazard risk to the crew. In May 2022, European Space Agency astronaut Samantha Cristoforetti demonstrated the rHEALTH ONE aboard ISS for its sample loading, flow cytometry, and data collection capability in microgravity. The JSC Immunology Lab provided flow cytometry expertise, designed the sample test protocol, and manufactured and benchmarked the flight samples on a ‘gold-standard’ flow cytometer. The analyzer was primed with water, purged of air, and four calibration solutions of polystyrene microparticles were tested to characterize its performance. Tightly controlled procedures and excellent execution prevented air bubble interference during air/water separation, fluid transfer, sample mixing and loading. Data showed a slight increase in signal noise on 3 of 5 channels and an anomaly of fluorophores migrating in one sample (confirmed by JSC post-flight). Flight results correctly detected the change in sample, matched the analyzer’s ground performance, and were consistent with the gold-standard’s ground results. Although areas were noted for improvement, these outcomes signified complete mission success.

R. S. Miller↗
DOE OSTI2015

Effect of synthesis methods on the Ca{sub 3}Co{sub 4}O{sub 9} thermoelectric ceramic performances

Three different synthesis methods producing nanometric grain sizes, coprecipitation with ammonium carbonate, oxalic acid, and by attrition milling have been studied to produce Ca{sub 3}Co{sub 4}O{sub 9} ceramics and compared with the classical solid state route. These three processes have produced high reactive precursors and all the organic material and CaCO{sub 3}·have been decomposed in a single thermal treatment. Coprecipitation leads to pure Ca{sub 3}Co{sub 4}O{sub 9} phase, while attrition milling and classical solid state produce small amounts of Ca{sub 3}Co{sub 2}O{sub 6} secondary phase. Power factor values are similar for all three samples, being slightly lower for the ones produced by attrition milling. These values are much higher than the obtained in samples prepared by the classical solid state method, used as reference. The maximum power factor values determined at 800 °C (∼0.43 mW/K{sup 2} m) are slightly higher than the best reported values obtained in textured ones which also show much higher density values. - Graphical abstract: Impressive raise of PF in Ca{sub 3}Co{sub 4}O{sub 9} thermoelectric materials obtained from nanometric grains. - Highlights: • Ca{sub 3}Co{sub 4}O{sub 9} has been produced by four different methods. • Precursors particle sizes influences on the final performances. • Coprecipitation methods produce single Ca{sub 3}Co{sub 4}O{sub 9} phase. • Power factor reaches values comparable to high density textured materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1990

Chemical differences between small subsamples of Apollo 15 olivine-normative basalts

Results are presented on the chemical and petrological characterization of nine samples of an Apollo 15 mare basalt suite. The results show that all nine samples are low-silica olivine normative basalts (ONBs) similar to those described earlier for low-silica ONBs from Apollo 15 site. The samples were found to vary in texture and grain size, from fine-grained intergranular or subophitic basalts to coarse-grained granular 'microgabbros'. Several displayed macroscopic heterogeneity. Variation diagrams show that the overall trend of the data is consistent with the fractionation of olivine (plus minor Cr-spinel) from a high-MgO parent magma.

Shervais, J. W.↗
DOE OSTI2024

Reaching the potential of electron diffraction

Microcrystal electron diffraction (MicroED) is an emerging structural technique in which submicron crystals are used to generate diffraction data for structural studies. Structures allow for the study of molecular-level architecture and drive hypotheses about modes of action, mechanisms, dynamics, and interactions with other molecules. Combining cryoelectron microscopy (cryo-EM) instrumentation with crystallographic techniques, MicroED has led to three-dimensional structural models of small molecules, peptides, and proteins and has generated tremendous interest due to its ability to use vanishingly small crystals. In this perspective, we describe the current state of the field for MicroED methodologies, including making and detecting crystals of the appropriate size for the technique, as well as ways to best handle and characterize these crystals. Our perspective provides insight into ways to unlock the full range of potential for MicroED to access previously intractable samples and describes areas of future development.

3D ED↗
DOE OSTI2015

Quiescent compact galaxies at intermediate Redshift in the cosmos field. The number density

We investigate the evolution of compact galaxy number density over the redshift range 0.2 1 for equivalently selected compact samples. Small variations in the abundance of the COSMOS compact sources as a function of redshift correspond to known structures in the field. The constancy of the compact galaxy number density is robust and insensitive to the compactness threshold or the stellar mass range (for M{sub ∗}>10{sup 10} M{sub ⊙}). To maintain constant number density any size growth of high-redshift compact systems with decreasing redshift must be balanced by a formation of quiescent compact systems at z<1.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1991

Determination of the half-life of Ca-41 from measurements of Antarctic meteorites

Accelerator mass spectrometry is utilized to determine the half-life of Ca-41 from the decrease of its concentration with terrestrial age in five Antarctic meteorites and a recent fall. The meteorites were selected on the basis of their Cl-36 concentrations, which showed a span of terrestrial ages of about 600 ka, and on the basis of other cosmogenic nuclide concentrations which indicated that the meteorites had small preatmospheric sizes, and sufficiently long irradiation times in space that the concentrations of Ca-41 and Cl-36 were in secular equilibrium prior to the meteorites' fall to earth. The half-life of Ca-41 is determined at 103 + or - 7 ka. Topics discussed include the effects of undersaturation (short exposure time in space), shielding (the samples are from the interior of a large meteorite), and weathering on the cosmogenic nuclide concentrations in meteorites.

Klein, Jeffrey↗
NASA NTRSDate not supplied

Europa Luminescence Microscope

The Europa Luminescence Microscope (ELM) is an automated fluorescence and dark-field mi-croscope designed to meet key objectives defined in the NASA Europa Lander Study Report, includ-ing the identification and characterization of morphological biosignatures. ELM’s heritage stems from a 2U cubesat fluorescence microscope, the Fluorescence Analysis for In-situ Research (FLAIR) im-ager, designed and built at NASA Ames Research Center, for the autonomous study of microbial bi-ology in low Earth orbit. For the ELM implementation, a sample is autonomously manipulated with a microfluidic system using in-line filter sets to capture successively smaller particles on 10, 1.0, and 0.1 µm pore-size filters for imaging. For darkfield imaging, ELM uses ultraviolet and visible light to image organic and inorganic structures with submicron resolution. The ability to detect structural and chemical biosignatures as small as 0.2 µm in size is achieved by imaging native fluorescence and us-ing fluorescence microscopy stains to identify key molecular and structural indicators of microbial life (proteins, lipids, nucleic acids). To excite fluorescence, ELM uses LEDs with wavelengths centered near 265, 370, 470, and 530 nm and five emission bands. The use of multiple excitation and emission wavelengths for native fluorescence imaging not only enables the detection of different molecular species, but also their rough classification. Excitation at 265 nm allows for the detection of smaller polyaromatic hydrocarbons (PAHs; 1-5 rings), aromatic amino acids, and proteins with little to no interference from mineral fluorescence, given proper emission band selection. 370 and 470 nm light excites increasingly larger PAH structures (e.g., coronene) and larger aromatic biomolecules that may be present (e.g., protective pigments). Similarly, inorganic fluorescence can be characterized and sep-arated from organic fluorescence, allowing the recognition and in some cases classification, of miner-als and other abiotic particles. ELM is based upon work supported by the NASA COLDTech and ICEE-2 programs.

Microscope↗
NASA NTRS1996

Microstructural Preparation and Examination of Polymer-Matrix Composites

Adapting procedures widely used in the metallographic characterization of metals and alloys, the microstructural preparation and examination of three polymer-matrix composites (PMC's) is described. The materials investigated contained either hollow ceramic filler particles or woven, continuous carbon/graphite fibers. Since the two particulate composites were considered to be isotropic, only one sample orientation was prepared. For the fiber composite, both longitudinal and planar orientations were studied. Once prepared, the samples were examined using reflected light microscopy. A number of microstructural features were evaluated qualitatively, including porosity and cracks, filler-matrix interfacial bonding, filler particle characteristics (shape, size, size distribution, and loading variation) and fiber characteristics (orientation, packing variation, and discontinuities).

Elban, Wayne L.↗
DOE OSTI2017

Aqueous phase hydrogenation of phenol catalyzed by Pd and PdAg on ZrO 2

The impact of particle size of ZrO 2 -supported Pd and of alloying with Ag was explored for hydrogenation of phenol in aqueous phase. Kinetic assessments were performed in a batch reactor, on monometallic Pd/ZrO 2 samples with different Pd loadings (0.5%, 1% and 2%), as well as on a 1% PdAg/ZrO 2 sample. The catalytic activity normalized to accessible Pd (turnover frequency, TOF) increased with the particle size of Pd. Reaction orders in phenol and H 2 and lower activation energies suggest that smaller particles bind the reacting substrates more strongly, leading to higher surface coverages by phenol. But, surprisingly, smaller Pd particles exhibited lower TOFs. The lower activity of the small Pd particles is attributed to lower activation entropies for the strongly bound species. Furthermore, the presence of Ag increased the catalyst activity by decreasing the apparent energy of activation and increasing the coverages of phenol and H 2 , without negatively affecting the activation entropy.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Scalable and efficient learning from crowds with Gaussian processes

Over the last few years, multiply-annotated data has become a very popular source of information. Online platforms such as Amazon Mechanical Turk have revolutionized the labelling process needed for any classification task, sharing the effort between a number of annotators (instead of the classical single expert). This crowdsourcing approach has introduced new challenging problems, such as handling disagreements on the annotated samples or combining the unknown expertise of the annotators. Probabilistic methods, such as Gaussian Processes (GP), have proven successful to model this new crowdsourcing scenario. However, GPs do not scale up well with the training set size, which makes them prohibitive for medium-to-large datasets (beyond 10K training instances). This constitutes a serious limitation for current real-world applications. In this work, we introduce two scalable and efficient GP-based crowdsourcing methods that allow for processing previously-prohibitive datasets. The first one is an efficient and fast approximation to GP with squared exponential (SE) kernel. The second allows for learning a more flexible kernel at the expense of a heavier training (but still scalable to large datasets). Since the latter is not a GP-SE approximation, it can be also considered as a whole new scalable and efficient crowdsourcing method, useful for any dataset size. Both methods use Fourier features and variational inference, can predict the class of new samples, and estimate the expertise of the involved annotators. A complete experimentation compares them with state-of-the-art probabilistic approaches in synthetic and real crowdsourcing datasets of different sizes. Finally, they stand out as the best performing approach for large scale problems. Moreover, the second method is competitive with the current state-of-the-art for small datasets.

96 KNOWLEDGE MANAGEMENT AND PRESERVATION↗
DOE OSTI2015

Measuring precipitation kinetics of sparingly soluble salts using Shock-Freeze Cryo-TEM

The precipitation of barium sulphate in a rapid mixing device (Y-mixer) coupled with an instantaneous sample freezing device was studied in order to accurately measure the nucleation rate. In this Shock-Freeze Cryo-TEM (SFCT) approach, a small volume of solution was removed from the stream directly into liquid ethane by means of a gravitational guillotine. Using different pipe lengths to measure various residence times, it was possible to determine the particle size, morphology, numbers of primary particles and thus nucleation rate. The nucleation rates were found to be a strong function of Reynolds number, up to Re=3×10 4 ( Re being inversely related to the micromixing time). The measured nucleation rates ranged between 2.79×10 14 and 3.81×10 17 # m -3 s -1 at supersaturations between 4×10 6 and 1×10 8 . The measured particle sizes in this work (ranging from 62 to 320 nm for supersaturation values between 4×10 6 and 1×10 8 ) were smaller than those measured by previous researchers, possibly because previous work did not quench the reaction sufficiently fast and thus allowed the particles a longer time to increase in size. Finally, in summary, this work successfully achieved a fast, accurate and reproducible nucleation rate measurement that could also give information about particle morphology.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1975

Study of the thermal degradation mechanism of a composite propellant

The current experimental program was designed to systematically investigate the role of the oxidizer in the thermal degradation process of composite propellants. The scanning electron microscope (SEM) was used to examine the failure sites in thermally degraded propellant samples. The formulation variables tested were oxidizer purity, oxidizer particle size, and oxidizer to binder bonding agent. The binder, a saturated hydrocarbon, was kept constant throughout the experiments. The oxidizers were: AP, chlorate-doped AP, arsenate-doped AP, and phosphate-doped AP. The oxidizer particle size distribution was 60% of the large fraction and 40% of the small fraction. The bonding agent, when present, was used at the 0.15% level. The data showed that both the oxidizer purity and particle size had an important affect on the thermal degradation process. The affect of the oxidizer particle size was more noticeable at the higher temperature and stress levels. An examination of the failure site, by SEM, of propellants subject to these latter conditions indicated that the fracturing of the large oxidizer particles led to the propellant cracking.

Schmidt, W. G.↗
NASA NTRS1993

Carbonaceous chondrite clasts in the howardites Bholghati and EET87513

Twenty-two carbonaceous chondrite clasts from the two howardites Bholghati and EET87513 were analyzed. Clast N from EET87513 is a fragment classified as CM2 material on the basis of texture, bulk composition, mineralogy, and bulk O isotopic composition. Carbonaceous chondrite clasts from Bholghati, for which less data are available because of their small size, can be divided into two petrologic types: C1 and C2. C1 clasts are composed of opaque matrix with rare coarse-grained silicates as individual mineral fragments; textures resemble CI meteorites and some dark inclusions from CR meteorites. Opaque matrix is predominantly composed of flaky saponite; unlike typical CI and CR meteorites, serpentine is absent in the samples we analyzed. C2 clasts contain chondrules, aggregates, and individual fragments of coarse-grained silicates in an opaque matrix principally composed of saponite and anhydrous ferromagnesian silicates with flaky textures similar to phyllosilicates. These anhydrous ferromagnesian silicates are interpreted as the product of heating of pre-existing serpentine. The carbonaceous chondrite clasts we have studied from these two howardites are, with one notable exception (clast N from EET87513), mineralogically distinct from typical carbonaceous chondrites. However, these clasts have very close affinities to carbonaceous chondrites and have also experienced thermal metamorphism and aqueous alteration, but to different degrees.

Buchanan, P. C.↗
NASA NTRS1996

Gravitational Effects on Closed-Cellular-Foam Microstructure

Polyurethane foam has been produced in low gravity for the first time. The cause and distribution of different void or pore sizes are elucidated from direct comparison of unit-gravity and low-gravity samples. Low gravity is found to increase the pore roundness by 17% and reduce the void size by 50%. The standard deviation for pores becomes narrower (a more homogeneous foam is produced) in low gravity. Both a Gaussian and a Weibull model fail to describe the statistical distribution of void areas, and hence the governing dynamics do not combine small voids in either a uniform or a dependent fashion to make larger voids. Instead, the void areas follow an exponential law, which effectively randomizes the production of void sizes in a nondependent fashion consistent more with single nucleation than with multiple or combining events.

Noever, David A.↗
DOE OSTI2022

Inline small-angle X-ray scattering-coupled chromatography under extreme hydrostatic pressure

As continuing discoveries highlight the surprising abundance and resilience of deep ocean and subsurface microbial life, the effects of extreme hydrostatic pressure on biological structure and function have attracted renewed interest. Biological small-angle X-ray scattering (BioSAXS) is a widely used method of obtaining structural information from biomolecules in solution under a wide range of solution conditions. Due to its ability to reduce radiation damage, remove aggregates, and separate monodisperse components from complex mixtures, size-exclusion chromatography-coupled SAXS (SEC-SAXS) is now the dominant form of BioSAXS at many synchrotron beamlines. While BioSAXS can currently be performed with some difficulty under pressure with non-flowing samples, it has not been clear how, or even if, continuously flowing SEC-SAXS, with its fragile media-packed columns, might work in an extreme high-pressure environment. Here we show, for the first time, that reproducible chromatographic separations coupled directly to high-pressure BioSAXS can be achieved at pressures up to at least 100 MPa and that pressure-induced changes in folding and oligomeric state and other properties can be observed. Further, the apparatus described here functions at a range of temperatures (0°C–50°C), expanding opportunities for understanding biomolecular rules of life in deep ocean and subsurface environments.

59 BASIC BIOLOGICAL SCIENCES↗
NASA NTRS2014

X-Ray Micro-Computed Tomography of Apollo Samples as a Curation Technique Enabling Better Research

X-ray micro-computed tomography (micro-CT) is a technique that has been used to research meteorites for some time and many others], and recently it is becoming a more common tool for the curation of meteorites and Apollo samples. Micro-CT is ideally suited to the characterization of astromaterials in the curation process as it can provide textural and compositional information at a small spatial resolution rapidly, nondestructively, and without compromising the cleanliness of the samples (e.g., samples can be scanned sealed in Teflon bags). This data can then inform scientists and curators when making and processing future sample requests for meteorites and Apollo samples. Here we present some preliminary results on micro-CT scans of four Apollo regolith breccias. Methods: Portions of four Apollo samples were used in this study: 14321, 15205, 15405, and 60639. All samples were 8-10 cm in their longest dimension and approximately equant. These samples were micro-CT scanned on the Nikon HMXST 225 System at the Natural History Museum in London. Scans were made at 205-220 kV, 135-160 microamps beam current, with an effective voxel size of 21-44 microns. Results: Initial examination of the data identify a variety of mineral clasts (including sub-voxel FeNi metal grains) and lithic clasts within the regolith breccias. Textural information within some of the lithic clasts was also discernable. Of particular interest was a large basalt clast (approx.1.3 cc) found within sample 60639, which appears to have a sub-ophitic texture. Additionally, internal void space, e.g., fractures and voids, is readily identifiable. Discussion: It is clear from the preliminary data that micro-CT analyses are able to identify important "new" clasts within the Apollo breccias, and better characterize previously described clasts or igneous samples. For example, the 60639 basalt clast was previously believed to be quite small based on its approx.0.5 sq cm exposure on the surface of the main mass. These scans show the clast to be approx.4.5 g, however (assuming a density of approx.3.5 g/cc). This is large enough for detailed studies including multiple geo-chronometers. This basalt clast is of particular interest as it is the largest Apollo 16 basalt, and it is the only mid-TiO2 basalt in the Apollo sample suite. By identifying the location of interesting clasts or grains within a sample, we will be able to make more informed decisions about where to cut a sample in order to best expose clasts of interest for future study. Moreover, knowing the location of internal defects (e.g., fractures) will allow more precise chipping and extraction of clasts or grains. By combining micro-CT scans with compositional techniques like micro x-ray fluorescence (particularly on sawn slabs), we will be able to provide even more comprehensive information to scientists trying to best select samples that fit their scientific needs.

Ziegler, R. A.↗
DOE OSTI2022

NSUF Report EBSD analyses on metallic fuel samples: Tips and improvements in sample preparation

EBSD (Electron Backscatter Diffraction) is an analytic technique used to evaluate the microstructure of a specimen. It can be used to evaluate interesting crystallographic data ranging from grains size to grain orientation, from grain boundaries to stresses. Moreover, in the field of nuclear materials EBSD can aid in estimating properties important for reactor performance, such as dislocation and residual strain [1]. EBSD patterns are generated by focusing a stationary electron gun on the specimen and detecting the backscatter electrons that satisfy Bragg conditions, and thus are diffracted by the crystal planes forming the so called Kikuchi bands. These bands correspond to each of the lattice diffracting crystal planes and are detected on a phosphor screen via a low-light CCD camera [2]. The patterns are generated within a small interaction volume located at a depth of less than 50-100 nm. Because of this, the data quality is extremely sensitive to the integrity of the crystallographic lattice order at the surface of the sample and thus to sample preparation and to oxidation [3]. While conventional sample preparation methods are widely used in materials science, they often do not lead to quality electron backscatter patterns needed for the characterization of nuclear materials [4]. This has been related by many authors to the challenges associated with handling radiological specimens, their rapid oxidation, and artifacts in such samples (e.g. high number of defects, precipitates and/or porosities) [4-8]. EBSD, however, can be an invaluable technique to evaluate microstructural evolution during fabrication and after irradiation at the required mesoscale for model and simulation. It also provides important information on grain structure necessary to understand irradiation effect, such as grain subdivision. Thus, it is of paramount importance to further the application of this technique to nuclear materials and especially irradiated fuels.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2017

Effects of management and pore characteristics on organic matter composition of macroaggregates: evidence from characterization of organic matter and imaging

Macroaggregates are of interest because of their fast response to land management and their role in the loss or restoration of soil organic carbon (SOC). The study included two experiments. In Experiment I, we investigated the effect of long-term (27 years) land management on the chemical composition of organic matter (OM) of macroaggregates. Macroaggregates were sampled from topsoil under conventional cropping, cover cropping and natural succession systems. The OM of macroaggregates from conventional cropping was more decomposed than that of cover cropping and especially natural succession, based on larger δ 15 N values and decomposition indices determined by multiple magic-angle spinning nuclear magnetic resonance ( 13 C CP/MAS NMR) and Fourier transform infrared (FTIR) spectroscopy. Previous research at the sites studied suggested that this was mainly because of reduced diversity and activity of the decomposer community, change in nutrient stoichiometry from fertilization and contrasting formation pathways of macroaggregates in conventional cropping compared with cover cropping and, specifically, natural succession. In Experiment II, we investigated the relation between OM composition and pore characteristics of macroaggregates. Macroaggregates from the natural succession system only were studied. We determined 3-D pore-size distribution of macroaggregates with X-ray microtomography, for which we cut the macroaggregates into sections that had contrasting dominant pore sizes. Then, we characterized the OM of macroaggregate sections with FTIR and δ 15 N methods. The results showed that within a macroaggregate, the OM was less decomposed in areas where the small (13–32 µm) or large (136–260 µm) pores were abundant. This was attributed to the role of large pores in supplying fresh OM and small pores in the effective protection of OM in macroaggregates. Finally, previous research at the site studied had shown increased abundance of large and small intra-aggregate pores following adoption of less intensive management systems. It appears that land management can alter the OM composition of macroaggregates, partly by the regulation of OM turnover at the intra-aggregate scale.

54 ENVIRONMENTAL SCIENCES↗