Microaggregate Formation Research Articles

Electrochemistry of TiO2–iron hexacyanocobaltate composite electrodes

Abstract In this paper we investigate the electrochemical behavior of iron hexacyanocobaltate (FeHCC) in comparison to the cobalt hexacyanoferrate (CoHCF). The best results were achieved on electrochemical synthesized film of FeHCC on the TiO2 modified electrodes. The chemical and physical characterizations confirm the formation of the FeHCC with the classical cubic crystal structure of the Prussian blue analogs, with cell parameter a very close to 10 A, as well as the formation of micro aggregates of TiO2 covered by FeHCC. The synthesis was performed on various substrates such as glassy carbon (GC), graphite foil (GF) and indium tin oxide (ITO) in order to develop new technological applications.

Effects of long-term fertilization on soil organic carbon content and aggregate composition under continuous maize cropping in Northeast China

SUMMARYFertilizer application can play an important role in soil organic carbon (SOC) retention and dynamics. The mechanisms underlying long-term accumulation and protection of SOC in intensive maize cropping systems, however, have not been well documented for cool high-latitude rainfed areas. Based on a 23-year fertilization experiment under a continuous maize cropping system at Gongzhuling, Jilin Province, China, the effects of fertilization regimes on SOC content and soil aggregate-associated carbon (C) composition were investigated. Results showed that, within the 0–1·0 m soil profile, SOC contents decreased significantly with soil depth in all treatments. In the topsoil layer (0–0·2 m), SOC concentrations in balanced inorganic fertilizers plus farmyard manure (MNPK), fallow system (FAL) and balanced inorganic fertilizers plus maize straw residue (SNPK) treatments were significantly greater than initial levels by 61·0, 34·1 and 20·1%, respectively. The MNPK and SNPK treatments increased SOC content by 50·7 and 12·4% compared to the unfertilized control in the topsoil layer, whereas no significant differences were found between balanced inorganic nitrogen, phosphorus and potassium fertilizers (NPK) and the unfertilized control treatment. There were no significant differences in aggregate-size distribution among the unfertilized control, NPK and MNPK treatments, whereas the SNPK treatment significantly enhanced the formation of micro-aggregates (53–250 μm) and decreased the formation of silt+clay aggregates (<53 μm) compared to the unfertilized control, NPK and MNPK treatments. Moreover, SOC concentrations in all aggregate fractions in the MNPK treatment were the highest among treatments. Furthermore, the MNPK treatment significantly increased SOC stock in micro- and silt+clay aggregates, which may slow down C decomposition in the soil. These results indicate that long-term manure amendment can benefit SOC sequestration and stability in the black soil of Northeast China.

Dynamic surface elasticity of mixed poly(diallyldimethylammonium chloride)/sodium dodecyl sulfate/NaCl solutions

The non-monotonic concentration and kinetic dependencies of the dynamic dilational surface elasticity of mixed poly(diallyldimethylammonium chloride)/sodium dodecyl sulfate(SDS)/NaCl solutions indicate the formation of micro-aggregates in the surface layer. The increase of the solution's ionic strength results in strong changes of the surface dilational rheological properties and thereby of the interfacial layer structure at lower SDS concentrations due to the large increase of the adsorption rate. Close to the charge match point of the components, however, the rheological properties are influenced by the presence of micro-aggregates embedded in the adsorption layer. In this case, effects of changing the sample history on the rheological behavior and heterogeneity of the interface were further investigated.

Gelatine enhances drug dispersion in alginate bilayer film via the formation of crystalline microaggregates

In our previous study, a novel alginate-based bilayer film for slow-release wound dressings was successfully developed. We found that alginate alone yielded poor films; however, the addition of gelatine had significantly enhanced the drug dispersion as well as the physical properties. Here, an investigation of the drug-polymer interactions in the bilayer films was carried out. Drug content uniformity test and microscopy observation revealed that the addition of gelatine generated bilayer films with a homogenous drug distribution within the matrix. The FTIR and XRD data showed an increase in film crystallinity which might infer the presence of drug-polymer crystalline microaggregates in the films. DSC confirmed the drug-polymer interaction and indicated that the gelatine has no effect on the thermal behaviour of the microaggregates, suggesting the compatibility of the drug and excipients in the bilayer films. In conclusion, the addition of gelatine can promote homogenous dispersion of hydrophobic drugs in alginate films possibly through the formation of crystalline microaggregates.

Aggregate-associated soil organic matter as an ecosystem property and a measurement tool

Our 2000 paper Soil macroaggregate turnover and microaggregate formation: A mechanism for C sequestration under no-tillage agriculture had its genesis in attempts to identify and isolate soil organic matter (SOM) fractions that reflect the impacts of climate, soil physiochemical properties and physical disturbance on the soil organic carbon balance. A key prerequisite for the investigation was the development of a simple device to isolate the microaggregates (53–250 μm) contained within stable (i.e., resistant to slaking) macroaggregates (>250 μm) obtained by conventional wet-sieving. By comparing the abundance and C content of micro-within-macroaggregates, the size distribution of intra-aggregate particulate organic matter (iPOM) and isotopically-based estimates of the age of the organic matter in the different fractions, we were able to corroborate our hypothesis that the absence of tillage (i.e., in no-till and native soils) promotes greater longevity of newly-formed macroaggregates, resulting in greater SOM stabilization in microaggregates formed within stable macroaggregates. Follow-up research has indicated that the microaggregate-within-macroaggregate fraction is 1) potentially a robust indicator for management-induced SOC changes over decadal time scales, 2) of biological origin and therefore useful in interpreting impacts of soil biota on soil C and N dynamics, but not in-situ CO2 and N2O fluxes, 3) useful in complimentary chemical and spectroscopic approaches to relate SOM dynamics to soil structure and attributes of the soil pore space, and 4) a good candidate for being incorporated into models as a measurable fraction.

AggModel: A soil organic matter model with measurable pools for use in incubation studies

Current soil organic matter (SOM) models are empirical in nature by employing few conceptual SOM pools that have a specific turnover time, but that are not measurable and have no direct relationship with soil structural properties. Most soil particles are held together in aggregates and the number, size and stability of these aggregates significantly affect the size and amount of organic matter contained in these aggregates, and its susceptibility to decomposition. While it has been shown that soil aggregates and their dynamics can be measured directly in the laboratory and in the field, the impact of soil aggregate dynamics on SOM decomposition has not been explicitly incorporated in ecosystem models. Here, we present AggModel, a conceptual and simulation model that integrates soil aggregate and SOM dynamics. In AggModel, we consider unaggregated and microaggregated soil that can exist within or external to macroaggregated soil. Each of the four aggregate size classes contains particulate organic matter and mineral-associated organic matter fractions. We used published data from laboratory incubations to calibrate and validate the biological and environmental effects on the rate of formation and breakdown of macroaggregates and microaggregates, and the organic matter dynamics within these different aggregate fractions. After calibration, AggModel explained more than 60% of the variation in aggregate masses and over 70% of the variation in aggregate-associated carbon. The model estimated the turnover time of macroaggregates as 31 and 181 days for microaggregates. Sensitivity analysis of AggModel parameterization supported the notion that macroaggregate turnover rate has a strong control over microaggregate masses and, hence, carbon sequestration. In conclusion, AggModel successfully incorporates the explicit representation for the turnover of soil aggregates and their influence on SOM dynamics and can form the basis for new SOM modules within existing ecosystem models.

Glycoprotein Ibα and FcγRIIa play key roles in platelet activation by the colonizing bacterium, Streptococcus oralis

Infective endocarditis (IE) is characterized by thrombus formation on a cardiac valve. The oral bacterium, Streptococcus oralis, is recognized for its ability to colonize damaged heart valves and is frequently isolated from patients with IE. Platelet interaction with S.oralis leads to the development of a thrombotic vegetation on heart valves, which results in valvular incompetence and congestive heart failure. To investigate the mechanism through which platelets become activated upon binding S.oralis. Platelet interactions with immobilized bacteria under shear conditions were assessed using a parallel flow chamber. S.oralis-inducible platelet reactivity was determined using light transmission aggregometry. Dense granule secretion was measured by luminometry using a luciferin/luciferase assay. Using shear rates that mimic physiological conditions, we demonstrated that S.oralis was able to support platelet adhesion under venous (50-200s(-1) ) and arterial shear conditions (800s(-1) ). Platelets rolled along immobilized S.oralis through an interaction with GPIbα. Following rolling, platelet microaggregate formation was observed on immobilized S.oralis. Aggregate formation was dependent on S.oralis binding IgG, which cross-links to platelet FcγRIIa. This interaction led to phosphorylation of the ITAM domain on FcγRIIa, resulting in dense granule secretion, amplification through the ADP receptor and activation of RAP1, culminating in platelet microaggregate formation. These results suggest a model of interaction between S.oralis and platelets that leads to the formation of a stable septic vegetation on damaged heart valves.

Fluorochrome-Functionalized Nanoparticles for Imaging DNA in Biological Systems

Attaching DNA binding fluorochromes to nanoparticles (NPs) provides a way of obtaining NPs that bind to DNA through fluorochrome mediated interactions. To obtain a nanoparticle (NP) that bound to the DNA in biological systems, we attached the DNA binding fluorochrome, TO-PRO 1 (TO), to the surface of the Feraheme (FH) NP, to obtain a fluorochrome-functionalized NP denoted TO-FH. When reacted with DNA in vitro, TO-FH formed microaggregates that were characterized by fluorescence, light scattering, and T2 changes. The formation of DNA/TO-FH microaggregates was also characterized by AFM, with microaggregates exhibiting a median size of 200 nm, and consisting of DNA and multiple TO-FH NPs whose individual diameters were only 25-35 nm. TO-FH failed to bind normal cells in culture, but treatment with chemotherapeutic agents or detergents yielded necrotic cells that bound TO-FH and vital fluorochromes similarly. The uptake of TO-FH by HT-29 xenografts (treated with 5-FU and oxaliplatin) was evident by surface fluorescence and MRI. Attaching multiple DNA binding fluorochromes to magnetic nanoparticles provides a way of generating DNA binding NPs that can be used to detect DNA detection by microaggregate formation in vitro, for imaging the DNA of necrotic cells in culture, and for imaging the DNA of a tumor treated with a chemotherapeutic agent. Fluorochrome functionalized NPs are a multimodal (magnetic and fluorescent), highly multivalent (n ≈ 10 fluorochromes/NP) nanomaterials useful for imaging the DNA of biological systems.

Humus Substances and Soil Structure

Abstract In this study, the soil structure of two soil types (Haplic Chernozems and Eutric Fluvisols) in four ecosystems (forest, meadow, urban and agro-ecosystem) with dependence on humus substances were compared. The stability of dry-sieved and waterresistant macro-aggregates and micro-aggregates with a dependence on the proportion of humus substance fractions was determined. Quantity of humus substances influenced mainly water-resistant aggregates. A positive correlation was recorded between size fraction of 2.3 mm and contents of humus substances (P < 0.01; r = +0.710) and fulvic acids (P < 0.05; r = +0.634), and negative correlation between size fraction of 0.5.1 mm and contents of humus substances (P < 0.05; r = -0.613) and fulvic acids (P < 0.01; r = -0.711). Humic acids influenced mainly the formation of dry-sieved aggregates and fulvic acids played an important role in micro-aggregate formation. The quality of humus substances influenced more intensively the formation of dry-sieved aggregates. There were positive correlations between optical parameters of humus substances and humic acids and larger dry-sieved aggregates (3.7 mm) and negative correlations with smaller (0.5.3 mm). The highest proportions of larger size of water-resistant aggregates (1. 20 mm) were in forest ecosystem, but smaller (0.25.1 mm) agreggates were dominated in agro-ecosystem.

Formation of ultrafine microaggregates in laser ablation of polymers

Experimental results on polymer laser ablation are considered using the regularities of gas dynamics and flow of viscous fluids. It is shown that a significant role in the ablation of polymers is played by destructive and thermochemical processes of modification of the molecular structure during irradiation, leading in some cases to a reduction and in others to an increase in the viscosity of the melt. The intensity of the laser irradiation, optical properties, and physicochemical state of the surface layer of the polymer determine the nature and quantitative parameters of the formation and the shape of the microaggregates in the ablation flow. The emphasis is placed on polytetrafluorethylene (PTFE), the capabilities of which in terms of formation of a microaggregate fraction can be raised to high values, which makes it possible to use laser ablation (LA) for production of fiber-porous PTFE with unique properties. Fiber-porous PTFE is used in the manufacture of high performance filters that can operate in corrosive environments and under high temperatures.

Physical, chemical, and biochemical mechanisms of soil organic matter stabilization under conservation tillage systems: A central role for microbes and microbial by-products in C sequestration

Conservation tillage practices that entail no or reduced soil disturbance are known to help preserve or accumulate soil organic matter (OM). However, the underlying mechanisms especially at the molecular level are not well understood. In this study soil samples from 25-year-old experimental plots continuously cropped with barley (Hordeum vulgare L.) under no-tillage (NT) and chisel tillage (CT) were subjected to a new physical fractionation method to isolate dissolved OM, mineral-free particulate OM located outside aggregates (physically and chemically unprotected), OM occluded within both macroaggregates and microaggregates (weakly and strongly protected by physical mechanisms, respectively), and OM in intimate association with minerals (protected by chemical mechanisms). The whole soils and OM fractions were analyzed for organic C and N content and by modern nuclear magnetic resonance (NMR) techniques. The soil under NT stored 16% more organic C and 5% more N than the soil under CT. Compared to CT, NT increased free organic C content by 7%, intra-macroaggregate organic C content by 20%, intra-microaggregate organic C content by 63%, and mineral-associated organic C content by 16% and decreased dissolved organic C content by 11%. The mineral-associated OM pool accounted for 65% of the difference in total organic C content between NT and CT, whereas the intra-microaggregate OM only explained 18%, intra-macroaggregate OM 14%, and free OM 11%. The NMR experiments revealed that the free and intra-aggregate OM fractions were dominated by crop-derived materials at different stages of decomposition, whereas the mineral-associated OM pool was predominately of microbial origin. Overall, our results indicate that microbes and microbial by-products associated with mineral surfaces and likely physically protected by entrapment within very small microaggregates constitute the most important pool of OM stabilization and C sequestration in soils under NT. Most probably the slower macroaggregate turnover in NT relative to CT boosts not only the formation of microaggregates and thereby the physical protection of crop-derived particulate OM, but more importantly the interaction between mineral particles and microbial material that results in the formation of very stable organo-mineral complexes.

Long-term application of organic manure and mineral fertilizers on aggregation and aggregate-associated carbon in a sandy loam soil

A long-term field experiment was established in September 1989 to examine the influence of mineral fertilizer and compost on soil fertility in an intensively cultivated fluvo-aquic soil in the North China plain. The study involved seven treatments: compost (CM), half-compost plus half-fertilizer N (HCM), fertilizer NPK (NPK), fertilizer NP (NP), fertilizer NK (NK), fertilizer PK (PK) and control without any fertilizer (CK). The mass and organic C concentration of aggregates (>250μm macroaggregate, 53–250μm microaggregate, and <53μm free silt+clay fraction) and subfractions within aggregates, including intra particulate organic matter (iPOM) and silt+clay subfraction, were measured to evaluate the process of aggregation and organic C accumulation. The long-term application of mineral fertilizers had no obvious effects on the mass proportion of aggregate, and in contrast, compost significantly increased the mass proportion of macroaggregates from 8.8% in CK soil to 17.7–30.8% in compost-added soils. This increase was at the expense of microaggregates and free silt+clay fraction. Soil organic C concentrations were significantly increased in the CM and HCM treatments by 124% and 72%, respectively, but less than 27% increase was observed in mineral fertilizer-added soils over an 18-year period (compared with CK soil). Amendment with mineral fertilizer NPK mainly increased organic C concentrations in macroaggregates and particularly in the free silt+clay fraction. In contrast, compost application mainly accelerated organic C accumulation in macroaggregates by increasing the amount of C in the silt+clay subfraction rather than iPOM, which accounted for 47–58% of the increased organic C in soil. This was because of the mass proportion increase in macroaggregates. The mass proportion of macroaggregates was significantly related with organic C concentration in microaggregate and free silt+clay fractions (P=0.005). The mass ratio of macroaggregates plus microaggregates to the free silt+clay fraction (P=0.015) and macroaggregates to microaggregates (P=0.003) was significantly correlated with organic C concentration in the free silt+clay fraction. These results indicated that the increase of soil organic C in compost-added soil was possibly due first to the enhancement of organic C concentration in the free silt+clay fraction, which in turn promoted the formation of microaggregates and/or macroaggregates. We consider that the increase of organic C concentration in the free silt+clay fraction is likely to play a key role in aggregation and C sequestration.

High throughput generated micro-aggregates of chondrocytes stimulate cartilage formation in vitro and in vivo

Cell-based cartilage repair strategies such as matrix-induced autologous chondrocyte implantation (MACI) could be improved by enhancing cell performance. We hypothesised that micro-aggregates of chondrocytes generated in high-throughput prior to implantation in a defect could stimulate cartilaginous matrix deposition and remodelling. To address this issue, we designed a micro-mould to enable controlled high-throughput formation of micro-aggregates. Morphology, stability, gene expression profiles and chondrogenic potential of micro-aggregates of human and bovine chondrocytes were evaluated and compared to single-cells cultured in micro-wells and in 3D after encapsulation in Dextran-Tyramine (Dex-TA) hydrogels in vitro and in vivo. We successfully formed micro-aggregates of human and bovine chondrocytes with highly controlled size, stability and viability within 24 hours. Micro-aggregates of 100 cells presented a superior balance in Collagen type I and Collagen type II gene expression over single cells and micro-aggregates of 50 and 200 cells. Matrix metalloproteinases 1, 9 and 13 mRNA levels were decreased in micro-aggregates compared to single-cells. Histological and biochemical analysis demonstrated enhanced matrix deposition in constructs seeded with micro-aggregates cultured in vitro and in vivo, compared to single-cell seeded constructs. Whole genome microarray analysis and single gene expression profiles using human chondrocytes confirmed increased expression of cartilage-related genes when chondrocytes were cultured in micro-aggregates. In conclusion, we succeeded in controlled high-throughput formation of micro-aggregates of chondrocytes. Compared to single cell-seeded constructs, seeding of constructs with micro-aggregates greatly improved neo-cartilage formation. Therefore, micro-aggregation prior to chondrocyte implantation in current MACI procedures, may effectively accelerate hyaline cartilage formation.

Cytoskeleton keratin regulation of FasR signaling through modulation of actin/ezrin interplay at lipid rafts in hepatocytes

FasR stimulation by Fas ligand leads to rapid formation of FasR microaggregates, which become signaling protein oligomerization transduction structures (SPOTS), through interactions with actin and ezrin, a structural step that triggers death-inducing signaling complex formation, in association with procaspase-8 activation. In some cells, designated as type I, caspase 8 directly activates effector caspases, whereas in others, known as type II, the caspase-mediated death signaling is amplified through mitochondria. Keratins are the intermediate filament (IF) proteins of epithelial cells, expressed as pairs in a lineage/differentiation manner. Hepatocyte IFs are made solely of keratins 8/18 (K8/K18), the hallmark of all simple epithelia. We have shown recently that in comparison to type II wild-type (WT) mouse hepatocytes, the absence of K8/K18 IFs in K8-null hepatocytes leads to more efficient FasR-mediated apoptosis, in link with a type II/type I-like switch in FasR-death signaling. Here, we demonstrate that the apoptotic process occurring in type I-like K8-null hepatocytes is associated with accelerated SPOTS elaboration at surface membrane, along with manifestation of FasR cap formation and internalization. In addition, the lipid raft organization is altered in K8-null hepatocytes. While lipid raft inhibition impairs SPOTS formation in both WT and K8-null hepatocytes, the absence of K8/K18 IFs in the latter sensitizes SPOTS to actin de-polymerization, and perturbs ezrin compartmentalization. Overall, the results indicate that the K8/K18 IF loss in hepatocytes alters the initial FasR activation steps through perturbation of ezrin/actin interplay and lipid raft organization, which leads to a type II/type I switch in FasR-death signaling.

Characterizations of Blend Gels of Carboxymethylated Polysaccharides and their Use for the Controlled Release of Herbicide

Polysaccharide-derived polymers are good carriers for encapsulating bioactive compounds for their controlled delivery. Aiming at extending the release of herbicide metolachlor in water, the novel gel beads were prepared by crosslinking the blend solutions of carboxymethyl cellulose (CM-cell) and carboxymethyl chitosan (CM-chit), and then used as the carrier in controlled release formulations (CRFs). SEM observation showed the formation of micro-aggregates in blend gel beads, and FT-IR analysis confirmed strong interactions between the two polymers. DSC analysis indicated stronger bonding of water by blend gels than that by pure CM-cell gel. The higher encapsulation efficiency and slower release of herbicide metolachlor were obtained by using the blend gel beads as the carrier of CRFs instead of the commercial emulsible oil formulation. The release of herbicide from the blend gels was dominated by a mechanism of Fickian diffusion, and related to their rapid and low swelling in water.

0–20 μm aggregate typology based on the nature of aggregative organic materials in a cultivated silty topsoil

Soil structure is a key characteristic of soil functioning. More, silt and clay-sized organo-mineral fractions are usually considered as the most reactive soil fractions. As transmission electronic microscopy (TEM) appeared to be well-suited to study soil microstructures, it was used to test the sensitivity of micro-aggregates to land use and cultivation practices. The aims of this study were therefore (1) to use TEM to characterize the organic materials involved in the stable 0–20 μm micro-aggregates of a cultivated topsoil and (2) to highlight any impact of cultivation practices and of inter-annual variation on micro-aggregation. For this, the study was conducted on a Calcic Cambisol (pH of 7.1, organic matter content of 33 g kg −1, C-to-N ratio around 10), developed in Eastern France and sampled two consecutive years before and after the exceptional dry period occurring in 2003. It was cropped under maize following moderate tillage and fertilisation within an experimental station. Digested sewage sludge has been applied since 1996 according to regulations for environmental controls at the rate of 10 Mg ha −1 year −1 for 4 years. Granulometric soil fractionations of samples from plots with or without sewage sludge addition were performed. Then, morphological and analytical characterization by TEM of the water-stable <20 μm micro-aggregates were carried out to specify their aggregative organic matter. 0–20 μm water-stable fraction contained more than 60% of the soil carbon. A typology of <20 μm micro-aggregates was established according the nature of aggregative organic matter. It appeared that among 0–2 μm and 2–20 μm micro-aggregates, those containing young organic matter of plant and microbial origin were involved in the most stable associations. The impact of the dry period on soil microstructure between 2002 and 2003 corresponded to both an increase in the 0–2 μm water-dispersibility and a decrease in the mean size of the 2–20 μm water-stable aggregates. Sewage sludge addition slightly increased the proportion of the 50–200 μm water-stable macro-aggregates and induced the formation of micro-aggregates containing sludge flocs, which could be considered as specific sewage sludge fingerprints. Descriptors of 0–20 μm stable micro-aggregates, i.e. size, stability and typology, appeared as a promising tool to specify the dynamics of the organo-mineral associations in soils subjected to environmental changes or cultivation practices.

P-selectin ligation induces platelet activation and enhances microaggregate and thrombus formation

Introduction Platelet P-selectin is a thrombo-inflammatory molecule involved in platelet activation and aggregation. This may occur via the adhesive function of P-selectin and its potential capacity to trigger intracellular signaling. However, its impact on platelet function remains elusive. This study was therefore designed to investigate the relationship between the signaling potential of platelet P-selectin and its function in platelet physiology. Methods and Results Human and mouse platelets were freshly isolated from whole blood. Platelet activation was assessed using flow cytometry and western blot analysis, while platelet physiological responses were evaluated through aggregation, microaggregate formation and in a thrombosis model in wild-type and P-selectin-deficient (CD62P −/−) mice. Interaction of P-selectin with its high-affinity ligand, a recombinant soluble form of P-Selectin Glycoprotein Ligand-1 (rPSGL-1), enhances platelet activation, adhesion and microaggregate formation. This augmented platelet microaggregates requires an intact cytoskeleton, but occurs independently of platelet α IIbβ 3. Thrombus formation and microaggregate were both enhanced by rPSGL-1 in wild-type, but not in CD62P −/− mice. In addition, CD62P −/− mice exhibited thrombosis abnormalities without an α IIbβ 3 activation defect. Conclusions This study demonstrates that the role of platelet P-selectin is not solely adhesive; its binding to PSGL-1 induces platelet activation that enhances platelet aggregation and thrombus formation. Therefore, targeting platelet P-selectin or its ligand PSGL-1 could provide a potential therapeutic approach in the management of thrombotic disorders.

Dilational rheology of polymer/surfactant mixtures at water/hexane interface

The present work is devoted to the interaction in mixed solutions of the cationic polyelectrolyte PAH and the counter-charged anionic surfactant SDS at the water/oil interface. For this purpose interfacial tension and dilational rheology studies were performed to describe the formation of complexes of PAH and SDS. The dilational elasticity values depend on the concentrations of surfactant and polyelectrolyte and their mixing ratio. As expected the concentration dependence of the dilational elasticities of SDS and PAH-SDS mixtures are similar in the SDS concentration range 10 −5 M till 10 −3 M (at a fixed amount of PAH) and have a maximum which correlates with an observed minimum in the interfacial tension isotherm. The sharp decrease of the dilational elasticity values in a narrow range is presumably due to the destruction of two-dimensional rigid structures and the formation of microaggregates in the interfacial layer. From dilational viscosity measurements as a function of oscillation frequency one can conclude that with increasing surfactant concentration the SDS dominates in the interfacial adsorption layer whereas the polyelectrolyte–surfactant complexes remain in the bulk phase.

Soil Carbon Storage in Silvopasture and Related Land-Use Systems in the Brazilian Cerrado

Silvopastoral management of fast-growing tree plantations is becoming popular in the Brazilian Cerrado (savanna). To understand the influence of such systems on soil carbon (C) storage, we studied C content in three aggregate size classes in six land-use systems (LUS) on Oxisols in Minas Gerais, Brazil. The systems were a native forest, a treeless pasture, 24- and 4-yr-old eucalyptus ( sp.) plantations, and 15- and 4-yr-old silvopastures of fodder grass plus animals under eucalyptus. From each system, replicated soil samples were collected from four depths (0-10, 10-20, 20-50, and 50-100 cm), fractionated into 2000- to 250-, 250- to 53-, and <53-μm size classes representing macroaggregates, microaggregates, and silt + clay, respectively, and their C contents determined. Macroaggregate was the predominant size fraction under all LUS, especially in the surface soil layers of tree-based systems. In general, C concentrations (g kg soil) in the different aggregate size fractions did not vary within the same depth. The soil organic carbon (SOC) stock (Mg C ha) to 1-m depth was highest under pasture compared with other LUS owing to its higher soil bulk density. The soils under all LUS had higher C stock compared with other reported values for managed tropical ecosystems: down to 1 m, total SOC stock values ranged from 461 Mg ha under pasture to 393 Mg ha under old eucalyptus. Considering the possibility for formation and retention of microaggregates within macroggregates in low management-intensive systems such as silvopasture, the macroaggregate dynamics in the soil seem to be a good indicator of its C storage potential.

Organic matter stabilization in soil aggregates: Understanding the biogeochemical mechanisms that determine the fate of carbon inputs in soils

We studied the biochemical and biophysical processes of carbon sequestration in an intensive agroforestry system on two soils (Feralsol – Luero; Arenosol – Teso) in W. Kenya to elucidate the mechanisms associated with long-term carbon storage. Specifically, we looked at a top-down model (macro-aggregates form around organic matter particles and micro-aggregates form within the macro-aggregates) and a bottom-up model (micro-aggregates form independently and are incorporated into macro-aggregates) of soil aggregate formation. Soil samples were collected from experiments on improved tree fallows using different species and two tillage treatments; water-stable aggregates were extracted and sorted into three size classes: macro-aggregates (> 212 μm), meso-aggregates (53–212 μm) and micro-aggregates (20–53 μm). Organic matter characterization of each fraction was based on 13C isotope abundance, Fourier transform infrared (FTIR) spectroscopy and the abundance of polysaccharides. Improved fallows increased soil C by 0.28 and 0.26 kg m ­2 in the top 20 cm of the soil profile in Luero and Teso, respectively. Tillage altered the distribution of aggregates among size classes. Changes in the δ 13C signature in each fraction indicated that more of the new carbon was found in the macro-aggregates (35–70%) and meso-aggregates (18–49%) in Luero and less (9–17%) was found in the micro-aggregates. In Teso, about 40–80% of the new aggregate C was found in the meso-aggregates, 14–45% was found in the micro-aggregates and only 4–26% was found in the macro-aggregates. The meso-aggregates and macro-aggregates to a lesser extent, in both sites, were enriched in carboxylic-C and aromatic-C, indicating the importance of OM decomposition and plant-derived C in the stabilization of larger aggregates, supporting the top-down model of aggregate formation. Microbially derived polysaccharides play a leading role in the formation of stable micro-aggregates and carboxylic-C promotes stabilization through surface occlusion. This bottom-up process is essential to promote long-term carbon sequestration in soils. Additionally, the micro-aggregates at both sites were enriched in polysaccharides and had elevated ratios of galactose + mannose:arabinose + xylose than the other aggregate fractions, indicating the importance of microbial processes in the formation of stable micro-aggregates and supporting the bottom-up model.