Variabilidade de goethita e hematita via dissolu����o redutiva em solos de regi��o tropical e subtropical

Dezenove amostras de horizontes B latoss��licos, uma de B pl��ntico, uma de B incipiente e uma de horizonte Cr foram submetidas a dissolu����es seletivas da goethita e da hematita por ditionito-citratro-bicarbonato de s��dio a 20 ��C em intervalos de tempo entre 5 e 3.840 minutos. O di��metro m��dio do cristal (DMC), ��rea superficial espec��fica (ASE) e substitui����o do ferro por Al (substitui����o por Al3+) da goethita e da hematita nas amostras foram estimados, por difratometria de raios X (DRX), em subamostras n��o tratadas e em subamostras tratadas selecionadas ao longo das dissolu����es seletivas. O controle das fases de goethita e hematita que persistiram ap��s cada etapa da dissolu����o seletiva foi realizado pelo m��todo da difratometria diferencial de raios X (DDRX). A cin��tica dissolutiva dos cristais foi descrita por meio da segmenta����o das curvas de dissolu����o do ferro no tempo em intervalos em que o comportamento dissolutivo foi linear. As taxas de dissolu����o nos diferentes segmentos lineares foram estimadas a partir do coeficiente angular da reta relativa a cada segmento. Em amostras goeth��ticas latoss��licas, a metodologia discriminou diferentes fases de goethita (popula����es heterog��neas) quanto �� substitui����o por Al3+, DMC e ASE e ofereceu condi����es para classificar o grau de estabilidade da goethita em baixa, m��dia, alta e muito alta estabilidade. Amostras goeth��ticas oriundas de ambientes pedog��nicos jovens e amostras hemat��ticas latoss��licas caracterizaram-se pela maior homogeneidade de seus cristais, classificados como de baixa e m��dia estabilidade. Nas amostras goeth��ticas, a propor����o de cristais com maior estabilidade (alta substitui����o por Al3+ e cristalinidade) em cada popula����o mostrou associa����o positiva com a propor����o de gibbsita em rela����o a caulinita nas amostras. A hematita apresentou menor resist��ncia �� dissolu����o por redu����o em rela����o �� goethita, exceto quando a ��ltima combinou baixa substitui����o por Al3+ e alta ASE. A caracteriza����o de popula����es heterog��neas de goethita pode ampliar a utiliza����o deste ��xido como indicador de processos pedog��nicos em solos poligen��ticos, bem como contribuir para um melhor entendimento do comportamento qu��mico e f��sico desses solos.

Cristallochemical characterization of synthetic Zn-substituted maghemites (g-Fe2-xZn xO3)

Maghemite (g-Fe2O3) is the most usually found ferrimagnetic oxide in red basalt-derived soils. The variable degrees of ionic substitution of Fe3+ for different metals (e.g. Ti4+, Al3+, Mg2+, Zn2+, and Mn2+) and non-metals in the maghemite structure influence some cristallochemical features of this iron oxide. In this study, synthetic Zn-substituted maghemites were prepared by co-precipitation in alkaline aqueous media of FeSO4.7H2O with increasing amounts of ZnSO4.7H2O to obtain the following sequence of Fe3+ for Zn2+ substitutions: 0.0, 0.025, 0.05, 0.10, 0.15, 0.20, and 0.30 mol mol-1. The objective of this work was to evaluate the cristallochemical alterations of synthetic Zn-substituted maghemites. The dark black synthetic precipitated material was heated to 250 ��C during 4 h forming a brownish maghemite that was characterized by chemical analysis as well as X ray diffraction (XRD), specific surface area and mass-specific magnetic susceptibility. The isomorphic substitution levels observed were of 0.0013, 0.0297, 0.0590, 0.1145, 0.1764, 0.2292 and 0.3404 mol mol-1, with the formation of a series of maghemites from Fe2Zn0O3 to Fe(1.49)Zn(0.770)O3 . The increase in Fe3+ for Zn2+ substitution, [Zn mol mol-1] increased the dimension a0 of the cubic unit cells of the studied maghemites according to the regression equation: a0 = 0.8343 + 0.02591Zn (R�� = 0.98). On the other hand, the mean crystallite dimension and mass-specific magnetic susceptibility of the studied maghemites decreased with increasing isomorphic substitution.

Reduction of symmetry in grandite solid solution

The results of a crystal structure refinement of an anisotropic grandite garnet specimen with composition Gro36-4 And63-6 are given. The structure obtained has orthorrombic symmetry (space group Fddd) and is compared with similar results obtained by other authors. In all cases the reduction of symmetry is due to the ordering of Fe3+ and Al in octahedral sites. Non cubic structures of grandites are discussed in connection with optical, morphological an grou-th features of these minerals.

Cation distribution and magnetization of BaFe12-2xCoxSnxO19 (x=0.9,1.28) single crystals

The distribution of Sn4+ cations within the five crystallographic sites of the magnetoplumbite (M) ���like compound BaFe12���2xCoxSnxO19 has been analyzed using single���crystal x���ray���diffraction data. The species Fe3+ and Co2+ cannot be distinguished using x rays because of their very similar atomic numbers; however, the calculation of the apparent valencies for the different sites allows an insight into the Co2+ cation segregation. The use of previous data from neutron powder diffraction allows a precise picture of the cation distribution, which indicates a pronounced site selectivity for both Sn4+ and Co2+ cations. The Sn4+ cations prefer the 4f2 sites and to a much lower extent the 12k sites, while they do not enter the octahedral 2a sites at all. Co2+ cations are distributed among tetrahedral and octahedral sites displaying a clear preference for the tetrahedral 4f1 sites. Magnetic measurements indicate that the compound still exhibits uniaxial anisotropy with the easy direction parallel to the c axis. Nevertheless, the magnetic structure shows a considerable degree of noncolinearity. A strong reduction of the magnetic anisotropy regarding that of the undoped compound is also detected.

Reversibilidade de f��sforo n��o-l��bil em solos submetidos �� redu����o microbiana e qu��mica: II - extra����es sucessivas do f��sforo pela resina de troca ani��nica

A forma����o de P n��o-l��bil a partir de formas l��beis, adsorvidas aos oxihidr��xidos de Fe e Al, �� raz��o para o baixo efeito residual da fertiliza����o fosfatada em solos tropicais. A reversibilidade do P n��o-l��bil para formas l��beis nesses solos pode ser favorecida pela redu����o do Fe3+ e, ou, pela diminui����o da atividade dos oxihidr��xidos de Fe e Al. Neste trabalho, objetivou-se avaliar a adsor����o e dessor����o de P, sua transforma����o em n��o-l��bil e reversibilidade dessa forma em amostras de solos submetidas �� redu����o microbiana ou qu��mica. Para isso, amostras de 11 solos foram homogeneizadas com 300 mg kg-1 de P na forma de NaH2PO4 em solu����o, incubadas por 30 dias e, ent��o, submetidas a dez extra����es sucessivas de P com resina de troca ani��nica (RTA) (tratamento sem redu����o). Em seguida, outras amostras dos solos foram submetidas �� ambiente redutor em solu����o de sacarose 0,1 mol L-1 com posterior aplica����o da doses de P (300 mg kg-1) e as dez extra����es sucessivas de P (tratamento redu����o microbiana). Depois, outras amostras foram reduzidas/complexadas com oxalato de am��nio (Ox) ou com citrato-ditionito-bicarbonato (CDB), e os solos/res��duos receberam a mesma dose de P dos experimentos anteriores. Foram ent��o incubados por 30 dias e submetidos ��s extra����es sucessivas com RTA (tratamento redu����o qu��mica). A capacidade m��xima de adsor����o de P (CMAP) dos solos mostrou-se mais dependente da goethita (com 70,8 % de contribui����o para seu valor) do que da gibbsita (com contribui����o de 29,2 %). A correla����o negativa entre os teores de P obtidos na segunda extra����o com a RTA, nos solos em condi����o natural, sem redu����o, e os teores de gibbsita sugere que este, e n��o a goethita, �� o oxihidr��xido respons��vel pela maior restri����o �� dessor����o do P. Os valores de CMAP, estimados por meio do P remanescente (P-rem), mostraram, nas amostras submetidas ao tratamento com redu����o microbiana, pequena altera����o para o grupo dos solos menos ox��dicos, com menor CMAP. Todavia, nos solos mais ox��dicos, com maior CMAP, o efeito pr��vio da sacarose foi o aumenta do P-rem (diminui����o da CMAP) 10 vezes em rela����o aos solos para o tratamento sem redu����o. Entretanto, a redu����o gerada pela sacarose n��o alterou a dessor����o do P anteriormente adsorvido. A expectativa de que ocorreria significativa reversibilidade de P n��o-l��bil com a redu����o microbiana ou qu��mica dos solos n��o se concretizou, demonstrando a grande estabilidade dessas formas.

Biomass production and phosphorus use of forage grasses fertilized with two phosphorus sources

A major constraint to agricultural production in acid soils of tropical regions is the low soil P availability, due to the high adsorption capacity, low P level in the source material and low efficiency of P uptake and use by most of the modern varieties grown commercially. This study was carried out to evaluate the biomass production and P use by forage grasses on two soils fertilized with two P sources of different solubility. Two experiments were carried out, one for each soil (Cambisol and Latosol), using pots filled with 4 dm�� soil in a completely randomized design and a 4 x 2 factorial scheme. The treatments consisted of a combination of four forage plants (Brachiaria decumbens, Brachiaria brizantha, Pennisetum glaucum and Sorghum bicolor) with two P sources (Triple Superphosphate - TSP and Arad Reactive Phosphate - ARP), with four replications. The forage grasses were harvested at pre-flowering, when dry matter weight and P concentrations were measured. Based on the P concentration and dry matter production, the total P accumulation was calculated. With these data, the following indices were calculated: the P uptake efficiency of roots, P use efficiency, use efficiency of available P, use efficiency of applied P and agronomic efficiency. The use of the source with higher solubility (TSP) resulted, generally, in higher total dry matter and total P accumulation in the forage grasses, in both soils. For the less reactive source (ARP), the means found in the forage grasses, for use efficiency and efficient use of available P, were always higher when grown in Latosol, indicating favorable conditions for the solubility of ARP. The total dry matter of Brachiaria brizantha was generally higher, with low P uptake, accumulation and translocation, which indicated good P use efficiency for both P sources and soils. The forage plants differed in the P use potential, due to the sources of the applied P and of the soils used. Less than 10 % of the applied P was immobilized in the forage dry matter. Highest values were observed for TSP, but this was not reflected in a higher use efficiency of P from this source.

Double-tagged fluorescent bacterial bioreporter for the study of polycyclic aromatic hydrocarbon diffusion and bioavailability.

Bacterial degradation of polycyclic aromatic hydrocarbons (PAHs), ubiquitous contaminants from oil and coal, is typically limited by poor accessibility of the contaminant to the bacteria. In order to measure PAH availability in complex systems, we designed a number of diffusion-based assays with a double-tagged bacterial reporter strain Burkholderia sartisoli RP037-mChe. The reporter strain is capable of mineralizing phenanthrene (PHE) and induces the expression of enhanced green fluorescent protein (eGFP) as a function of the PAH flux to the cell. At the same time, it produces a second autofluorescent protein (mCherry) in constitutive manner. Quantitative epifluorescence imaging was deployed in order to record reporter signals as a function of PAH availability. The reporter strain expressed eGFP proportionally to dosages of naphthalene or PHE in batch liquid cultures. To detect PAH diffusion from solid materials the reporter cells were embedded in 2 cm-sized agarose gel patches, and fluorescence was recorded over time for both markers as a function of distance to the PAH source. eGFP fluorescence gradients measured on known amounts of naphthalene or PHE served as calibration for quantifying PAH availability from contaminated soils. To detect reporter gene expression at even smaller diffusion distances, we mixed and immobilized cells with contaminated soils in an agarose gel. eGFP fluorescence measurements confirmed gel patch diffusion results that exposure to 2-3 mg lampblack soil gave four times higher expression than to material contaminated with 10 or 1 (mg PHE) g(-1).

Interstitial dendritic cell guidance by haptotactic chemokine gradients.

Directional guidance of cells via gradients of chemokines is considered crucial for embryonic development, cancer dissemination, and immune responses. Nevertheless, the concept still lacks direct experimental confirmation in vivo. Here, we identify endogenous gradients of the chemokine CCL21 within mouse skin and show that they guide dendritic cells toward lymphatic vessels. Quantitative imaging reveals depots of CCL21 within lymphatic endothelial cells and steeply decaying gradients within the perilymphatic interstitium. These gradients match the migratory patterns of the dendritic cells, which directionally approach vessels from a distance of up to 90-micrometers. Interstitial CCL21 is immobilized to heparan sulfates, and its experimental delocalization or swamping the endogenous gradients abolishes directed migration. These findings functionally establish the concept of haptotaxis, directed migration along immobilized gradients, in tissues.

Din��mica da emiss��o de metano em solos sob cultivo de arroz irrigado no sul do Brasil

Solos de v��rzea sob cultivo de arroz irrigado contribuem com aproximadamente 18 % das emiss��es totais de metano (CH4) do Estado do Rio Grande do Sul. Entretanto, a libera����o de CH4 depende do curso de redu����o de cada solo. O objetivo do presente estudo foi avaliar a din��mica da emiss��o desse g��s de efeito estufa (GEE) em seis solos: Gleissolo (2), Planossolo (2), Chernossolo e Neossolo, representativos do cultivo de arroz irrigado no Sul do Brasil, visando identificar tamb��m sua rela����o com propriedades do solo e as altera����es eletroqu��micas da solu����o ap��s o alagamento. O experimento foi realizado em casa de vegeta����o, com tr��s repeti����es, segundo delineamento de blocos casualizados. Os solos foram dispostos em vasos de PVC mantidos com uma l��mina de ��gua de 10 cm de altura e cultivados com arroz. A avalia����o das emiss��es de CH4 foi realizada semanalmente, do 3�� ao 66�� dia ap��s o alagamento (DAA) do solo, com o aux��lio de uma c��mara de PVC acoplada ao topo dos vasos. As amostras de ar foram coletadas em quatro intervalos de 5 min, para estimativa das taxas de emiss��o de CH4. A solu����o do solo tamb��m foi coletada e caracterizada. O in��cio da emiss��o de CH4 variou entre os solos e, normalmente, ocorreu ap��s a quase total redu����o do Fe3+ (em torno de 90 % da maior libera����o de Fe2+) e estabiliza����o dos valores de pH e de Eh da solu����o. A emiss��o total de CH4 variou de 8,5 a 44,2 g m-2 e apresentou rela����o sigmoidal com os teores de C org��nico dos solos (r��=0,83, p < 0,05), sugerindo que a disponibilidade de C somente foi limitante para o processo de metanog��nese em teores inferiores a 8 g kg-1 de C no solo. Os resultados mostram que a din��mica e as quantidades totais de CH4 emitidas s��o influenciadas pelo tipo de solo e que esfor��os devem ser direcionados para determina����o dos fatores de emiss��o de CH4 para os diferentes solos representativos da produ����o de arroz no Sul do Brasil, bem como na avalia����o do efeito de pr��ticas agr��colas na mitiga����o das emiss��es desse GEE nos diferentes solos.

Phosphatase activity in sandy soil influenced by mycorrhizal and non-mycorrhizal cover crops

Cover crops may difffer in the way they affect rhizosphere microbiota nutrient dynamics. The purpose of this study was to evaluate the effect of mycorrhizal and non-mycorrhizal cover crops on soil phosphatase activity and its persistence in subsequent crops. A three-year experiment was carried out with a Typic Quartzipsamment. Treatments were winter species, either mycorrhizal black oat (Avena strigosa Schreb) or the non-mycorrhizal species oilseed radish (Raphanus sativus L. var. oleiferus Metzg) and corn spurry (Spergula arvensis L.). The control treatment consisted of resident vegetation (fallow in the winter season). In the summer, a mixture of pearl millet (Pennisetum americanum L.) with sunnhemp (Crotalaria juncea L.) or with soybean (Glycine max L.) was sown in all plots. Soil cores (0-10 cm) and root samples were collected in six growing seasons (winter and summer of each year). Microbial biomass P was determined by the fumigation-extraction method and phosphatase activity using p-nitrophenyl-phosphate as enzyme substrate. During the flowering stage of the winter cover crops, acid phosphatase activity was 30-35 % higher in soils with the non-mycorrhizal species oilseed radish, than in the control plots, regardless of the amount of P immobilized in microbial biomass. The values of enzyme activity were intermediate in the plots with corn spurry and black oat. Alkaline phosphatase activity was 10-fold lower and less sensitive to the treatments, despite the significant relationship between the two phosphatase activities. The effect of plant species on the soil enzyme profile continued in the subsequent periods, during the growth of mycorrhizal summer crops, after completion of the life cycle of the cover crops.

Nutrient relations during an eucalyptus cycle at different population densities

To synchronize nutrient availability with the requirements of eucalyptus during a cultivation cycle, the nutrient flow of this system must be well understood. Essential, for example, is information about nutrient dynamics in eucalyptus plantations throughout a cultivation cycle, as well as impacts on soil nutrient reserves caused by the accumulation and subsequent export of nutrients via biomass. It is also important to quantify the effect of some management practices, such as tree population density (PD) on these fluxes. Some nutrient relations in an experiment with Eucalyptus grandis, grown at different PDs in Santa Barbara, state of Minas Gerais, Brazil, were evaluated for one cultivation cycle. At forest ages of 0.25, 2.5, 4.5, and 6.75 years, evaluations were carried out in the stands at seven different PDs (between 500 and 5,000 trees ha-1) which consisted in chemical analyses of plant tissue sampled from components of the aboveground parts of the tree, from the forest floor and the litterfall. Nutrient contents and allocations of the different biomass components were estimated. In general, there were only small and statistically insignificant effects of PD on the nutrient concentration in trees. With increasing forest age, P, K, Ca and Mg concentrations were reduced in the aboveground components and the forest floor. The magnitud of biochemical nutrient cycling followed the sequence: P > K > N > Mg. At the end of the cycle, the quantities of N, P, Ca and Mg immobilized in the forest floor were higher than in the other components.

Kinetic parameters of soil &#946;-glucosidase response to environmental temperature and moisture regimes

Soil &#946;-glucosidase participates in the final step of cellulose biodegradation. It is significant in the soil C cycle and is used as an indicator of the biological fertility of soil. However, the response of its kinetic parameters to environmental temperature and moisture regimes is not well understood. This study tested the &#946;-glucosidase response in the main agricultural soils (black soil, albic soil, brown soil, and cinnamon soil) of Northeast China. Incubation tests were conducted to measure the kinetic parameters Km, Vmax or Vmax/Km of soil &#946;-glucosidase at environmental temperatures of 10, 20 and 30 ��C and at 10, 20 and 30 % soil moisture content. The insensitive response of the kinetic parameters to temperature changes indicates that soil &#946;-glucosidase was present primarily in immobilized form. The significant response of the kinetic parameters of soil &#946;-glucosidase to soil moisture rather than to environmental temperatures suggests that the catalytic ability of soil &#946;-glucosidase was sensitive to changing soil moisture regimes.

Aggregation of integrins and RhoA activation are required for Thy-1-induced morphological changes in astrocytes.

Thy-1, a cell adhesion molecule abundantly expressed in mammalian neurons, binds to a beta(3)-containing integrin on astrocytes and thereby stimulates the assembly of focal adhesions and stress fibers. Such events lead to morphological changes in astrocytes that resemble those occurring upon injury in the brain. Extracellular matrix proteins, typical integrin ligands, bind to integrins and promote receptor clustering as well as signal transduction events that involve small G proteins and cytoskeletal changes. Here we investigated the possibility that the cell surface protein Thy-1, when interacting with a beta(3)-containing integrin on astrocytes, could trigger signaling events similar to those generated by extracellular matrix proteins. DI-TNC(1) astrocytes were stimulated with Thy-1-Fc immobilized on beads, and increased RhoA activity was confirmed using an affinity precipitation assay. The effect of various inhibitors on the cellular response was also studied. The presence of Y-27632, an inhibitor of Rho kinase (p160ROCK), a key downstream effector of RhoA, significantly reduced focal adhesion and stress fiber formation induced by Thy-1. Similar effects were obtained when astrocytes were treated with C3 transferase, an inhibitor of RhoA. Alternatively, astrocytes were transfected with an expression vector encoding fusion proteins of enhanced green fluorescent protein with either the Rho-binding domain of Rhotekin, which blocks RhoA function, or the dominant-negative N19RhoA mutant. In both cases, Thy-1-induced focal adhesion formation was inhibited. Furthermore, we observed that RhoA activity after stimulation with soluble Thy-1-Fc molecule was augmented upon further cross-linking using protein A-Sepharose beads. The same was shown by cross-linking beta(3)-containing integrin with anti-beta(3) antibodies. Together, these results indicate that Thy-1-mediated astrocyte stimulation depended on beta(3) integrin clustering and the resulting increase in RhoA activity.

Lead and zinc in the structure of organic and mineral soil components

In addition to the more reactive forms, metals can occur in the structure of minerals, and the sum of all these forms defines their total contents in different soil fractions. The isomorphic substitution of heavy metals for example alters the dimensions of the unit cell and mineral size. This study proposed a method of chemical fractionation of heavy metals, using more powerful extraction methods, to remove the organic and different mineral phases completely. Soil samples were taken from eight soil profiles (0-10, 10-20 and 20-40 cm) in a Pb mining and metallurgy area in Adrian��polis, Paran��, Brazil. The Pb and Zn concentrations were determined in the following fractions (complete phase removal in each sequential extraction): exchangeable; carbonates; organic matter; amorphous and crystalline Fe oxides; Al oxide, amorphous aluminosilicates and kaolinite; and residual fractions. The complete removal of organic matter and mineral phases in sequential extractions resulted in low participation of residual forms of Pb and Zn in the total concentrations of these metals in the soils: there was lower association of metals with primary and 2:1 minerals and refractory oxides. The powerful methods used here allow an identification of the complete metal-mineral associations, such as the occurrence of Pb and Zn in the structure of the minerals. The higher incidence of Zn than Pb in the structure of Fe oxides, due to isomorphic substitution, was attributed to a smaller difference between the ionic radius of Zn2+ and Fe3+.

Structural and magnetic characterization of maghemites prepared from Al-substituted magnetites

Synthetic aluminum-substituted maghemites were characterized by total chemical analysis, powder X-ray diffraction (XRD), M��ssbauer spectroscopy (ME), and vibrating sample magnetometry (VSM). The aim was to determine the structural, magnetic, and hyperfine properties of &#947;-Fe2-xAl xO3 as the Al concentration is varied. The XRD results of the synthetic products were indexed exclusively as maghemite. Increasing Al for Fe substitution decreased the mean crystalline dimension and shifted all diffraction peaks to higher ��2&#952; angles. The a0 dimension of the cubic unit cell decreased with increasing Al according to the equation a o = 0.8385 - 3.63 x 10-5 Al (R��= 0.94). Most M��ssbauer spectra were composed of one sextet, but at the highest substitution rate of 142.5 mmol mol-1 Al, both a doublet and sextet were obtained at 300 K. All hyperfine parameters from the sub-spectra were consistent with high-spin Fe3+ (0.2 a 0.7 mms-1) and suggested a strong superparamagnetic component associated with the doublet. The magnetic hyperfine field of the sextets decreased with the amount of Al-substitution [Bhf (T) = 49.751 - 0.1202Al; R�� = 0.94] while the linewidth increased linearly. The saturation magnetization also decreased with increasing isomorphous substitution.