Microscale Spatial Distribution Research Articles

NanoSIMS Analysis of Trace Element Segregation during the Al-Si Eutectic Reaction

The purpose of the present article was to test, for the first time, the potential of the NanoSIMS for quantifying the microscale spatial distribution of trace elements in aluminum alloys. As a suitable test case, two variants of an as-cast industrial A356 alloy have been chosen and analyzed quantitatively. The results show that Sr segregates preferentially to the α-Si particles rather than to the Al matrix. The Na segregates preferentially to α-Si, but in contrast to Sr, Na is inhomogeneously distributed in the particles and tends to accumulate at parts of their surfaces. The Ca segregates preferentially to the Al matrix and was not found in α-Si. This suggests that impurity-induced twinning is probably not the mechanism responsible for the flake-to-fibrous transition observed in Ca-modified Al-Si-alloys. The P was difficult to analyze as the 31 P-peak was close to the peak of the molecule fragment 31 Si + 1 H. The complication can probably be resolved in future studies. The present work demonstrated that NanoSIMS is a tool with great potential for providing new insight into microscale trace element segregation in Al alloys.

Epifluorescence microscopy and image analysis of high‐level polycyclic aromatic hydrocarbon contamination in soils

Interactions between polycyclic aromatic hydrocarbons (PAHs) and soil are an important determinant of their chemical availability and transport. Laboratory examination of microscale PAH-soil interaction is limited by the availability of methods for particle-scale observation. Inverted epifluorescence microscopy, combined with digital photography and computer image analysis, was evaluated for specificity and linearity using dissolved PAHs. A pyrene filter (excitation wavelength, 360-400 nm; emission wavelength, 450-520 nm) gave nonspecific PAH fluorescence, and bias for fluoranthene, benzo[b]fluoranthene, benzo[g,h,i]perylene, and benz[a]anthracene was quantified in comparison to that for pyrene. Concentrations ranging from 1 to 10 mM for anthracene, fluoranthene, and pyrene and from 1 to 50 mM for naphthalene produced a linear response with low interpixel variability. Liquid-phase analyses validated use of the technique for the descriptive analysis of PAH distribution in solid samples, but liquid-phase calibration was not quantitative for spiked or field-contaminated soils. The mean luminance for three field soils was proportional to the values predicted from their chemically measured concentrations and to values from spiked, aged, uncontaminated materials. Image analysis of laboratory- and field-contaminated samples determined the area distribution of fluorescent intensity and the size of fluorescent areas exceeding a threshold luminance. These qualitative descriptions of the microscale spatial distribution of PAH contamination are presented as potential endpoints for future research on biogeochemical interactions in heavily contaminated solids.

Impact of the Microscale Distribution of a Pseudomonas Strain Introduced into Soil on Potential Contacts with Indigenous Bacteria

Soil bioaugmentation is a promising approach in soil bioremediation and agriculture. Nevertheless, our knowledge of the fate and activity of introduced bacteria in soil and thus of their impact on the soil environment is still limited. The microscale spatial distribution of introduced bacteria has rarely been studied, although it determines the encounter probability between introduced cells and any components of the soil ecosystem and thus plays a role in the ecology of introduced bacteria. For example, conjugal gene transfer from introduced bacteria to indigenous bacteria requires cell-to-cell contact, the probability of which depends on their spatial distribution. To quantitatively characterize the microscale distribution of an introduced bacterial population and its dynamics, a gfp-tagged derivative of Pseudomonas putida KT2440 was introduced by percolation in repacked soil columns. Initially, the introduced population was less widely spread at the microscale level than two model indigenous functional communities: the 2,4-dichlorophenoxyacetic acid degraders and the nitrifiers (each at 10(6) CFU g(-1) soil). When the soil was percolated with a substrate metabolizable by P. putida or incubated for 1 month, the microscale distribution of introduced bacteria was modified towards a more widely dispersed distribution. The quantitative data indicate that the microscale spatial distribution of an introduced strain may strongly limit its contacts with the members of an indigenous bacterial community. This could constitute an explanation to the low number of indigenous transconjugants found most of time when a plasmid-donor strain is introduced into soil.

Modification of spatial distribution of 2,4-dichlorophenoxyacetic acid degrader microhabitats during growth in soil columns.

Bacterial processes in soil, including biodegradation, require contact between bacteria and substrates. Knowledge of the three-dimensional spatial distribution of bacteria at the microscale is necessary to understand and predict such processes. Using a soil microsampling strategy combined with a mathematical spatial analysis, we studied the spatial distribution of 2,4-dichlorophenoxyacetic acid (2,4-D) degrader microhabitats as a function of 2,4-D degrader abundance. Soil columns that allowed natural flow were percolated with 2,4-D to increase the 2,4-D degrader abundance. Hundreds of soil microsamples (minimum diameter, 125 microm) were collected and transferred to culture medium to check for the presence of 2,4-D degraders. Spatial distributions of bacterial microhabitats were characterized by determining the average size of colonized soil patches and the average number of patches per gram of soil. The spatial distribution of 2,4-D degrader microhabitats was not affected by water flow, but there was an overall increase in colonized patch sizes after 2,4-D amendment; colonized microsamples were dispersed in the soil at low 2,4-D degrader densities and clustered in patches that were more than 0.5 mm in diameter at higher densities. During growth, spreading of 2,4-D degraders within the soil and an increase in 2,4-D degradation were observed. We hypothesized that spreading of the bacteria increased the probability of encounters with 2,4-D and resulted in better interception of the degradable substrate. This work showed that characterization of bacterial microscale spatial distribution is relevant to microbial ecology studies. It improved quantitative bacterial microhabitat description and suggested that sporadic movement of cells occurs. Furthermore, it offered perspectives for linking microbial function to the soil physicochemical environment.

Micro-scale spatial heterogeneity and the loss of carbon, nitrogen and phosphorus in degraded grassland in Ordos Plateau, northwestern China

The micro-scale spatial distribution and loss of carbon, nitrogen and phosphorus were examined in degraded grassland near Ordos, in the Mu Us Sand-land, northwestern China. Five communities that represented a series of successionally degraded stages in desertification were chosen for the work. The dominant plant of Community 1 was the steppe grass Stipa bungeana; Community 2 was dominated by a mix of S. bungeana and the shrub Artemisia ordosia; Community 3 was A. ordosia; Community 4 was a mix of A. ordosia and the desert grass Cynanchum komorovii; and Community 5 was C. komorovii. The soils in root-spheres and in the bare openings between plants in five successionally degraded plant communities were analyzed for total organic carbon (TOC), total nitrogen (TN), inorganic nitrogen (IN), total phosphorus (TP), and available phosphorus (AP). The results showed that the heterogeneity process of the soil chemistry was characterized first by TOC heterogeneity and later by TN heterogeneity. The heterogeneity process of TP was only characterized in the community 3. No significant heterogeneity was present for AP in the five community stages. At the beginning of degradation, invasion by the shrub A. ordosia of S. bungeana grassland was found to lead to competition for soil elements between S. bungeana and A. ordosia and made the Community 2 soil environment temporarily homogeneous. In Community 3, however, the soil elements became spatially heterogeneous, and this led to the development of `islands of fertility'. The concentrations of soil elements (TOC, TN and IN) were greatest in the shrub root-spheres. With further desertification (from Community 4 to Community 5), the islands of fertility began to collapse, and the concentrations of the main soil elements declined rapidly. The dynamics of soil phosphorus under progressive desertification were different from those of the other soil elements. TP decreased from form Community 1 to Community 5, while the AP concentration did not change in mid-level desertification (Community 3), but increased with serious desertification (Community 5).

A novel method for characterizing the microscale 3D spatial distribution of bacteria in soil

Knowledge of the microscale spatial distribution of bacteria in soil would improve our understanding of microbial ecology and function. A specific 3D method has been developed to characterize the spatial distribution of bacterial microhabitats in soil. The method combines a specific microsampling strategy with a statistical data analysis. The sampling strategy involves several different unit volumes (minimum sample side: length of 50 μm) subsequently tested for the presence or absence of the targeted microorganisms. The statistical analysis is based on the comparison of experimental sampling data with data expected from the limited sampling of numerous theoretical spatial distributions. Since the exact spatial pattern of the zones colonized by bacteria requires the spatial coordinates of the samples, this approach only identifies possible distributions. Distributions are characterized by descriptors relevant to soil, such as the average size of colonized patches and their average number g −1 of soil. These descriptors provide the basis for comparing spatial patterns. Computer simulations confirmed the method's ability to distinguish between different spatial patterns. The spatial patterns in soil of two bacterial types, NH 4 + oxidizers and 2,4-D degraders, were studied independently in repacked soil columns percolated with solutions containing either NH 4 + or 2,4-D. The spatial analysis method distinguished between the two microbial spatial distributions ( P≤0.025). Thus, this method constitutes a useful tool for identifying factors regulating bacterial spatial distributions in soil at the microscale.

Microscale Spatial Distribution of Dislocations and Long Range Internal Stresses in Cold Worked bcc Fe

Microscale Spatial Distribution of Dislocations and Long Range Internal Stresses in Cold Worked bcc Fe

Microscale Spatial Distribution of Phragmites australis (Common Reed) Invasion into Spartina patens (Salt Hay)-dominated Communities in Brackish Tidal Marsh

Over the last century, Phragmites australis (common reed) has been expanding rapidly from the marsh–upland boundary into Spartina patens (salt hay)-dominated high marsh communities of the eastern US coast. Whereas direct and indirect human disturbances and changes in hydrology or salinity are likely to influence rates of spread at the landscape scale, the susceptibility of specific plant communities to invasion also influence rates of Phragmites expansion at the local scale. I measured microscale (0.25 m2) spatial patterns of culms (emerging buds and mature stems) in October 1993 at both expanding and stable boundaries of Phragmites populations within a S. patens-dominant matrix. In both expanding and stable plots, Phragmites culms were observed more frequently than expected on hummocks that were created by S. patens tussock-forming root structure. Culm density within a plot was correlated with the percent hummock cover within a plot. Further, Phragmites culms, particularly mature stems, were concentrated along the perimeter of the hummocks. Because the culms were not evenly distributed between hummocks and hollows, I suggest that invasion rates of Phragmites are limited in S. patens communities by microscale differences in hummock availability. The pattern of emergence suggests that expanding rhizomes of Phragmites encounter both competition with S. patens roots on the hummocks and physiological stressors (salinity, anoxia, sulfide concentrations) in the hollows.

Estimating Microscale Spatial Distribution of Conductivity and Pore Continuity using Computed Tomography

AbstractDespite the importance of information on the spatial distribution of unsaturated hydraulic conductivity (Kus) at microscale in soil, experimental determination of this property is difficult. The objectives of this study were to: (i) seek a simple, but reliable, procedure for the estimation of Kus at microscale and (ii) determine the sensitivity of the estimates to wetting‐induced changes in selected structural parameters (porosity, ɛ, surface fractal dimension, D, and pore continuity, PC). Using computer‐assisted tomography (CAT), spatial distributions of soil water content (θ) and changes in ɛ that occurred during wetting were monitored at 2 by 2 mm resolution at 1‐cm depth increments in water‐stable (WSA) and unstable aggregate (USA) columns. The Fuentes theoretical equation, which requires data on saturated hydraulic conductivity (Kɛ), θ, ɛ, and D, was used for the estimation of the spatial distribution of Kus. The spatial distribution of θ in WSA columns ranged from 0.113 to 0.327 cm3 cm‐3 and from 0.175 to 0.567 cm3 cm‐3 in the USA columns. The spatial distribution of ɛus ranged from 0.46 to 0.74 and was used in the computation of D and PC. Values of Kɛ ranged from 0.006 to 0.745 cm h‐1. The spatial distribution of Kus ranged from 6.87 × 10‐4 to 1.49 × 10‐2 cm h‐1 in WSA compared with 7.3 × 10‐4 to 4.11 × 10‐2 cm h‐1 in USA. Pore continuity, θ, D, and initial aggregate diameter (x) accounted for 94 to 95% of the variability in Kus distributions. The results indicate that reliable estimates of Kus distributions at microscale can be computed from single‐source CAT‐derived data on θ and ɛ.