We investigated the influence of root border cells on the colonisation of seedling Zea mays roots by Pseudomonas fluorescens SBW25 in sandy loam soil packed at two dry bulk densities. Numbers of colony forming units (CFU) were counted on sequential sections of root for intact and decapped inoculated roots grown in loose (1.0 mg m(-3)) and compacted (1.3 mg m(-3)) soil. After two days of root growth, the numbers of P. fluorescens (CFU cm(-1)) were highest on the section of root just below the seed with progressively fewer bacteria near the tip, irrespective of density. The decapped roots had significantly more colonies of P. fluorescens at the tip compared with the intact roots: approximately 100-fold more in the loose and 30-fold more in the compact soil. In addition, confocal images of the root tips grown in agar showed that P. fluorescens could only be detected on the tips of the decapped roots. These results indicated that border cells, and their associated mucilage, prevented complete colonization of the root tip by the biocontrol agent P. fluorescens, possibly by acting as a disposable surface or sheath around the cap.
Summary The limit to the precision with which moisture‐release data may be related to soil structure is examined. Thin sections in two orthogonal planes were prepared from cores for which moisture release data and saturated conductivity measurements were also collected. Structural parameters are inferred indirectly through estimation of the mass fractal dimension from the moisture‐release curve, and directly through measurement of the scaling properties of the pore space and solid matrix from the thin sections using image analysis. Theoretical results are presented which show that, in the absence of additional information, the interpretation of the moisture‐release curve is ambiguous for several reasons. A power‐law exponent is a consequence of either a fractal pore volume; a fractal solid volume; a fractal pore wall; or a non‐fractal, self‐similar pore wall, and one cannot infer from the measurement which is the case. Experimental results are presented which confirm that direct measurement of the fractal dimension of the solid matrix is a good predictor of the Brooks–Corey exponent for the soils studied here. Therefore, although a specific structural parameter characterizes the moisture‐release curve, the latter cannot be used as a detailed measure of soil structure.
Subcritical water repellency is a poorly acknowledged physical property of soil. It refers to soil where water uptake appears to occur readily, yet is impeded to some extent by the presence of hydrophobic surface films. It was only after the recent development of a sensitive testing technique that subcritical water repellency was shown to be a common feature of many soils. It is a fundamental physical property of soil and has implications for the resistance of soil structure against disruption by wetting, bypass flow, and surface runoff. Using a technique adapted by Hallett and Young (1999), we assessed a water repellency index, R, of individual soil aggregates from a range of cultivation practices with different fertilizer inputs and depths. The parameter R is extremely powerful since it is directly proportional to the decrease in water sorptivity caused by repellency. The hypotheses tested are (i) that soil disturbance reduces R and (ii) that high levels of plant nutrients (fertilizer) will enhance R Cultivation was found to cause a twofold decrease in R for all soils tested except one pasture treatment. Pasture soil from another site had an R value that was three times higher to a depth of 60 cm than an adjacent plowed soil. Soil aggregates were more repellent from no-till than plowed treatments. Higher levels of N added to field soil did not affect R
The sedimentary basins of Bass Strait display a marked asymmetry of structure. This is particularly evident in the Gippsland Basin and Torquay Sub-basin where Tertiary structures are better developed along their northern margins. The origins of this asymmetry are reviewed and a structural model for the evolution of Bass Strait basins set out. Mapping of the regional structures developed at three stages of the region's evolution demonstrates that dextral strike slip movement was important throughout. During the Cretaceous the regional stress regime was dextral transtensive, changing to transpressive by the mid-Eocene. Despite early similarities in these basins' formation, their Tertiary histories diverged, contributing to the wide variations in hydrocarbon potential seen today. It is suggested that the present day structural asymmetry is a consequence of partial inversion of initial half-graben basin geometries.
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