ALBERT

Notes: Abstract Photoreceptors and neurons at various levels to cortex have been counted in mouse and rat. The ratios of neuron numbers (rat/mouse) are similar to the ratio of retinal areas or the squared ratio of eye sizes; so to a first approximation the two species have linearly scaled eyes, equal photoreceptor spacings (in μm), and visual pathways scaled numerically by the number of photoreceptors. With supplementary data from the literature, some of the functional implications of the design can be evaluated level by level. Overall, there is structural and computational economy, or even parsimony.

Notes: Stability and resilience of a variety of soil properties and processes are emerging as key components of soil quality. We applied recently developed measures of biological and physical resilience to soils from an experimental site treated with metal-contaminated sewage sludge. Soils treated with cadmium-, copper- or zinc-contaminated, digested or undigested sewage sludge were studied. Biological stability and resilience indices were: (i) the time-dependent effects of either a transient stress (heating to 40°C for 18 hours) or a persistent stress (amendment with CuSO4) on decomposition, and (ii) the mineralization of dissolved organic carbon (DOC) released by drying–rewetting cycles. Physical stability and resilience measures were: (i) compression and expansion indices of the soils, and (ii) resistance to prolonged wetting and structural regeneration through drying–rewetting cycles. Soil total carbon and DOC levels were greater in the sludge-amended soils, but there were no differential effects due to metal contamination of the sewage sludge. Effects of metals on physical resilience were greater than effects on soil C, there being marked reductions in the expansion indices with Cd- and Cu-contaminated sludge, and pointed to changes in soil aggregation. The rate of mineralization of DOC released by drying and wetting was reduced by Zn contamination, while biological resilience was increased in the Zn-contaminated soil and reduced by Cd contamination. We argue that physical and biological resilience are potentially coupled through the microbial community. This needs to be tested in a wider range of soils, but demonstrates the benefits from a combined approach to the biological and physical resilience of soils.

Notes: Crack development is predominant in soil structure formation. A number of fracture mechanics models have been applied to soil to describe cracking, but most are not applicable for soil in a wet, plastic state. We address this weakness by applying a new elastic–plastic fracture mechanics approach to describe crack formation in plastic soil. Samples are fractured using a deep-notch (modified four-point) bend test, with data on load transmission, sample bending, crack growth, and crack-mouth opening collected to assess the crack-tip opening angle (CTOA). CTOA provides a powerful parameter for describing soil cracking since it can be induced by soil shrinkage (an easily measured parameter) and can be used to describe elastic–plastic fracture in numerical approximations, such as finite element modelling. The test variables we studied were the direction of the applied consolidation stress, clay content, and pore water salinity. All samples were formed by consolidating soil slurry one-dimensionally with a 120-kPa vertical effective stress. Tests on pure kaolinite showed that the direction of the consolidation stress did not affect CTOA, which was 0.23 ± 0.02 m m−1 for specimens cut both in a horizontal and in a vertical direction to the applied stress. Soil clay content had a marked influence, however, with silica sand:kaolinite mixtures by weight of 20:80 and 40:60 reducing CTOA to 0.14 ± 0.02 m m−1 and 0.12 ± 0.01 m m−1, respectively. These smaller values of CTOA indicate that less strain is required to induce fracture when the amount of clay is less. Salinity (0.5 m NaCl) caused a reduction in the CTOA of pure kaolinite from 0.23 ± 0.02 m m−1 to 0.17 ± 0.03 m m−1.

Notes: The production of exudates by plant roots and microbes in the rhizosphere, together with intense wetting and drying cycles due to evapotranspiration, stimulate changes in soil structure. We have attempted to separate these two processes using an experimental model with bacterial exopolysaccharides (dextran and xanthan) and root mucilage analogues (polygalacturonic acid, PGA), and up to 10 cycles of wetting and drying. To characterize the soil structure, tensile strength, water sorptivity and ethanol sorptivity of the amended soils were measured, and thin sections were made. Xanthan and PGA induced greater tensile strength of the amended soil, suggesting that they increased the bond energy between particles. Porosity increased with each cycle of wetting and drying, and this increase was less pronounced for the PGA 2 g l−1 than for the xanthan and dextran. This suggests that PGA stabilized the soil against the disruptive effect caused by the wetting and drying. The PGA was the only polysaccharide that influenced water sorptivity and repellency, resulting in slower wetting of the treated soil. Wetting and drying led to an increase of the sorptivity and a decrease of the repellency for all treatments with the exception of the PGA-amended soils. The PGA may therefore stabilize the soil structure in the rhizosphere by increasing the strength of bonds between particles and decreasing the wetting rate.〈section xml:id="abs1-2"〉〈title type="main"〉Influence de mucilages racinaire et microbiens modèles sur la structure du sol et le transport d'eau〈section xml:id="abs1-3"〉〈title type="main"〉RésuméLa production d'exsudats par les plantes et les microbes de la rhizosphère ainsi que les cycles d'humectation–dessiccation très intense due à l'évapotranspiration, entraînent des modifications de la structure du sol. Notre objectif a été de séparer ces deux processus en utilisant un modèle expérimental avec des polysaccharides bactériens (dextran et xanthan) et un analogue d'exsudat racinaire (acide polygalacturonique, APG), et jusqu'à dix cycles d'humectation et dessiccation. Afin de caractériser la structure du sol, la résistance en traction ainsi que l'infiltration de l'eau et de l'éthanol dans le sol amendé par les différents polymères ont été mesurés, et des lames minces ont été réalisées. Le xanthan et l'APG ont provoqué la plus forte augmentation de la résistance en traction, ce qui serait attribuable à une plus grande énergie de liaison entre les particules de sol. La porosité a augmenté avec chaque cycle d'humectation–dessiccation pour tous les traitements et cette augmentation a été moins prononcée pour l'APG 2 g l−1 par rapport au xanthan et au dextran. Cela suggère que l'APG a stabilisé le sol contre la déstructuration provoquée par les cycles d'humectation–dessiccation. L'APG a été le seul polysaccharide qui a influencé– dans le sens d'une diminution – l'infiltration de l'eau dans le sol amendé. Les cycles d'humectation–dessiccation ont entraîné une augmentation de l'infiltration de l'eau dans le sol amendé par les différents polymères à l'exception de l'APG. Ce dernier stabiliserait donc la structure du sol dans la rhizosphère en augmentant la force de liaison entre les particules et en diminuant la vitesse d'humectation du sol.

Notes: Soil microbes produce exudates which upon drying become water-repellent, thus altering hydraulic properties. The influence of microbial activity caused by adding plant nutrients on the hydraulic characteristics of soil aggregates is reported. Soil aggregates were collected from a field that had been fertilized with different amounts of nitrogen. Aggregates were also incubated with different nutrient treatments in the laboratory. Their sorptivity, hydraulic conductivity and water repellency were measured with a new device. Adding nitrogen was found to decrease sorptivity and hydraulic conductivity because of increased water repellency in the field. In the laboratory studies, the addition of nutrients caused severe water repellency in the soil aggregates. Respiration studies identified a large increase in biological activity following nutrient amendment which produces water-repellent materials.

Notes: One mechanism in the restoration of severely degraded soil by vegetation might be the movement of dissolved organic carbon (DOC) to macropore and aggregate surfaces. We propose that this lowers the soil wetting rate and subsequently its slaking resistance by creating a partially hydrophobic surface. In this study, we determined how wetting and drying (w/d) cycles redistribute DOC to soil surfaces, and how DOC affects hydrophobicity where it accumulates, in relation to the soil surface area to volume ratio and to different types of vegetation planted to restore a severely degraded soil. Repacked soil cores that simulate different soil aggregate sizes were tested. The results showed that w/d cycles increase surface DOC concentration through a depletion of DOC in the interior of the soil. Correspondingly, w/d cycles enhanced hydrophobicity, measured as a water repellency index, R, from 1.5–2.3 to 3.6–7.6, the values affected significantly by the type of vegetation. This index (R) did not change for a control soil with no vegetation. The link between the amount of DOC and water repellency was weak (coefficient of determination r2 = 0.06–0.26), indicating that DOC quality was probably more important than its quantity. Although increasing the core size resulted in a greater accumulation of DOC on the drying surface of the core, the impact of this on water repellency was minimal. Incubation caused a decrease in the amount of DOC, but had minimal influence on water repellency. This work improves the understanding of changes in soil wetting and soil stabilization under processes of natural weathering and vegetation restoration.