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  • 1
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: We simulated the distribution of water flowing over the soil surface in small plots using a lattice-gas approach. The numerical lattice-gas algorithm consists in simulating a fluid as a finite set of particles which move in a discrete lattice obeying deterministic rules. With this technique, complex boundary conditions and transient flow regimens are handled with ease. The amount of depressional storage and the onset of runoff were considered as functions of rainfall intensity, surface roughness, slope angle and infiltration dynamics. Rainfall was simulated by spherical raindrops of fixed diameter, which were generated by a space and time Poisson process of constant intensity. The effect of infiltration on surface flow was simulated by removing liquid particles at the solid surface with a chosen rate function. Depressional storage and runoff then result from the intrinsic hydrodynamic behaviour of the model under the imposed rainfall, infiltration rate and surface geometry. Results of numerical experiments are presented. They show that the lattice-gas approach is well-adapted for simulating the formation of excess water and the roles of roughness and slope in attenuating runoff for any arbitrary complex surface geometry.
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  • 2
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: The prismatic structure and soil behaviour were studied on a saline clayey soil in the polder of the Marais de ľQuest (France). Three profiles with different histories were examined, and the structure under wet and dry conditions, ionic concentrations, and changes in void ratio and water suction were measured on prisms sampled between 5 and 40 cm deep, in four places (base, centre, top and side). Seasonal dynamics were accompanied by reorganization within the prismatic structure, changing both the size and distinctiveness of the blocky substructure. Chemical and physical characteristics were closely related to the sharpness of these two imbricated structures. In the absence of blocky substructure the salinity increased horizontally from the centres of the prisms to their sides (the latter acting as exchange surface between the soil and the atmosphere), and no physical gradient was apparent. These results emphasized the permanence of lateral hydraulic continuity in individual prisms. On the other hand, where the blocky substructure was well developed there was no lateral variation of salinity within the prisms, whereas the swelling potential varied from the centres to the sides: the sharpness of the substructure therefore caused a decrease in lateral water transfer. The internal structure of the prisms appeared to determine the seasonal behaviour of the soil.
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  • 3
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: As French soils are naturally free of Bradyrhizobium japonicum, soya beans planted into new areas must be inoculated with this bacterium. Although, the B. japonicum inoculum can survive in soils for long periods of time even in the absence of a soya bean crop, re–inoculation may increase nodulation and grain yield. Thus, populations of B. japonicum can fall below optimum for plant growth. To identify the soil properties controlling survival of the inoculated bacteria samples of soil were collected from 52 sites from France that had previously grown soya beans. The samples were analysed for some physical and chemical characteristics and the B. japonicum population counted. The soil's CaCO3 content was the main factor affecting survival. The average B. japonicum numbers (per g soil) were 80 for calcareous soils and 15000 for non–calcareous soils. In the latter, silt and sand contents were correlated with the numbers of B. japonicum. The cropping frequency of soya bean and the time since the crop was last grown were other factors affecting Bradyrhizobium populations. Thus, there is a probability of enhancing economic benefit for farmers with re–inoculation of soya bean in calcareous and sandy soils.
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  • 4
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Copper sulphate (Bordeaux mixture) has been used as a fungicide against mildew in vineyards for more than a hundred years. This treatment has resulted in significant Cu accumulation in soils (from 100 to 1500 mg kg−1). It is desirable to determine the distribution of Cu in these soils to predict if this element is potentially toxic. Several speciation methods have been compared: sequential and single–step extractions, analytical transmission electron microscopy (ATEM, ASEM) and physical fractionation to study a profile of a vineyard acid soil in Beaujolais (France). Physical fractionation showed that copper is concentrated in the coarse organic fractions associated with plant residues and in the fine clay fraction. Analytical electron microscopy showed a great diversity of fixation sites (bacteria, amorphous organic and mineral compounds). Chemical sequential extractions showed that after the sequential extraction treatments, 60% of Cu was not extracted. Extraction data showed that in the case of an acid sandy soil, sequential chemical extraction did not seem to be sufficiently selective to speciate copper. In the single–step extractions study, the hydroxylamine treatment was the most selective. In the nonselective cases several phenomena may be responsible for the failure, the analytical electron microscopy study showed copper redistribution after certain single–step extractions were carried out.After comparing three different distribution methods we conclude that the distribution of this element in acid soils should be investigated by several analytical methods to establish the true speciation.
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  • 5
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Recent advances in measuring soil microbial biomass by chloroform fumigation–extraction (CFE) and microbial heterotrophic activity interpreted by quantitative concentration–activity relations (QCAR) have renewed interest in assessing side–effects of agricultural chemicals on soil microorganisms. We have studied the effects of a herbicide, 4,6-dinitroorthocresol (DNOC), taken as a test chemical, on the rate of microbial carbon turnover and the size of the soil microbial biomass. We used the CFE technique in combination with in-situ labelling of the soil biomass. Exposure of prelabelled soil samples to the herbicide resulted in a significant increase in 14CO2 production during which the radioactive carbon content of the microflora decreased exponentially without apparent reduction in the size of the biomass. The extra production of 14CO2 by DNOC-treated soil over control, or carbon-enriched soil, is the expression of an increased rate of endogenous metabolism to compensate for shortage in energy caused by a decoupling of ATP generation at the oxidative phosphorylations level by the DNOC. To assess the influence of DNOC on soil microbial communities we also compared the advantages of short-term respirometric tests with those resulting from application of heterotrophic activity measurements in connection with QCAR. Both procedures detected modifications in the metabolic behaviour of soil microorganisms when faced with chemical stress. Short-term respirometric tests showed that DNOC causes a decrease in the respirometric activity of the soil microflora. Measuring heterotrophic activity also makes it possible to interpret microbial responses in terms of changes in the physiological traits of the microbial communities. DNOC provokes an apparent enrichment in microorganisms with a smaller saturation constant, Km and, as a consequence, a greater affinity for carbon substrates.
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  • 6
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: The number of runoff and erosion studies in cultivated catchments in north west Europe is increasing. Measurements are usually costly and time-consuming, in particular of soil hydrological properties because agriculture causes considerable spatial and temporal variation. A subdivision of the catchment is necessary to optimize the choice of sampling sites and to define extrapolation procedures. Four sampling strategies were examined, consisting of subdividing the catchment according to the crops (1), or according to the surface structure (2), followed by increasing the number of sampled fields by adding fields that are presumed to have maximum runoff activity (3) or intermediate activity (4). These strategies were compared by modelling the runoff and erosion in a virtual catchment, containing 15 arable fields with various crop sequences, at three times during a winter season. Strategy 2 gave better results than 1, because of the large variability of surface structure of the bare fields, which was not correlated with the crop type. Runoff prediction benefited from extra sampling only when groups of fields with a large heterogeneity in hydrological behaviour were included. The quality of erosion prediction increased when the steeper fields were included in the sampling scheme. A monitoring strategy is proposed that consists of: (i) repeated, fast and exhaustive descriptions of the soil surface using surface structure classes, (ii) subdivision of the catchment into groups of fields with identical surface characteristics before measurements are made, and (iii) sampling and extrapolation according to these groups.
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  • 7
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: This paper discusses the effects of different horizons and soil solution compositions on dissolved organic matter retention in a moorland podzol and compares the results with previous studies of forest podzols. Adsorption isotherms were constructed for each of the major horizons of a freely draining, upland, moorland, humic podzol from north-east Scotland, to investigate processes of retention and release of dissolved organic matter (DOM). Carbon retention of a range of solute types was studied, and phthalate was chosen as a model compound to measure carbon retention at three different pH values (3, 4.5 and 6). Retention and release of DOM was related to chemical, physical and mineralogical characteristics of the different soil horizons. All the mineral horizons retained DOM, with the Bs horizon most retentive. Solution pH did not significantly affect DOM retention in the O and A horizons. At pH 3 and 4.5 organic matter was weakly retained in the Bhs horizon, but strongly retained in the Bs and the Cx horizons. At pH 6 reversal of surface charge occurred in the Bs and Cx horizons resulting in the release of similar amounts of organic matter to that released from the O horizon at the same pH. The results demonstrate how podzols act as a ‘valve’ in controlling the input of dissolved organic compounds into surface and ground water, and how sensitive the controlling mechanisms are to pH change.
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  • 8
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Dynamic simulation models of the nitrogen cycle are an important aid for predicting the effect of land management on nitrate concentration in ground and surface waters. These models often neglect, or treat in a highly empirical way, the process of denitrification. The solute leaching model SLIM, suitable for use in aggregated soil, was linked to two alternative denitrification submodels ('simple structure'and ‘aggregate assembly’ treatments). The linked models can be used in dynamic simulation of the fate of nitrate in soil. The models were calibrated using water release and soil respiration data from two field sites (a clay loam and a sandy loam). The fate of Br−1 and 99 atom percentage 15NO−3 tracers on drained plots was measured at these sites. Both models predicted less denitrification from plots receiving small inputs of fresh organic residue than was actually lost; both did better where residue inputs were large. The ‘aggregate assembly’ model predicted short-term losses immediately after incorporation of residues more successfully than did the ‘simple structured’ model. The simulated dynamics of denitrification during the autumn and winter in the two models were also very different. Overall N balances were better predicted in the clay loam than the sandy loam. Possible reasons for discrepancies between model output and real data include immobilization (especially when large inputs of residues were incorporated), denitrification in the subsoil, and an inadequate description of the way in which degradation of organic matter depends on temperature.
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  • 9
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Cation exchange reactions in clays and soils have repeatedly been shown in the literature to be hysteretic during typical laboratory experiments. Yet all existing models of solute transport in soils and aquifers assume these reactions to be thermodynamically reversible. To remedy this situation a mathematical description of cation exchange hysteresis is needed. In the present research isotherms were obtained for a K-Mg exchange on Camp Berteau montmorillonite. Scanning curves were also measured, corresponding to arbitrary reversals of the direction of the exchange reactions. Three different Preisach-hysteresis models were used to describe the scanning curves. The fit of each hysteresis model to the experimental observations was analysed in terms of the features of the model itself and, from a physical viewpoint, in terms of a recently proposed conceptual model of cation exchange hysteresis.
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  • 10
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Moisture content and bulk density largely characterize physical and mechanical soil status and behaviour. A nondestructive determination of these soil properties is essential. Time domain reflectometry (TDR), although widely accepted for determination of volumetric water content, θ, has its limitations, and recently a frequency domain (FD) sensor has been developed and tested. An equation relating relative permittivity, ɛ′, to gravimetric water content, w, and bulk density, p, was established for three soil types (sand, sandy loam and clay). If ɛ′ and w are known, our model can be used to calculate bulk density and associated volumetric water content, θ, keeping in mind that θ= pw. Utilization is found in long-term monitoring of moisture fluctuations or short-term detection of traffic-induced soil compaction.
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