ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1996.tb01842.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

SummaryCrusting and erosion of cultivated soils result from aggregate breakdown and the detachment of soil fragments by rain, and the susceptibility of soil to these processes is often inferred from measurements of aggregate stability. Here, theories of aggregate breakdown are reviewed and four main mechanisms (i.e. slaking, breakdown by differential swelling, mechanical breakdown by raindrop impact and physico–chemical dispersion) are defined. Their relative importance depends on the nature of the rain, as well as on the soil's physical and chemical properties. The relations between aggregate breakdown, crusting and water erosion are analysed, and existing methods for the assessment of aggregate stability are reviewed. A unified framework for the measurement of aggregate stability is proposed to assess a soil's susceptibility to crusting and erosion. It combines three treatments having various wetting conditions and energies (fast wetting, slow wetting, and stirring after pre‐wetting) and measures the resulting fragment size distribution after each treatment. It is designed to compare different soils, or different climatic conditions for a given soil, not to compare time‐dependent changes in that

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1996.tb01843.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

SummaryExamples of pore and solid chord distributions obtained for ideal porous media are presented, and the distributions of the porous and solid phases of a soil have been studied by pore and solid chord distributions. Serial sections, 100 μm apart, were cut in a soil core, impregnated with resin, and images were obtained of them. The 2D images from 160 sections were used to build a 3D reconstruction of the core. The initial 2D images, the 3D reconstructed medium and 2D computed images from the latter were studied. We found that the solid matrix of the particular soil is homogeneous and isotropic at the scale studied, and it could be characterized with a single 2D image. For pores ranging from 500 μm to 2 mm we also found similar pore chord distributions for the 2D images in the three orthogonal directions and the 3D medium. A single 2D image can be used to study these pores. For larger pores more than one 2D image is required, and we showed that eight 2D images are sufficient to describe these pore

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1996.tb01844.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

SummaryWe 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 g

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1996.tb01845.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

SummaryThe 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

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1996.tb01846.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

SummaryUniversal kriging with a topographic variable used as external drift was compared with ordinary kriging, ordinary kriging with external drift, and universal kriging for predicting the thickness of a superficial deposit on part of the Beauce of central France. Thickness was recorded at 62 sites and at another 25 sites to obtain a sample for validation. A Digital Elevation Model (DEM) was computed from 484 sites where relative elevation was measured. The slope gradient which is significantly correlated to thickness was used as an external drift variable for kriging thickness. The sample for validation was used for testing the four prediction methods. In testing each of the methods, two indices were computed from the validation sample (observed data) and predicted values. These two indices are the mean error(ME)and the root mean square error(RMSE).The results show that universal kriging with the slope gradient used as external drift was more precise than the other kriging methods. Further analysis based on the mean ranks of performance confirmed these results.

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1996.tb01847.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

SummarySoil organic matter (SOM) dynamics are usually described by compartmental models. We have sought SOM separates that might be related to SOM dynamic compartments. The turnover of C in various separates from long–term field experiments with maize was measured using the natural13C labelling technique. The Rothamsted carbon model gave a good prediction of the observed C turnover. Primary particle–size fractions coarser than 50 μm had short lives, and could be associated with the plant structural compartment of models. Water–extractable components are enriched in young C but cannot be associated with labile compartments. None of the chemical separates obtained by acid hydrolysis, wet oxidation, thermic oxidation, pyrolysis or alkaline extraction, were enriched either in young or old C. The results showed neither a sequential relation between fulvic acids and humic acids nor a resistance of nonhydrolysable material. The range of lifetimes of soil C seems to be determined more by physical position and protection than by the chemical nature

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1996.tb01848.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

SummaryParticulate organic matter (POM) is a labile fraction of soil organic matter which is thought to be physically protected from biodegradation when within soil aggregates. We have developed a fractionation method to separate POM located outside stable soil macroaggregates (>200 μm) and microaggregates (50–200 μm) from that within them, and applied it to a cultivation sequence of humic loamy soils. The natural abundance of13C was used to determine the amounts of POM derived from forest and that derived from crop in the free and occluded fractions. In the forest soil the free and occluded POM fractions had the same composition, morphology and isotopic signature. On cultivation the amounts of POM decreased sharply. The loss of C in the POM from forest was mainly from POM outside the aggregates. The POM occluded within microaggregates was found to turnover slowly. This may be due either to its recalcitrant chemical nature or to its physical protection within microaggreg

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1996.tb01849.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

SummaryAs French soils are naturally free ofBradyrhizobium japonicum, soya beans planted into new areas must be inoculated with this bacterium. Although, theB. japonicuminoculum 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 ofB. japonicumcan 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 theB. japonicumpopulation counted. The soil's CaCO3content was the main factor affecting survival. The averageB. japonicumnumbers (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 ofB. japonicum.The cropping frequency of soya bean and the time since the crop was last grown were other factors affectingBradyrhizobiumpopulations. Thus, there is a probability of enhancing economic benefit for farmers with re–inoculation of soya bean in calcareous and sandy

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1996.tb01850.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

SummaryThe interaction of extracellular enzymes with the solid phase of the soil affects their mobility and their catalytic properties. In particular, adsorption on clay minerals is known to shift the optimum pH of the catalytic activity towards alkaline values. Two conflicting interpretations of this phenomenon have been proposed: a surface pH effect and a pH–dependent modification of the conformation of the adsorbed enzyme. To assess the two mechanisms, we studied the adsorption on montmorillonite of two extracellular acid phosphatases of the ectomycorrhizal fungusHebeloma cylindrosporum, and its consequences on catalytic activities. The results obtained are better interpreted by a pH–dependent modification of enzyme conformation due mainly to electrostatic interactions with the clay surface. At low pH, the two positively charged enzymes unfold on the negatively charged montmorillonite surface. At high pH, both the enzymes and the clay are negatively charged, and adsorption decreases. Adsorption and modification of conformation are largely irreversible, which should be taken into account when considering the fate of enzymes in soil. Finally, the comparison with the effect of clays on the catalytic activities of intracellular enzymes raises the hypothesis of a selection pressure of the soil solid phase leading to more stable extracellular enzy

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1996.tb01851.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY