ummary
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.

Influence de mucilages racinaire et microbiens modeles sur la structure du sol et le transport d'eau
Resume
La production d'exsudats par les plantes et les microbes de la rhizosphere ainsi que les cycles d'humectation–dessiccation tres intense due a l'evapotranspiration, entrainent des modifications de la structure du sol. Notre objectif a ete de separer ces deux processus en utilisant un modele experimental avec des polysaccharides bacteriens (dextran et xanthan) et un analogue d'exsudat racinaire (acide polygalacturonique, APG), et jusqu'a dix cycles d'humectation et dessiccation. Afin de caracteriser la structure du sol, la resistance en traction ainsi que l'infiltration de l'eau et de l'ethanol dans le sol amende par les differents polymeres ont ete mesures, et des lames minces ont ete realisees. Le xanthan et l'APG ont provoque la plus forte augmentation de la resistance en traction, ce qui serait attribuable a une plus grande energie de liaison entre les particules de sol. La porosite a augmente avec chaque cycle d'humectation–dessiccation pour tous les traitements et cette augmentation a ete moins prononcee pour l'APG 2 g l−1 par rapport au xanthan et au dextran. Cela suggere que l'APG a stabilise le sol contre la destructuration provoquee par les cycles d'humectation–dessiccation. L'APG a ete le seul polysaccharide qui a influence– dans le sens d'une diminution – l'infiltration de l'eau dans le sol amende. Les cycles d'humectation–dessiccation ont entraine une augmentation de l'infiltration de l'eau dans le sol amende par les differents polymeres a l'exception de l'APG. Ce dernier stabiliserait donc la structure du sol dans la rhizosphere en augmentant la force de liaison entre les particules et en diminuant la vitesse d'humectation du sol.

Root‐ and microbial‐derived mucilages affect soil structure and water transport

Abstract Despite its widespread use in petrophysics, tortuosity remains a poorly understood concept. Tortuosity can have various meanings when used by physicists, engineers or geologists to describe different transport processes taking place in a porous material. Values for geometrical, electrical, diffusional and hydraulic tortuosity are in general different from one another. Electrical tortuosity is defined in terms of conductivity whereas hydraulic tortuosity is usually defined geometrically, and diffusional tortuosity is typically computed from temporal changes in concentration. A better approach may be to define tortuosity in terms of the underlying flux of material or electrical current with respect to the forces which drive this flow. Unsteady transport processes, including diffusion, can be described only by a population of tortuosities corresponding to the different flow paths taken by particles traversing the medium. In measurements of steady flow (e.g., those normally used to obtain resistivity or permeability), information about particle travel times is lost, and so the multiple values of tortuosity are homogenised. It can be shown that the maximum amount of information about pore structure is embedded in transport processes that combine advective and diffusive elements. Most existing formulations of tortuosity are model-dependent, and cannot be correlated with independently measurable pore-structure properties. Nevertheless, tortuosity underpins the rigorous relationships between transport processes in rocks, and ties them with the underlying geometry and topology of their pore spaces. Tortuosity can be redefined in terms of the energetic efficiency of a flow process. The efficiency is related to the rate of entropy dissipation (or isothermally, energy dissipation) with respect to a simple, non-tortuous model medium using the postulates of non-equilibrium thermodynamics. Through Onsager’s reciprocity relation for coupled flows it is possible to inter-relate efficiency for pairs of transport processes, and so go some way towards unifying tortuosity measures. In this way we can approach the goal of predicting the value of one transport parameter from measurements of another.

Tortuosity: a guide through the maze

Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Georgia Institute of Technology Simultaneous Inversion of Rayleigh Phase Velocity and Attenuation for Near-Surface Site

Simultaneous Inversion of Rayleigh Phase Velocity and Attenuation for Near-Surface Site Characterization

Formation and collapse of metastable particle packings and open structures in loess deposits

Soil Physics Introduction Markus Tuller Physical Properties of Primary Particles Joseph M. Skopp Soil Structure Teamrat A. Ghezzehei Mechanics of Unsaturated Soils for Agricultural Applications Rainer Horn and Stephan Peth Soil Water Content and Water Potential Relationships Dani Or, Jon M. Wraith, David A. Robinson, and Scott B. Jones Water Flow in Soils David E. Radcliffe and Jirka Simunek Water and Energy Balances in the Soil-Plant-Atmosphere Continuum Steven R. Evett, John H. Prueger, and Judy A. Tolk Solute Transport Feike J. Leij and Antonella Sciortino Gas Transport in Soils Dennis E. Rolston and Per Moldrup Soil Thermal Regime Robert Horton and Tyson Ochsner Soil Spatial Variability Ole Wendroth, Sylvia Koszinski, and Vicente Vasquez Soil Chemistry Introduction Donald L. Sparks Soil Organic Matter Jeffrey A. Baldock and Kris Broos Soil Solution Paul Schwab Kinetics and Mechanisms of Soil Chemical Reactions Donald L. Sparks Oxidation-Reduction Phenomena Bruce R. James and Dominic A. Brose Soil Colloidal Behavior Sabine Goldberg, Inmaculada Lebron, John C. Seaman, and Donald L. Suarez Ion Exchange Phenomena Ian C. Bourg and Garrison Sposito Chemisorption and Precipitation Reactions Robert G. Ford Role of Abiotic Catalysis in the Transformation of Organics, Metals, Metalloids, and Other Inorganics Pan Ming Huang and A.G. Hardie Soil pH and pH Buffering Paul R. Bloom and Ulf Skyllberg Soil Mineralogy Introduction Joseph W. Stucki Alteration, Formation, and Occurrence of Minerals in Soils G. Jock Churchman and David J. Lowe Phyllosilicates Hideomi Kodama Oxide Minerals in Soils Nestor Kampf, Andreas C. Scheinost, and Darrell G. Schulze Poorly Crystalline Aluminosilicate Clay Minerals James Harsh Soil Biology and Biochemistry: Soil Biology in Its Second Golden Age Introduction E.A. Paul and P. Nannipieri Microbiota Raffaella Balestrini, Valeria Bianciotto, Paola Bonfante, Michael Schloter, Sharath Srinivasiah, R. Greg Thorn, Kurt E. Williamson, and K. Eric Wommack Soil Fauna Michael Bonkowski, M.A. Callaham, Jr., Marianne Clarholm, David C. Coleman, D.A. Crossley, Jr., Bryan Griffiths, Paul F. Hendrix, Mark G. St. John, Robert McSorley, and P.C.J. van Vliet Microbially Mediated Processes Susumu Asakawa, Else K. Bunemann, Emmanuel Frossard, E.G. Gregorich, Jan Jansa, H.H. Janzen, Michael A. Kertesz, Makoto Kimura, Loretta Landi, David Long, Terence L. Marsh, Paolo Nannipieri, Astrid Oberson, Giancarlo Renella, and Thomas Voice Nitrogen Transformations Richa Anand, Jean-Claude Germon, Peter M. Groffman, Jeanette M. Norton, Laurent Philippot, James I. Prosser, and Joshua P. Schimel Molecular Techniques Judith Ascher, Yin Chen, Guo-Chun Ding, Holger Heuer, Jiri Jirout, Deepak Kumaresan, J. Colin Murrell, Giacomo Pietramellara, Kornelia Smalla, and Maria Teresa Ceccherini Pedology Introduction Larry T. West and Larry P. Wilding Geomorphology of Soil Landscapes Douglas A. Wysocki, Philip J. Schoeneberger, Daniel R. Hirmas, and Hannan E. LaGarry Pedogenic Processes Judith Turk, Oliver A. Chadwick, and Robert C. Graham Soil Taxonomy Robert J. Ahrens and Richard W. Arnold Other Systems of Soil Classification Erika Micheli and Otto C. Spaargaren Classification of Soils Olafur Arnalds, Fredrich H. Beinroth, J.C. Bell, J.G. Bockheim, Janis L. Boettinger, M.E. Collins, R.G. Darmody, Steven G. Driese, Hari Eswaran, Delvin S. Fanning, D.P. Franzmeier, C.T. Hallmark, Willie Harris, Wayne H. Hudnall, Randall K. Kolka, David J. Lowe, Paul A. McDaniel, D.G. McGahan, H. Curtis Monger, Lee C. Nordt, Chien-Lu Ping, Martin C. Rabenhorst, Paul F. Reich, Randall Schaetzl, Joey N. Shaw, Christopher W. Smith, Randal J. Southard, David Swanson, C. Tarnocai, Goro Uehara, Larry T. West, and Larry P. Wilding Land Evaluation for Landscape Units J. Bouma, J.J. Stoorvogel, and M.P.W. Sonneveld Hydropedology Phillip Owens, Henry Lin, and Zamir Libohova Subaqueous Soils Mark H. Stolt and Martin C. Rabenhorst Digital Soil Mapping Alex. B. McBratney, Budiman Minasny, Robert A. MacMillan, and Florence Carre Soil Change in the Anthropocene: Bridging Pedology, Land Use and Soil Management Daniel deB. Richter Jr. and Arlene J. Tugel Noninvasive Geophysical Methods Used in Soil Science James A. Doolittle Index

Handbook Of Soil Sciences: Properties And Processes

Fundamentals of Soil Behaviour

Six definable particle types need to be recognised in engineering soils. Failure to appreciate the true nature of soil and to treat soils crudely as a ‘black box’ has led to many different types of failures. The basic five types that need to be recognised are (related to the R-size diagram, see Figure 1) AA’ active clay minerals, BB’ inactive clay minerals, C fine cohesive primary minerals, D silt (at R approximately equals 1), E sand. We estimate that at about 200 microns and above the cohesive forces can be neglected, so that truly granular soils start at 200 microns and extend to 60 mm. We propose 60 mm as the upper limit of truly granular behaviour, which has been arbitrary chosen for the discussion purposes. From 50 to 200 microns we propose that a sixth definable sub-region type occurs, where R<l but cohesive forces are still significant.

Ian Jefferson

Papers

Fundamentals of Soil Behaviour

Six Definable Particle Types in Engineering Soils and Their Participation in Collapse Events: Proposals and Discussions

Distinctive Particle Types in Ground Materials: Implications for Engineering Geology