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  • Blackwell Science Ltd/Inc.  (4)
  • 1
    Electronic Resource
    Electronic Resource
    Oxford, UK; Malden, USA : Blackwell Science Ltd/Inc.
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: According to the literature, soil physical properties are linked mainly with organic constituents that are often considered as the first indicator of soil fertility. But the use of fertilizers and amendments can change soil properties independently of the organic matter content. In a long-term experiment at Versailles, fertilizers and amendments have been applied each year to uncultivated plots. After 70 years, the plots had the same low organic matter content except those which were treated with manure. However, the physico-chemical environment had become strongly differentiated. Physical properties, especially soil water relations, were also greatly affected. The use of ammoniacal fertilizers strongly decreased soil pH and cation exchange capacity (CECsoil). Plots treated in this way were more sensitive to the degradation of their hydraulic properties and became unstable in spite of the preservation of their porosity. Basic amendments (i.e. bases added as CO32–, OH–, O2– or silicate anions) increased soil pH, CECsoil and its saturation by exchangeable calcium. The increase in CECsoil improved soil structural cohesion and water flow properties. After basic treatment, there is greater structural stability, and water moves faster through the soil. In potassic and sodic plots, K+ and Na+ affected water movement and increased the soil's sensitivity to degradation. Manure treatment increased water retention and soil stability. The cation exchange capacity, measured at soil pH, can be used as a good indicator of soil stability, in combination with the organic matter content and the kinds of exchangeable cation (especially K+ and Na+).
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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: A sample of ombrotrophic peat from Moor House in northern England was extensively extracted with dilute nitric acid (pH 1) to free it of bound cations. Suspensions of the acid-washed peat (5–30 g l−1), prepared with different concentrations of background electrolyte (NaCl and KCl), were used to conduct batch acid–base titrations. A strong dependence of proton release on ionic strength (I) was observed, the apparent acid dissociation constant (pKapp) being found to decrease by approximately 1.0 for each tenfold increase in I. This behaviour could not be explained satisfactorily with Humic Ion-Binding Model VI, a discrete-site/electrostatic model of cation binding by humic substances, parameterized with data from laboratory studies on isolated samples. More success was obtained by abandoning the impermeable-sphere electrostatic submodel used in Model VI, and instead assuming the peat to consist of aggregates with fixed internal volume, and with counterion accumulation described by the Donnan model, as proposed by Marinsky and colleagues. The fixed-volume Donnan model (Model VI-FD) could also approximately explain other reported results from acid–base titrations of peat, including the effects on the titrations of complexing cations (Al, Ca, Cu). Copper titrations of the Moor House sample were performed using an ion-selective electrode, with peat suspensions in the acid pH range, at two ionic strengths, and in the presence of Al and Ca. The measured concentrations of Cu2+ were in the range 10−13−10−5 m. Model VI-FD provided reasonable fits of the experimental data, after optimization of the intrinsic binding constant for Cu, the optimized value being close to the default value derived previously from data referring to isolated humic substances. The optimized constants for Al and Ca, derived from their competition effects, were also close to their default values. Additional experiments were performed in which the centrifugation-depletion method was used to measure the binding of a cocktail of metals (Al, Ni, Cu, Zn, Cd, Eu, Pb) at a single pH. The model correctly predicted strong binding of Al, Cu, Eu and Pb, and weaker binding of Ni, Zn and Cd. For the strongly binding metals, the dissolved forms were calculated to be mainly due to complexes with dissolved humic matter, whereas the free ions (Ni2+, Zn2+, Cd2+) dominated for the weakly binding metals. Acid-washed soil appears to provide a valuable intermediate between isolated humic substances and untreated soil for the investigation of cation binding by natural organic matter in the natural environment.
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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: A prerequisite to investigate the importance of osmotic potential (Ψo) in relation to matric potential (Ψm) in the soil for water uptake is the existence of a method that measures the temporal and spatial dynamics of Ψo in the vicinity of roots. One method for measuring Ψoin situ is the collection of soil solution with micro suction cups, the spatial resolution of which is suitable for rhizosphere studies. A major drawback of soil solution sampling is the disturbance of soil solution equilibrium, which makes frequent measurements impossible, so another method is required to provide information on the temporal dynamics of Ψo. The time-domain reflectometry (TDR) technique might be suitable as the signal attenuation (σ) shows a close linear correlation with the salt concentration for a known soil water content. The temporal resolution of the TDR technique is high and the measurement has no impact on soil solution equilibrium. However, the spatial resolution of the TDR technique is too coarse to be used on its own in rhizosphere studies.We used a combination of TDR (fine temporal resolution) and micro suction cups (fine spatial resolution) to measure Ψo in a model system with Zea mays grown in quartz substrates. Osmotic potential changed continuously with time, and a steep gradient between bulk soil and the root compartment developed during the 39-day growing period. The steepest gradient measured over a distance of 6 mm across the nylon net, separating the bulk soil from the root compartment, was −365 kPa. The combination of both methods made it possible to extend the time interval between micro suction cup samplings and thus minimize the impact of sampling on soil solution equilibrium. Problems of separate calibration were avoided by calibrating the TDR measurements against the results obtained with the micro suction cups within the same experiment.
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  • 4
    Electronic Resource
    Electronic Resource
    Oxford, UK; Malden, USA : Blackwell Science Ltd/Inc.
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Granite inselbergs protrude from forest and savanna in the tropics. They are exposed to harsh climates (alternation of heavy rain and severe drought) and provide little nutrient for plants. Soil animals and humus components were investigated in cyanobacterial crusts close to patches of epilithic vegetation on the surface of the Nouragues inselberg (French Guiana). Three biological crust samples, corresponding to bromeliacean carpets of increasing size (supposed of increasing age), were sampled for faunal and micromorphological studies. Arthropods (mainly mites and insects) were abundant and highly diversified, the more so after enchytraeid worms ate and transformed the cyanobacterial mass. Below the superficial cyanobacterial crust, humus was made of a loose assemblage of enchytraeid faeces where these animals were present, or of a compact assemblage of cyanobacteria and amorphous organic matter where mites were the dominant animal group. Roots abounded in the humified part of the crust. We conclude that soil invertebrates, in particular enchytraeid worms, are important for the accumulation of organic matter on granite outcrops, and so therefore for the encroachment of plant succession.
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