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  • 1
    Electronic Resource
    Electronic Resource
    A mathematical model of root uptake of cations incorporating root turnover, distribution within the plant, and recycling of absorbed species (2000)
    Oxford, UK : Blackwell Science Ltd
    European journal of soil science 51 (2000), S. 0 
    Details
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Understanding the movement of cations in soil, particularly trace metals, is required in many applications such as phytoremediation and pollution control. A dynamic mechanistic model has been developed to describe the long-term root uptake of a surface-applied, strongly adsorbed, pollutant metal cation, such as radiocaesium, from soil. It consists of two submodels. The first calculates uptake per unit root length at a local scale over a root's lifetime, for various initial conditions. The second calculates cumulative uptake at a whole-plant scale for the entire rooting depth as a function of time. The model takes into account the renewal of roots which are considered to have a limited lifetime. Root density may be a function of soil depth and a proportion of roots need not contribute to uptake. Recycling from decaying, or grazed, roots and shoots is considered.Simulations show that removal of cations from soil is exaggerated unless some recycling by roots or shoots is considered or the entire root length does not contribute to uptake. Because of root turnover, uptake is not rapidly limited by diffusive flux of the cation from the bulk soil solution to the solution–root interface. Uptake is very sensitive to root architecture and plant physiology.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-2389.2000.00331.x
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  • 2
    Electronic Resource
    Electronic Resource
    The effect of high solute concentrations on nitrification rates in soil (1987)
    Springer
    Plant and soil 97 (1987), S. 37-45 
    Details
    ISSN: 1573-5036
    Keywords: Ammonium ; Chloride ; Inhibition ; Nitrification ; Osmotic pressure ; Sorbitol ; Sulphate
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Summary A short term nitrification assay (〈18 h) was used to assess the effect of high concentrations of different solutes on the rate of nitrate production. High solute concentrations were found to inhibit nitrification and the degree of inhibition was related both to the osmotic pressure of the soil solution and the osmoticum used. Ammonium chloride, ammonium sulphate and sorbitol were used as sources of osmotic pressure. The results showed that, with ammonium salts, no inhibition was observed with pressures less than 2 atm. Above these values, the severity of the inhibition followed the order ammonium chloride〉ammonium sulphate〉sorbitol up to the maximum osmotic pressure studied (25 atm). With ammonium chloride, a pressure of 3.5 atm. was sufficient to cause a 90% inhibition of nitrification rate. The inhibition produced by mixtures of ammonium chloride and sorbitol, each mixture generating an osmotic pressure of 5 atm. in the assay, was also investigated. The results suggest that inhibition by Cl-ion is disproportionate to its contribution to the osmotic pressure of the soil solution. The recovery of the nitrification rate, following exposure to high osmotic pressure solutions, was also investigated. It was found that the recovery of the nitrification rate was only partial, with the extent of the recovery diminishing as the severity of the initial osmotic stress applied increased. These results suggest that both reversible and irreversible mechanisms are involved in the inhibition of nitrification.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02149821
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  • 3
    Electronic Resource
    Electronic Resource
    A theoretical consideration of the implications of cell clustering for the prediction of nitrification in soil (1987)
    Springer
    Plant and soil 99 (1987), S. 387-400 
    Details
    ISSN: 1573-5036
    Keywords: Ammonium oxidation ; Clusters ; Modelling ; Nitrification ; pH effects
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Summary The time course of nitrate production is different in different soils; in some soils, nitrate is produced at a constant rate while in others, this rate increases with time, often exponentially. Mechanistic models, based on the Monod equations, cannot account for a constant rate of nitrification. All such mechanistic models make the implicit assumption that the nitrifying organisms are distributed uniformly as single cells throughout the soil volume, while in reality, the cells might be expected to occur in small clusters formed by repeated cell division. This paper examines the effects of allowing the ammonium oxidising cells to occur in evenly distributed clusters of cells of equal volume. One effect of clustering would be the lowering of soil pH around the cluster, caused by differences in the rates of acid production and diffusion. The effects of this pH depression were examined using a mathematical model. In general, it was found that the effect of clustering was to reduce the rate of ammonium oxidation. In extreme cases, in which the fraction of the soil volume occupied by the cells was assumed to be small, the model predicted a constant rate of ammonium oxidation with time in contrast to the increasing rate with time predicted by a model based on a uniform single cell distribution. The clustering model was therefore capable of reproducing the different time courses of ammonium oxidation reported in the literature. The differences between the time courses of ammonium oxidation predicted using the two different assumptions was affected by the initial pH of the soil. This observation suggested a possible experimental test of the clustering hypothesis.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02370884
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  • 4
    Electronic Resource
    Electronic Resource
    Re-sorption of organic components by roots of Zea mays L. and its consequences in the rhizosphere (1992)
    Springer
    Plant and soil 143 (1992), S. 259-266 
    Details
    ISSN: 1573-5036
    Keywords: maize ; organic components ; re-sorption ; rhizosphere ; root
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Abstract The re-sorption of carbon compounds from the rhizosphere was investigated using 14C-labelled glucose, mannose and citric acid. Uptake in roots of 5-day-old, intact Zea mays plants in sterile solution culture was determined over a period of 48 hours. Under optimal growth conditions significant re-absorption of glucose and mannose occurred with the uptake rates being 70.5 and 40.2 μg compound g-1 root DW h-1, respectively. For glucose and mannose approximately 25% of the 14C label taken up by the root was recovered inside the plant as low-MW compounds and 33% polymerized into high MW compounds. 42% was respired as 14C-CO2. Citric acid by comparison showed little accumulation within plant tissues (11.4%) with most being respired and recovered as 14C-CO2 in KOH traps (88%). The uptake rate for citric acid was 34.8 μg g-1 root DW h-1. Over the 48-hour period a net efflux (i.e. exudation) of labelled plus unlabelled C was observed at a rate of 608 μg C g-1 root DW h-1 (equivalent to 1520 μg glucose/mannose). Of the C released as root exudates, a minimum estimate of the amount of C taken back into the plant was therefore 9.5%. The two main C fluxes within the rhizosphere, namely release of C by the root and uptake by the microorganisms, have been well documented in recent years. It is now apparent however that a third flux term, re-sorption of C by roots, can also be identified. This may play an important but previously overlooked role within the rhizosphere, and further work is needed to determine its significance. A comparison between exudate release in static (permitting accumulation of C) and flowing culture (C removed as it is released) was also made with the respective rates being 15.36 and 45.18 mg C g-1 root DW in 2 days. The relative important of re-sorption in natural environments and laboratory experiments is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00007881
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  • 5
    Electronic Resource
    Electronic Resource
    Models of the rhizosphere (1991)
    Springer
    Plant and soil 133 (1991), S. 187-199 
    Details
    ISSN: 1573-5036
    Keywords: insoluble carbon ; mathematical model ; population dynamics ; rhizosphere ; soluble carbon
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Abstract A mathematical model has been developed which is capable of simulating the population dynamics of microbial biomass surrounding a root which is releasing soluble and insoluble carbon compounds into the soil. The model simulates the interconversions of C between different pools within the soil as well as the diffusion and mass flow of soluble carbon. Two main aspects of carbon release were examined: (i) a strategy where exudate was released uniformly over the root surface was compared to the case where exudation was confined to a small region behind the root tip; (ii) the situation in which all the C released was in soluble form was compared to the case of an approximately equal partition between soluble and insoluble forms. Substantial differences between the different simulations were found. It was shown that the maximum concentration and penetration of soluble exudates differed markedly between different simulations and the implications of this for micronutrient acquisition by phytosiderophores and for colonisation of the rhizosphere by root pathogens were discussed. The different simulations also predicted very different biomass distributions in the rhizosphere in both space and time.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00009191
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  • 6
    Electronic Resource
    Electronic Resource
    A fluorimetric method for measuring the activity of soil enzymes (1986)
    Springer
    Plant and soil 92 (1986), S. 81-88 
    Details
    ISSN: 1573-5036
    Keywords: Assay ; Enzymes ; Fluorimetric ; Kinetics ; Method ; 4-Methylumbelliferone ; Soil
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Summary A fluorimetric method is described for the measurement of the activity of a range of soil enzymes. The method is based on the measurement of 4-methylumbelliferone (MUB), a fluorescent product liberated on hydrolysis of the enzyme substrate. The main advantage of the method over colorimetric techniques is that separation of MUB from the soil is unnecessary and the method is therefore suitable for routine, automated analyses. The method was used to measure the activity of β-cellobiase, β-galactosaminidase, β-glucosidase and β-xylosidase over a wide range of substrate concentration and in a range of soils. Kinetic parameters are reported for these enzymes. The method was also shown to be suitable for the assay of arylsulphatase and acid and alkaline phosphatase in soil. The technique should be applicable to a wide range of soil hydrolases, using the same assay methods.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02372269
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  • 7
    Electronic Resource
    Electronic Resource
    Modelling growth responses of soil nitrifiers to additions of ammonium sulphate and ammonium chloride (1985)
    Springer
    Plant and soil 86 (1985), S. 425-439 
    Details
    ISSN: 1573-5036
    Keywords: Ammonium ; Chloride ; Growth ; Inhibition ; Monod model ; Nitrification ; Soil ; Sulphate
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Summary Following the addition of 0–75 μmole N g−1 as ammonium chloride or ammonium sulphate to a sandy loam soil the nitrate formed was measured daily for a period of 15–17 days. The nitrate produced as a function of time was described using the Monod equation for microbial growth. An optimisation technique is described for obtaining, from the nitrification time course data, the maximum specific growth rate, the affinity constantant and an index limited by the concentration of ammonium in soil solution. Additions of more than 7.3 μmoles N g−1 soil as ammonium chloride were found to inhibit nitrification. The inhibition was interpreted as being caused by osmotic pressure or by chloride ion. A similar effect was not found with ammonium sulphate, because the salt concentration in the soil solution was restricted by the precipitation of calcium sulphate. The model developed was capable of accounting for nitrate production in the soil under non-steady state conditions of substrate concentrations and nitrifier biomass.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02145463
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  • 8
    Electronic Resource
    Electronic Resource
    Role of proteinaceous amino acids released in root exudates in nutrient acquisition from the rhizosphere (1994)
    Springer
    Plant and soil 158 (1994), S. 183-192 
    Details
    ISSN: 1573-5036
    Keywords: amino acids ; nutrient acquisition ; rhizosphere ; root exudates
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Abstract The role of proteinaceous amino acids in rhizosphere nutrient mobilization was assessed both experimentally and theoretically. The degree of adsorption onto the soil's solid phase was dependent on both the amino acid species and on soil properties. On addition of amino acids to both soil and freshly precipitated Fe(OH)3, no detectable mobilization of nutrients (K, Na, Ca, Mg, Cu, Mn, Zn, Fe, S, P, Si and Al) was observed, indicating a very low complexation ability of the acidic, neutral and basic amino acids. This was supported by results from a solution equilibria computer model which also predicted low levels of amino acid complexation with solutes present in the soil solution. On comparison with the Fe(OH)3 and equilibria data obtained for the organic acid, citrate, it was concluded that amino acids released into the rhizosphere have a limited role in the direct acquisition of nutrients by plants. The effectiveness of root exudates such as amino acids, phytosiderophores and organic acids in nutrient mobilization from the rhizosphere is discussed with reference to rhizosphere diffusion distances, microbial degradation, rate of complexation and the root's capacity to recapture exudate-metal complexes from the soil.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00009493
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  • 9
    Electronic Resource
    Electronic Resource
    Amino-acid influx at the soil-root interface of Zea mays L. and its implications in the rhizosphere (1994)
    Springer
    Plant and soil 163 (1994), S. 1-12 
    Details
    ISSN: 1573-5036
    Keywords: amino-acids ; maize ; rhizosphere ; root exudates
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Abstract The aim of the study was to investigate the ability of intact Zea mays. L. roots to regulate the amount of free amino-acids present in the rhizosphere. Using metabolic inhibitors it was demonstrated that the release of amino-acids from the root occurred by passive diffusion, whilst free amino-acids outside the root could be re-captured by an active transport mechanism. The influx of amino-acids into the root was shown to be relatively independent of spatial location along the root and was little affected by the presence of other organic compounds in solution. It was deduced from root concentration gradients that the main site of amino-acid exudation was at root tips. Amino-acid uptake by the root was shown to be independent of both inorganic-N concentration and the presence of other organic solutes in solution. A computer simulation model was constructed to assess the contribution of organic-N uptake (acidic, basic and neutral amino-acids) to the plant's N budget, in comparison to the inorganic solutes NO3 and NH4. Simulations of N uptake from a 0.5 mm radius rhizosphere indicated that when inorganic-N concentrations in soil were limiting (≤0.1 μmoles cm-3 soil), the uptake of amino-N accounted for up to 90% the total N taken up by the roots. In situations where fertilizer inputs are high, and levels of organic matter in soil are low, the contribution of amino-N might still be expected to form 〈30% of the total N taken up by the root system. It was concluded that the uptake of amino-acids from the rhizosphere may be important in both N nutrition and in the minimization of root C and N losses to the soil. Consequently this may be important in governing the size of the rhizosphere microbial population.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00033935
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  • 10
    Electronic Resource
    Electronic Resource
    Re-sorption of organic compounds by roots of Zea mays L. and its consequences in the rhizosphere (1993)
    Springer
    Plant and soil 153 (1993), S. 47-59 
    Details
    ISSN: 1573-5036
    Keywords: maize ; mathematical model ; re-sorption ; rhizosphere ; root exudates
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Abstract The exudation of soluble carbon compounds from Zea mays roots was investigated over a 10 day growth period under sterile and non-sterile solution culture conditions. The results showed that plants grown in sterile static solution culture, where C was allowed to accumulate, released 8 times less C than plants grown under culture conditions in which the solutions were replaced daily. The increased C loss from plant cultures in which exudates were removed daily was attributable to, (a) the reduced potential for root re-sorption of previously lost C, and (b), increasing diffusion gradients between the root and the surrounding bathing solution increasing passive leakage of exudates from the roots. In treatments where C was removed daily from the root-bathing solution, 86% of the total C lost was of a soluble low molecular weight nature, whereas, in sterile and non-sterile static cultures, allowing the accumulation of C over 10 days, this was reduced to 67.5 and 48% respectively. The main C fluxes operating in a solution culture system (efflux and influx of C by both roots and microorganisms) were examined using a computer simulation model to describe movement of soluble sugar-C in both sterile and non-sterile conditions. In sterile static cultures where C was allowed to accumulate in solution over a 10 day growth period, 98% of the C exuded was re-absorbed by the plant. Where C was removed daily from the root-bathing solution this was reduced to 86%. The predicted patterns of C accumulation were similar to those found in the experiments. Simulations showed that the pattern of accumulation and final equilibrium concentrations were dependent on the rate of exudation, the spatial characteristics of exudation, solution volume, root growth rate and the presence of a microbial population. Simulations under non-sterile conditions showed that roots can compete with microorganisms for exudates in solution indicating the possible importance of re-sorption in a soil environment. The results clearly indicate that roots are capable of regulating the net amount of C released into a solution culture with the amount of C collected being highly dependent on the experimental conditions employed. The possible implications of soluble C influx on processes operating within the rhizosphere and in experimental systems is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00010543
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