bluuebunny/crossref_metadata_2025 huggingface dataset:
Ion chromatographic determination of nitrate in 2 M KC1 soil extracts
Authors: STEWART B. M.
Published: July 2006

A method for the analysis of NO3‐N in a 2 m KCL soil extract, using ion chromatography, is discussed. It involves the treatment of the KCl extract with Ag+‐activated cation exchange resin to remove excess Cl–, followed by analysis using a Dionex ion chromatograph. The results of NO3‐N in soil extracts agreed closely with those obtained by steam distillation. The method was linear up to 20 mg N dm–3 with a detection limit of 0.1 mg Ndm–3. Quantitative recovery of NO3‐N added to soil extracts over the linear range were observed.

Standard deviations of samples containing 2.57 and 15.11 mgN dm–3 were found to be 0.06 and 0.11 respectively.
Copper‐induced indefinite aggregation of humic substances: theoretical consequences for copper‐binding behaviour
Authors: TEASDALE R. D.
Published: July 2006

An important factor in plant availability of copper is its anomalous binding to soil humic substances, for which aggregation phenomena have been implicated. Two alternative models of copper‐induced humate aggregation are theoretically explored to characterize binding behaviour. In the first model the copper modifies the humic acid so that it self‐associates indefinitely. Copper binding is found to be dramatically enhanced, except at the very low concentrations corresponding to copper‐deficiency. In the second model bivalent copper forms bridges between different humate molecules to yield an array of large cross‐linked complexes. In the latter cross‐linking model, binding of copper is markedly enhanced at low copper concentrations, a behaviour still found when only relatively few sites sterically permit cross‐linking. The cross‐linking model accounts, at least in part, for the low availability and immobility of copper in some problem soils and is consistent with several items of experimental evidence.
Migration of phosphate into aggregated particles of ferrihydrite
Authors: WILLETT I. R. , CHARTRES C. J. , NGUYEN T. T.
Published: July 2006

The slow reaction of phosphate with aggregated particles of ferrihydrite, after initial rapid phosphate sorption, was investigated by measuring the changes, with time and temperature, in the amount of phosphate sorbed, and the extractability of the sorbed phosphate. The ferrihydrite was, subsequently, recovered and examined by infra–red spectrometry (IR) and electron probe micro–analysis.

Phosphate continued to react with ferrihydrite for at least 90 d at 25°C, but was completely recovered by extraction with 0.1 m NaOH. The IR spectra of sorbed phosphate was insensitive to the temperature and duration of the reaction. Electron probe micro–analysis of the aggregates showed that phosphate migrated to surface sorption sites within the aggregated particles of ferrihydrite.

There was no evidence for the formation of surface coatings of ferric phosphate, for changes in the type of bonding, or for penetration of phosphate into the crystal lattice. The slow reaction was attributed to the migration of phosphate to surface sorption sites of decreasing accessibility within aggregates.
The influence of sand‐and silt‐size particles on the cracking during drying of small clay‐dominated aggregates
Authors: TOWNER G. D.
Published: July 2006

Shrinkage through drying is an important natural process contributing to the disintegration of clay aggregates in the field. Sand‐size particles distort the shrinkage of the surrounding clay matrix, causing the latter to shrink anisotropically until cracking ensues.

Small artificial aggregates of kaolinite clay and glass beads were observed for cracking during shrinkage under ambient laboratory drying. In tests in which only single‐sized beads were present (1, 2, 3, 4 or 5 mm) the aggregates passed from a state where none cracked to that where all cracked as the concentration of beads was increased, except for 1 mm beads for which no cracking was observed. Furthermore, the concentration corresponding to the onset of cracking increased as the bead size decreased. In tests in which 3 or 5 mm beads were mixed with 1 mm beads, the same trends were observed, but increasing the 1 mm bead fraction tended to inhibit cracking. Some observations on thin clay annuli formed round solid discs complemented the aggregate results.

The results provide a framework for a further study on natural soils.
Electrophoretic evidence of the integrity of humic substances separated by means of electrofocusing
Authors: DE NOBILI MARIA
Published: July 2006

The alkaline extracts of an andic soil, a Bhir horizon of a spodic soil and leonardite which, after electrofocusing, displayed bands in the neutral region of the pH gradient, were fractionated by electrophoresis in phosphate buffer at pH 6.5. The non‐migrated fraction gave an electrofocusing profile containing only bands focused at pH values above that of the electrophoretic separation. Electrophoresis in sodium acetate buffer pH 4.8 of humic substances extracted from peat permitted isolation of bands focusing above pH4.8. The distribution of metals after electrophoresis suggests that a great part of humic substances which originate bands above pH 6.5 consists of negatively charged Fe and Al complexes. Addition of EDTA to solutions of humic substances extracted from the samples resulted in the disappearance of bands in the neutral region of the pH gradient in the electrofocusing profile of the Bhir sample, while a consistent lessening of these bands was observed in the leonardite and andic soil samples.
Accumulation of pyrite and Fe‐rich carbonate and phosphate minerals in a lowland moor area
Authors: JAKOBSEN B. H.
Published: July 2006

Pyrite and iron‐rich carbonate and phosphate minerals are found in large quantities in an inland moor in western Jutland, Denmark. Sediments with up to 10.4% pyrite (FeS2) occur well‐separated from sediments with iron‐rich carbonate and phosphate minerals both reaching up to ∼6–7%. Analysis of the ground water showed that water enters the moor from different sources. Water of high alkalinity (up to 2.05 meq l−1) as well as water rich in sulphate (up to 79 mg l−1), sulphide (up to 1.75 mg l−1) and Fe2+ (up to 22.4 mg l−1) were sampled from the upper ground water flowing into the moor. On entering the moor processes within moor sediments control authigenic mineralogy, resulting in segregated mineral precipitation.
Changes caused by Norway spruce in an ochreous brown earth, assessed by the isoquartz method
Authors: SOHET K. , HERBAUTS J. , GRUBER W.
Published: July 2006

Previous investigations had shown that about 80 years of Norway spruce monoculture on an ochreous brown earth of the Belgian Ardennes were sufficient to accentuate the weak podzolization process already present in the climax beechwood soil and to increase the weathering of several minerals, e.g. 2:2 clay minerals. In order to evaluate more accurately mineral weathering and spruce‐induced losses of nutrient elements, an isoquartz assessment method has been applied to a pair of soil profiles, one developed under beech, the other under spruce.

Results show a clear decrease of several weatherable minerals in the soil of the spruce stand which was more marked than in the beechwood, especially for chloritic minerals: losses reach about 30% of the initial chlorite content in the cambic horizon and regularly increase up to 70% in the humic layer. There were severe spruce‐induced losses of Mg and Na: 27% of the initial Mg reserve was removed from the whole soil and the losses reached 60% in the upper 20 cm.

It is concluded that 86 years of Norway spruce monoculture have appreciably increased weathering processes and removal of elements.
Aggregation in soils with small amounts of swelling clays. II. Chemistry and surface properties of Na resin stable soil aggregates
Authors: BARTOLI F. , PATERSON E. , PHILIPPY R. , DEMAI J. J. , DOIRISSE M.
Published: July 2006

Stable aggregates of soils with small amounts of swelling clays were characterized after 16 h shaking with Na resin. Different kinds of organic material operate at various levels of micro‐aggregate size, being more aromatic in the coarser aggregates and combined with poorly‐ordered Al and Fe hydrous oxides.

Effects on aggregate surface properties are described. For each size of resin stable aggregate, the mesopore volume increased with both increasing aggregating organic materials and poorly‐ordered Al and Fe hydrous oxides.
Source minerals and formation of partially interlayered vermiculites in Dystrochrepts derived from Tertiary sediments
Authors: MATSUE N. , WADA K.
Published: July 2006

The formation of partially interlayered vermiculite (PIV) was studied in six Dystrochrepts derived from Tertiary sediments. Mineralogy of silt and clay fractions of gravel and fine earth separated from surface and subsurface samples were determined by X‐ray diffraction. PIV, mica, vermiculite and regularly interstratified 1:1 PIV/chlorite (PIV/Ch(l:l)) were the dominant clay minerals. The contents of these minerals were compared between the specific particle‐size fractions of gravel and fine earth for each sample and the difference was interpreted in terms of mineral transformation associated with soil formation. PIV was formed from mica and PIV/Ch(l:l) in soils with a pH(KCl) of 3.5 to 4.0 and little organic matter. Vermiculite was formed from mica in surface soils with a low pH (pH(KCl) 3.5) and abundant organic matter. PIV would form directly from mica without an intermediary phase of vermiculite and form from PIV/Ch(l:l) by partial dissolution of interlayers in chlorite layers.
Quasi‐thermodynamic basis of competitive‐adsorption coefficients for anionic mixtures in soils
Authors: ROY W. R. , HASSETT J. J. , GRIFFIN R. A.
Published: July 2006

Equilibrium competitive‐adsorption coefficients derived from the application of the multi‐component Freundlich‐type Sheindorf‐Rebhun‐Sheintuch (SRS) equation appeared to have a soil‐specific, quasi‐thermodynamic basis. Using three soils, single‐solute adsorption isotherms at 288, 298, and 308 K were constructed with arsenate, phosphate, and molybdate. A Clausius‐Clapeyron‐type equation was used to estimate apparent isosteric heats of adsorption values (qiso). The adsorption of the three anions was characterized by different qiso values, and was generally in the order of H2PO4‐>H2AsO4‐>MoO42‐. The magnitudes of the SRS competitive coefficients derived from binary‐solute mixtures were in agreement with the distributions of the heats of adsorption of the individual anion‐soil systems. Higher competitive coefficients, indicating the preferential adsorption of solute i in a binary‐solute mixture containing solutes i and j, tended to associate with higher qiso values for solute i relative to solute j. However, the qiso values for the adsorption of molybdate by one soil underestimated the extent of competitive interactions between arsenate and molybdate.
On predicting the fate of radioactive caesium in soil beneath grassland
Authors: KIRK G. J. D. , STAUNTON S.
Published: July 2006

A simple mechanistic model is developed for predicting the fate of radioactive caesium in the soil beneath grassland. A continuity equation is derived describing the diffusion, leaching and reaction of caesium in soil, with uptake by a uniformly distributed root system. The soil reaction is characterized by three phases: an instantaneous exchange, a moderately‐fast reversible reaction and a slow reversible reaction. The time‐dependent reactions and root uptake have first‐order kinetics. It is shown that caesium is unlikely to be removed from the root zone by leaching and diffusion, even under prolonged high rainfall. Therefore, since the rates of the soil reactions and root uptake are much greater than the rates of diffusion and leaching, it is not necessary to calculate the concentration profile to find the rate of uptake. The computer program for the model is just 30 lines of fortran, and requires only a few seconds of micro‐computer CPU time for each year of simulation.

Some predicted rates of plant uptake and distributions in the soil are given. It is seen that the fate of the caesium is markedly influenced by soil parameters dependent on soil clay content, clay mineralogy, organic matter content, cation saturation and pH, and by the rooting density. The predicted rates of uptake vary from about 0.5% to 50% per annum, which is compatible with data reported elsewhere. The predictions are intended to expose gaps in knowledge of the fate of caesium in soil: in particular, the mobility of sorbed radio‐nuclides on soil surfaces needs further study. Predictions are not given for the fate of caesium in any real situations because insufficient data are available.
Aluminium contamination and other changes of acid soil solution isolated by means of porcelain suction‐cups
Authors: RAULUND‐RASMUSSEN KARSTEN
Published: July 2006

Comparison was made between the chemical composition of soil solutions isolated by means of a suction method using porcelain cups and by centrifugation. The soil solutions were isolated from three depths of field plots, where the soil (Typic Haplohumod) had been subjected to various pretreatments.

The cups were made of mullite and corundum as shown by X‐ray diffraction analysis. The material when powdered had a cation exchange capacity of about 10meq kg−1. Solutions with similar ionic strengths were obtained by the two methods, but the cups were found to release substantial amounts of Al and to adsorb H, Ca, K, Na and organic matter.

After the cups had been placed in the soil for more than 7 months, calculations suggested that the Al activity in cup solutions was controlled by amorphous gibbsite. This amorphous material was probably produced by proton‐induced decomposition of part of the cup material. It is concluded that such cups are improper for isolation of soil solution from acid soils.
The application of classical physics transport theory to water movement in soil: development and deficiencies
Authors: TOWNER G. D.
Published: July 2006

The development of soil‐water transport theory is essentially that of the application of classical physical theories that were originally derived for the perceived analogous transport of fluids, heat and electricity in continua. Much of soil‐water transport theory was developed by Buckingham at the turn of the century, but it took several decades for it to become well understood and established. An inability to solve partial differential equations with water‐dependent transport coefficients hampered a rigorous analysis and a proper interpretation of early experiments. The theory has been well proven in the laboratory, but application to the field has been less than successful because of hetereogeneity and macropores. More recent theories based on other principles have not yet received adequate testing. Future work should extend the established theory as well as develop and test new ones.
Effect of farmyard manure and its humic fractions on the aggregate stability of a sandy–loam soil
Authors: FORTUN A. , FORTUN C. , ORTEGA C.
Published: July 2006

The effect of cattle manure and its humic fractions on the aggregate stability of a sandyloam soil has been studied. Cattle manure was extracted with 0.1 m NaOH and the extract was dialysed to obtain a combined fulvic acid + humic acid fraction (FHA). Humic acid (HA) was precipitated from this extract, redissolved in 0.1 m NaOH solution and dialysed. The manure and the organic fractions were added at two rates to a sandy–loam soil and the mixtures incubated for 2 and 17 weeks.

Changes in the water‐stable aggregation of the soil were followed. In these tests, the effects on water stability of pre‐wetting with alcohol or benzene were also examined. Addition of manure, by itself, was rather ineffectual, but stability was significantly improved after two weeks of incubation by addition of the organic fractions. FHA was much more effective than HA, particularly in increasing stability after the benzene pretreatment. There were only slight changes in aggregation after 17 weeks as compared with 2 weeks.
Measuring unsaturated sorptivity and hydraulic conductivity using multiple disc permeameters
Authors: SMETTEM K. R. J. , CLOTHIER B. E.
Published: July 2006

A new field method of obtaining, with minimal disturbance, the unsaturated hydraulic conductivity (K0) and sorptivity (S0) from unconfined disc permeameter measurements is introduced. Conventionally, the sorptivity is obtained from the initial square‐root‐of‐time behaviour of discharge from the disc permeameter. In some cases this can be difficult to measure, in part because multi‐dimensional flows approach quasi‐steady state very rapidly. As an alternative we extend the ponded twin‐ring method to unsaturated discs of several radii. This method has the advantage that only long‐time, quasi‐steady discharges are needed to obtain K0 and S0. In the present study, the two methods are tested using data obtained for a fine sandy loam at a supply potential (Ψ0) of – 35 mm and agreement is good.
Frost action in clay soils. II. Ice and water location and suction of unfrozen water in clays below 0°C
Authors: BROWN S. C. , PAYNE D.
Published: July 2006

The relation between temperature, unfrozen water content (UWC) and shrinkage of a number of clays, and the distribution of ice and water within the clays, was studied. The clays included sodium and calcium Wyoming bentonites, < 1 μm and 1–2 μm kaolinite and three English subsoil clays. On freezing blocks of these clays (initial size up to approximately 1 cm3) over a period of months, they shrank progressively with reduction in temperature studied except for the < 1 μm and 1–2 μm kaolinites, which ceased shrinking at – 0.24°C and – 0.73°C respectively. During shrinkage, the water in the clay blocks migrated to freeze outside the blocks where ice was already nucleated. After equilibration for a month at – 0.19°C and at – 0.73°C, the water contents of the finer clays, excluding ice bands, equalled the UWCs measured previously using a differential scanning calorimeter. Two stages of ice formation were apparent in the curves of UWC vs temperature and were attributed to the different locations of the ice in the clays. Initially, ice formed in bands outside the clay matrix, accompanied by clay shrinkage (Stage 1). After a shrinkage limit had been reached ice began to form within the pores of the clay (Stage 2). Comparisons of the results of these freezing experiments with conventional moisture‐release curves showed that the suction of the unfrozen water in clays was, within experimental error, as predicted by the Schofield equation. Results are also discussed in terms of ice‐water and air‐water interfacial tensions.
Determination of effective diffusion coefficients for gaseous and dissolved organic substances in soil material using a ‘stopped elution’ method with HPLC and GC
Authors: GOSS K‐U. , SCHRAMM K‐W.
Published: July 2006

Non‐steady‐state diffusion of chemicals in soil can be described by Fick's second law using suitable effective diffusion coefficients. In this study a ‘stopped elution’ method was used to determine experimentally these effective diffusion coefficients for gaseous and dissolved substances in dry and water‐saturated soil. For this purpose a high‐performance liquid chromatography (HPLC) apparatus and a gas chromatograph (GS) were used in combination with soil‐filled columns. The method is based on the observed spreading of a substance moving through a soil column. This broadening is the result of several processes, one of which is diffusion. A suitable experimental design permits the separate measurement of that part of the widening caused by diffusion.

The measured effective diffusion coefficients were compared with theoretical values calculated using predictive algorithms from the literature. For liquid‐phase diffusion the maximum deviation between the calculated and the measured values was 34%. The experimental results for gaseous diffusion showed systematic upward deviation from the calculated Coefficients. This might be caused by the diffusion of molecules in the adsorbed state, a phenomenon that had been neglected in the calculations.
The extraction of soil water by the suction‐cup method: a review
Authors: GROSSMANN J. , UDLUFT P.
Published: July 2006

This article deals with the extraction of soil water using the suction‐cup method and its accompanying problems. This method has become well‐developed over the past 20 years. It allows continuous sampling during any period and, if necessary, at several different depths of a soil profile. The installation of the suction probe is easy and the profile is only negligibly disturbed. Some problems may occur when this technique is used. The spatial variability of the properties investigated is often underestimated and must be clarified by sufficient replication. An important point of discussion is the by passing of water flowing through macropores. The problems of alteration of the sample by the sampling system also deserve attention. These can be the ‘filter effect’ as regards macromolecules and colloids, the sorption of trace substances, and the gas exchange between the sample and the atmosphere of the sampling system.
Relationship between equivalent salt solution series of different soils
Authors: JAYAWARDANE N. S. , BLACKWELL P. S.
Published: July 2006

Equivalent salt solution series have been previously defined as solutions with combinations of sodium absorption ratio (SAR) and electrolyte concentration (Ec) producing the same extent of clay swelling in a given soil. The present study shows that there is a high (r2>0.96) positive correlation between log Ec and log SAR of equivalent salt solutions series, in the equation:

image

where a1 and b1 are constants for each equivalent salt solution series for a given soil.

Log a1 could also be represented as a linear function of b1 resulting in the equation:

image

where a2 and b2 are constants for a given soil. Solving this equation using any given value of b1 yields the combinations of SAR and Ec which make up each equivalent salt solution series for a given soil. The relationship between log a1 and b2 for three soils from western United States, namely Waukena, Pachappa and Grangeville, was similar, with their combined data having a r2 value of 0.96. This indicated that a single set of equivalent salt solution series values could be used for these three soils which have different clay contents and clay mineralogy.

Prediction of hydraulic conductivity decreases with Ec reduction at given values of SAR in red‐brown and alluvial soils from southern Tasmania, using the equivalent salt solution series values for Waukena soil, showed similar patterns to measured values and also to those predicted using the equivalent salt solution values applicable to the respective Tasmanian soils.

Thus, available data indicate that the same set of equivalent salt series could be applied to the five soils studied. If further testing shows that a single set of equivalent salt solutions values could be applied to all or large groups of soils, this would facilitate the application of the equivalent salt solution concept to predict salt solution flow in the field.
A model of ammonia volatilization from applied urea. V. The effects of steady‐state drainage and evaporation
Authors: KIRK G. J. D. , NYE P. H.
Published: July 2006

Rachhpal‐Singh & Nye's model of ammonia volatilization is expanded to account for the effects of steady‐state water movement by drainage or evaporation when the soil does not dry out to any great extent. The model shows how upward movement of water during evaporation increases volatilization by carrying urea‐derived NH4+ and HCO3− ions upward, thereby increasing the concentration of ammonia gas at the surface. Conversely, water drainage reduces volatilization by carrying the dissolved solutes into the soil. The model is used to assess the effects on volatilization of evaporating conditions and of irrigation or rainfall.
A model of ammonia volatilization from applied urea. VI. The effects of transient‐state water evaporation
Authors: KIRK G. J. D. , NYE P. H.
Published: July 2006

Rachhpal‐Singh & Nye's model of ammonia volatilization is further expanded to account for the effects of transient‐state water evaporation when the soil surface dries significantly. Full details are given of the derivation and numerical solution of equations describing transient‐state water movement, and the diffusion and convection in soil of urea and its hydrolysis products, and of acid generated by ammonia volatilization. For the wide range of soil hydraulic properties considered, the effects of a dry soil layer on the rate of volatilization supplement the effects of increased convective supply of NH+4 and HCO−3 ions to the soil surface. The dry layer results in increased gaseous NH3 diffusion through the soil, and thereby increases the flux of NH3 across the soil surface and the neutralization of H+ ions generated by volatilization.
The fractal structure of soil aggregates: its measurement and interpretation
Authors: YOUNG I. M. , CRAWFORD J. W.
Published: July 2006

The theory of fractal geometry is presented with reference to soil structure. Recent work on relating fractal structure to pore structure in soils is reviewed. It is suggested that the connection made in previous work between the fractal dimension and soil moisture retention curves is based on simplified assumptions that complicate the interpretation of results. A simple method for estimating the fractal dimension, D, of natural aggregates which circumvents some of these assumptions is presented. Preliminary results of aggregates from soils under different management systems show that, for the soils examined, D ranged from 2.75 to 2.93. The use of D to quantify heterogeneity in soil is explored.
The influence of soil deformations on the permeability to air
Authors: KIRBY J. M.
Published: July 2006

Samples were reconstituted in the laboratory from a cracking grey clay soil. Some samples were created with a granular structure in which there was no preferred macropore orientation, others were created with a strongly preferred macropore orientation. The samples were compressed and their air permeability measured. They were then sheared and their air permeability measured again.

In the samples with a granular structure the change in permeability was dominated by the change in the volume of the pore space. Volume increase led to permeability increase and volume decrease led to permeability decrease. At low shear strains and low volume increases there was some loss of pore continuity leading to small permeability decreases. In contrast, permeability change in the samples with a preferred macropore orientation was dominated by macropore continuity. Permeability decreased both when volume decreased and also over a wide range of volume increases. These results are interpreted in terms of critical‐state soil mechanics and used to construct ‘critical state for permeability’ lines analogous to the usual critical‐state lines.

It is suggested that these forms of behaviour encompass the likely bounds of behaviour of soils in many circumstances.
A comparison of three methods of organic carbon determination in some New Zealand soils
Authors: GREWAL K. S. , BUCHAN G. D. , SHERLOCK R. R.
Published: July 2006

A dry combustion (Dumas) reference method for measuring total organic C (T), is compared with two other methods: the wet acidified dichromate oxidation, Walkley‐Black procedure (an estimator of organic C) and loss on ignition (L) (an estimator of organic matter) for 40 samples from various depths from five New Zealand mineral soils, spanning a range of textures and cultivation treatments. Regression analyses reveal the following: (1) an excellent linear relation exists for all samples between Walkley‐Black procedure and reference method results, with a mean conversion factor 1.25, which compares with the original value 1.32 recommended by Walkley & Black (1934).

(2)LandTare less well‐related by an equation of the formL=a+bT.In contrast to the traditional assumption thatb≃ 1.72 anda≃ 0, a strong ‘intercept effect’ (a> 0) is revealed, reflecting removal on ignition of non‐organic soil constituents. An improved regression of the formL=a+bT+cC, whereC= percentage of clay, obtained by inclusion of the clay as an independent variable, points to the role of clay in releasing these constituents, chiefly adsorbed water.
Aggregation of humic substances in aqueous media as determined by light‐scattering methods
Authors: REID P. M. , WILKINSON A. E. , TIPPING E. , JONES M. N.
Published: July 2006

Samples of peat humic acid (PHA) and surface water humic (WBHA) and fulvic acids (WBFA) extracted from Whitray Beck in North Yorkshire, UK and previously studied by analytical (UV scanning) ultracentrifugation at low concentrations (10–20 mg dm−3) (Reid et al., 1990) have been investigated by dynamic and static light‐scattering methods at concentrations up to approximately 1700 mg dm−3. Dynamic light‐scattering gave very low diffusion coefficients for the humic acids (PHA and WBHA) suggesting the presence of aggregates. Static light‐scattering from the fulvic acid (WBFA) showed that the second virial coefficients of this material in various media were negative, which is characteristic of reversible aggregation. The aggregates, which consisted of approximately 20 smaller molecules, could not be dissociated completely by the detergents sodium n‐dodecylsulphate or Triton X‐100, although limited dissociation by the latter may occur. Sedimentation velocity measurements in the same concentration range as the light scattering gave sedimentation coefficients which increased with concentration also consistent with reversible aggregation.

The studies illustrate the problems inherent in the characterization of humic substances from natural sources and demonstrate that, at least for lower molecular weight fulvic acids, static light‐scattering from solutions of relatively high concentration in combination with sedimentation equilibrium measurements at low concentrations distinguishes a system having a broad molecular weight distribution from one in which aggregation occurs.
Local disjunctive kriging of soil properties with change of support
Authors: WEBSTER R.
Published: July 2006

Disjunctive kriging estimates from a set of measurements the conditional probability that true values of a regionalized variable such as a soil property exceed, or are less than critical thresholds. Estimates can be for sites equal in size to the supports of the samples and, by changing the support of the variogram, for larger blocks. The method also estimates the block averages and their associated variances.

The underlying theory and steps in the analysis are described. They are illustrated using a case study from south‐east Scotland where the aim is to estimate the likelihood of copper and cobalt deficiency over whole farms or 1 km2 blocks of land using data from individual fields of 5 to 10 ha only. Maps show the conditional probabilities for 1 km2 squares that the available copper in the soil is less than 1 mg kg−1 and that cobalt is less than 0.25 mg kg−1.
Measuring gross nitrogen mineralization, and nitrification by 15 N isotopic pool dilution in intact soil cores
Authors: DAVIDSON E. A. , HART S. C. , SHANKS C. A. , FIRESTONE M. K.
Published: July 2006

The isotope dilution method for measuring gross rates of N mineralization, immobilization, and nitrification was applied to intact soil cores so that the effects of soil mixing were avoided. Soil cores were injected with solutions of either 15NH4+ or 15NO4−; gross mineralization rates were calculated from the decline in “N enrichment of the NH: pool during a 24‐h incubation; gross nitrification rates were calculated from the decline in 15N enrichment of the NO−3 pool; gross rates of NH4+ and NO3− consumption were calculated from disappearance of the 15N label. The assumptions required for application of this method to intact cores are evaluated. Sensitivity analysis revealed that homogeneous mixing of added “N with ambient pools was not a necessary assumption but that bias in distribution of added label, coincident with a non‐random distribution of microbial processes, would cause significant errors in rate estimates. Rate estimates were also sensitive to errors in initial 15N and 14N pool size estimates, In a silt loam soil from a grassland site, abiotic processes consumed over 30% of the added 15NH4+ within minutes of adding the label to sterilized soil. Extracting a subset of soil cores at the beginning of an incubation is recommended for obtaining initial pool size estimates. Gross immobilization is probably stimulated by addition of inorganic 15N substrate and, therefore, is overestimated by the isotope dilution method. As an alternative method, a non‐linear equation is given for calculating the gross immobilization rate from the appearance of 15N in chloroform‐labile microbial biomass; but incomplete extraction of biomass N may result in low estimates. Details of the isotope dilution methodology (injection rates, concentrations, experimental artefacts, etc.) are described and discussed. When care is taken to understand the underlying assumptions and sources of error, the isotope dilution method provides a powerful tool for measuring gross rates of microbial transformations of soil nitrogen in intact soil cores.
Image analysis and three‐dimensional modelling of pores in soil aggregates
Authors: GLASBEY C. A. , HORGAN G. W. , DARBYSHIRE J. F.
Published: July 2006

Three cross‐sections of soil aggregates (2–5 mm diameter) were digitized at 5 μm resolution from montages (× 100) of scanning electron micrographs to produce binary images representing the soil pores and soil matrix. A three‐dimensional random Boolean process was chosen as a model of the soil pores and matrix. The soil matrix was simulated by randomly positioned, overlapping spheres with radii drawn from an exponential distribution. Simulation of a 1 mm cube of one soil aggregate showed that all but 0.1 % of the pore space was connected to the exterior, although only 50% appeared to be connected to the exterior in a cross‐sectional image. Pore spaces able to accommodate different sizes of microorganisms were also investigated. For example, a protozoan with a cross‐sectional diameter of 20 μm could be accommodated in 17% of the pores in a 1 mm3soil cube, although only 11 % of the pores would be accessible to those protozoa on the exterior of the cube.
Poorly ordered hydrous Fe oxides, colloidal dispersion and soil aggregation. I. Effect of humic macromolecules on surface and colloidal properties of Fe (III) polycations
Authors: BARTOLI F. , PHILIPPY R. , PORTAL J. M. , GERARD B.
Published: July 2006

Model humic macromolecules, obtained by auto‐oxidation of the catechol‐glycine system, were adsorbed on poorly ordered hydrous iron oxides, Fe(OH)2.8. The pH was nearly constant at c. 6 in order to simulate natural soil and river environments. When the amount of organic carbon adsorbed on 1 g of poorly ordered ferrihydrite increased from 0 to 70 mg, the ligand OH exchange was very small, but the zero point of charge of suspensions progressively shifted from 7.3 to 5.6, and net surface charge, which was initially positive, progressively became negative, suggesting an adsorption mechanism with positive charge neutralization. Consequently, colloidal stability changed from rapid to slow flocculation. The ferrihydrite‐haematite exothermic reaction partially shifted from 400°C to higher temperatures as a function of adsorbed organic matter, and ferrihydrite solubility progressively increased by formation of soluble iron‐organic complexes.

These results were explained more satisfactorily by the concept of colloidal organ‐ mineral association than by organic coatings on surfaces.
Evaluation of similar‐media scaling and a one‐parameter model for estimating the soil water characteristic
Authors: WILLIAMS R. D. , AHUJA L. R.
Published: July 2006

Two simple methods for estimating the soil water characteristic curve from saturated soil water content, or even just the soil bulk density, plus one other measured point on the curve (such as the water content at a matric potential of –33 kPa) were evaluated. A similar‐media scaling approach and the one‐parameter model of Gregson et al. were compared for eight diverse soils from within the USA. The scaling method also requires knowledge of one complete curve to serve as a reference. The one‐parameter model was extended to include the residual water content, and the soils were divided into three textural groups for the analysis. Visual comparison of the calculated vs. measured soil water content for both methods showed less scatter in the relationship to the 1:1 line, concomitant with smaller calculated error terms, for the one‐parameter model. These results demonstrate that the one‐parameter model is better than similar‐media scaling in estimating soil water content.
Effect of iron oxide on phosphate sorption by calcite and calcareous soils
Authors: HAMAD M. E. , RIMMER D. L. , SYERS J. K.
Published: July 2006

Pure calcite (AR grade CaCO3) was treated with ferrous perchlorate solution to give a surface coating of iron (Fe) oxide. Maximum sorption (xm) of phosphate (P) by the calcite increased from 18.2 to 160 mg P kg−1 as the amount of coating increased from 0.00 to 16.0 g Fe2O3, kg−1 CaCO3. Evidence for Fe oxide coatings on carbonate minerals in two Sudanese soils was obtained by optical microscopy and electron‐probe microanalysis. The relative contributions of carbonate and Fe oxide minerals, and Fe oxide coatings to P sorption in these soils were calculated, based on an assumed model of oxide distribution. Separate‐phase Fe oxide was the major contributor (30–40%) to P sorption in the soils; the Fe oxide coatings on carbonate minerals were only minor contributors (< 6%), and the contribution of uncoated carbonate minerals was found to be negligible (<1 %). These results suggest a very minor role for carbonate minerals, even when coated with Fe oxide, in the sorption of P by these calcareous, Sudanese soils.
Determination of aluminium activity from measurements of fluoride in acid soil solutions
Authors: MUNNS D. N. , HELYAR K. R. , CONYERS M.
Published: July 2006

Trivalent aluminium (Al) in solutions extracted Centrifugally from soils with pH ranging from 3.3 to 5.8 in 0.01 M CaCl2was estimated from: (i) iterative computation using total Al (pyrocatechol violet), pH, major ions and Al ligands (PCV + GEOCHEM); (ii) 15‐s reaction with 8‐quinolinol (8Q); and (iii) electrode measurements of free and Al‐complexed fluoride. Fluoride and hydroxyl complexes were major Al‐components. With decreasing pH, complexed and total F increased along with Al, but free F decreased. Results from PCV + GEOCHEM and 8Q were consistent with each other and with solubilities of the clay minerals in the soils. Relative to these, the F‐electrode method increasingly overestimated Al as pH increased and Al decreased; its evident failure was not attributable to La interference, OH interference, or the uncertainty of electrode calibration at F activities below 10−6M.
Selenium retention in the organic matter of Swedish forest soils
Authors: GUSTAFSSON J. P. , JOHNSSON L.
Published: July 2006

Fractions of selenium present in the soil profiles of three Swedish podzols were analysed using a sequential extraction scheme to characterize Se distribution among the organic and inorganic fractions. The process by which selenite deposited from the atmosphere is retained in a podzolic profile rich in organic matter was studied in a column experiment. Selenium present in organic fractions accounted for most of the Se extracted by Na4P2O7/NaOH. All soil organic matter fractions, particularly those in the B horizons, were considerably enriched with Se as compared with plant biomass. The most enriched fraction was that containing hydrophobic fulvates which had C to Se ratios ranging from 33 000 to 80 000. The distribution of Se among the organic fractions differed markedly from that of sulphur.

Selenite applied to columns continuously for 67 d was fixed very rapidly upon entering the forest floor layers, with 77% being recovered in the top 2 cm of the forest floor after the experiment. In column leachates from the surface layers, C to Se ratios decreased progressively following Se application. No effect specifically related to Se application was observed for leachates and soil horizons underlying Bs1. The mechanism responsible for the efficient and rapid Se immobilization by organic matter is unknown.
Denitrification losses of nitrogen fertilizer applied to winter wheat following ley and arable rotations as estimated by acetylene inhibition and 15N balance
Authors: GOULDING K. W. T. , WEBSTER C. P. , POWLSON D. S. , POULTON P. R.
Published: July 2006

We studied the fate of 222 kg N ha−1 applied in spring as K15NO3 to winter wheat test crops which followed either continuous arable cropping (Arable) or a rotation in which a 3‐year grass/clover ley preceded the wheat (Ley). Denitrification losses (measured by an acetylene‐inhibition method) of over 1 kg N ha−1 d−1 were measured for short periods following heavy rain in mid‐May. However the generally dry and cool weather resulted in accumulated losses by denitrification between fertilizer application and anthesis equivalent to only 5.3% and 3.6% (±2%) of the applied N for the arable and ley treatments respectively. The smaller loss from the ley was despite this treatment containing more inorganic N and available carbon.

15N balance indicated that, at anthesis, 1.5% and 11.5% (± 7%) of the labelled N was lost from the arable and ley treatments respectively. Given the precision of the 15N and the acetylene‐inhibition methods, the results are not significantly different. However, the larger difference between methods for losses from the ley treatment may be an underestimate because 15N balance does not measure losses of unlabelled N. These were probably very small on the arable treatment but could have increased total N loss by 25% to c. 32 kg ha−1 on the ley treatment compared with the 8 kg ha‐1 measured as denitrified. Such a large difference is unlikely to be an error but was probably due to ammonia volatilization from this crop which was severely infected by mildew. The results were thus a poor test of the acetylene‐inhibition method, but revealed another loss process which could be significant in some situations.
Influence of electrolyte composition and pH on cadmium sorption by an acid sandy soil
Authors: BOEKHOLD A. E. , TEMMINGHOFF E. J. M. , Van der ZEE S. E. A. T. M.
Published: July 2006

Extraction of soil with CaCl2, has been recommended as a measure of bioavailability of heavy metals. Interpretation of soil extraction data in terms of plant uptake potential may improve when the chemical behaviour of heavy metals in these extracts is ascertained. The effect of pH, Cd complexation by Cl, and competition between Cd and Ca on Cd sorption was studied at an ionic strength of 0.03 m. Sorption of cadmium was measured in 0.01 m CaCl2, in 0.01 m Ca(NO3)2, in a mixture of 0.02 m NaCl and 0.01 m NaNO3, and in 0.03 m NaNO3, at different values of pH ranging from 3.8 to 4.9. Adsorption isotherms were all linear, with a negative intercept on the y‐axis. This intercept indicated (linear) desorption of only part of the initial soil Cd content. About 50% of the Cd in solution was complexed in the presence of 0.02 m Cl at ionic strength of 0.03. Due to competition between Cd and Ca, sorption of Cd was reduced by 80% in the Ca‐electrolytes as compared with the Na‐electrolytes. Sorption was highly sensitive to pH as each 0.5 unit increase in pH resulted in twice as much sorption of Cd. An empirical factor in the sorption equation that accounts for this effect of pH showed a similar response to changes in pH as a mechanistic factor. This mechanistic factor was developed by assuming that Cd and protons sorb onto the same sites and that a two‐site Langmuir sorption isotherm for protons was able to describe the titration curve of the soil. This similarity may explain the successful application of the empirical factor in this and previous studies.
Incorporation of natural alcohols from plant residues into hydromorphic forest‐podzol
Authors: JAMBU P. , AMBLÈS A. , JACQUESY J. C. , SECOUET B. , PARLANTI E.
Published: July 2006

The proportion of alcohols in soil lipids was studied in an hydromorphic forest‐podzol. The L and F litter layers and the soil A l horizon were sampled and analysed for total lipid and total alcohol contents. Total alcohols were separated into four fractions: primary alkanols, secondary alkanols, sterols and triterpenic alcohols.

From the L layer to the soil A 1 horizon, all the fractions decreased sharply, except free primary alkanols and sterols. The decomposition of secondary alkanols was virtually complete. New odd‐chain primary alkanols were generated and new hopanoid alcohols (triterpenic alcohols), considered as markers of bacterial origin, were detected in the A 1 horizon. In the sterol fraction, the same components were observed in all the samples with changes concerning only their relative proportions.
Chemical characterization and decomposition of organic matter from two contrasting grassland soil profiles
Authors: HOPKINS D. W. , CHUDEK J. A. , SHIEL R. S.
Published: July 2006

Particle size fractions of soils from the surface 6 cm of two adjacent grassland plots which, as a result of different fertilizer treatments since 1897, have either a mor or a mull humus form were analysed using solid‐state13C nuclear magnetic resonance spectroscopy and fractionation of organic N by steam distillation. In the mor humus soil, which had received 180 kg (NH4)2SO4ha−1annually and was pH 4.3, there was more C and N in the larger particle size fractions than in the mull humus soil (pH 5.8). The NMR spectra of correspondingly sized soil fractions were similar for both soils. The intensities of NMR signals between 0 and 40 ppm (alkyl‐C) and between 160 and 200 ppm (carbonyl‐C) increased with decreasing particle size. The intensities of the NMR signals between 60 and 90 ppm (0‐alkyl‐C) and between 90 and 110 ppm (acetal‐ and ketal‐C) decreased with increasing particle size. Comparison of the NMR spectra of the >2000 um fractions from both soils with those of dried grass litter from the same plots indicated the exclusive plant origin of the C in the largest size fraction of the soils. NMR resonances between 40 and 60 ppm were attributed to alkyl‐amino‐C because their intensities agreed with the amino‐N determinations obtained during organic N fractionation. During incubation in soil microcosms, the larger sized fractions decomposed more rapidly than the smaller fractions. However, all the correspondingly sized particle fractions from the two soils decomposed at the same rate except the >2000 pm fractions. The largest size fraction from the mor humus soil decomposed faster than that from the mull humus soil. This difference in decomposition rate could not be attributed to differences in the chemical composition of the >2000 pm fraction.