bluuebunny/crossref_metadata_2025 huggingface dataset:
Dispersion and zeta potential of pure clays as related to net particle charge under varying pH, electrolyte concentration and cation type
Authors: CHOROM M. , RENGASAMY P.
Published: August 2005

The effect of changing pH and electrolyte concentration on the dispersion and zeta potential of Na‐and Ca‐forms of kaolinite, illite and smectite was investigated in relation to changes in their net negative charge.

The percentage of dispersible Na‐clay and the percentage increase in net negative charge was positively correlated with pH, but the slopes varied from clay to clay. In general, the net negative charge was the primary factor in clay dispersion, and the pH affected clay dispersion by changing the net charge on clay particles. Na‐smectite had larger net charge at all pHs than Na‐illite and Na‐kaolinite, and it always had larger flocculation values. The role of electrolyte concentration could be due to its effect both on flocculation and variable charge component of the clay minerals. The zeta potential at different pHs also reflected the same trend of clay dispersion with net particle charge.

In Ca‐clays the trends were similar to Na‐clays up to pH 7.0. In more alkaline solution CaCO3 formation led to charge reduction on clay particles, resulting in flocculation and reduction of zeta potential. At similar pHs the electrophoretic mobilities of all the clays showed constant potential behaviour. However, the zeta potentials of Ca‐clays were always smaller than those of sodic clays because the clays were more aggregated. Net particle charge was the most important factor in controlling clay dispersion for the whole range of pH and ionic strength and for all types of cations.
Estimating temporal change in soil monitoring: II. Sampling from simulated fields
Authors: PAPRITZ A. , WEBSTER R.
Published: August 2005

Design‐based and model‐based methods of estimating temporal change of soil properties over a finite area have been compared. Two large fields of auto‐ and cross‐correlated data were simulated, each representing the spatial distribution of a variable at one time. The fields were then sampled repeatedly. The means of stratified and systematic random samples and geostatistical global estimates were used to infer the mean difference between the fields. All estimators were unbiased, but their variances differed. Pairing the positions on the two occasions increased the precision of the design–based estimates. Systematic sampling was slightly more precise than stratified sampling. Kriging was less precise than both because some of the sample information must be used to estimate the variograms at short lags. Neither balanced differences nor the normal formula for simple random sampling predicted the estimation variances of small (n< 50) systematic samples accurately. For larger samples the method of balanced differences performed well. If the spatial variation is unknown in advance and only small samples can be taken then stratified random sampling with two observations per stratum is the preferred design. It resulted in the best combination of precision and accuracy in predicting the sampling error.
Land‐use effects on the composition of organic matter in particle‐size separates of soils: II. CPMAS and solution13C NMR analysis
Authors: GUGGENBERGER G. , ZECH W. , HAUMAIER L. , CHRISTENSEN B.T.
Published: August 2005

Soils from A horizons of Eutrochrepts under spruce forest (Sf), mixed deciduous forest (Df), permanent grassland (Gp), and arable rotation (Ar) were fractionated into clay‐ (<2 μm), silt‐(2–20 μm) and sand‐ (20–2000 μm) sized separates.13C NMR spectroscopy was used to compare SOM composition across size separates and between land‐use regimes.

CPMAS13C NMR spectroscopy showed that the intensity of signals assigned to carbohydrates (representing most O‐alkyl C) and lignin (phenolic and methoxyl C) declined with decreasing particle size. Concurrently, alkyl C and C‐substitution of aromatic C increased in the order sand, silt, clay. The amount of alkyl C correlated well with microbial resynthesis of carbohydrates. Solution13C NMR spectra suggested that humic acids (HA) extracted from the size separates were richer in carboxyl C and aromatic C than the bulk size separates. Also HA reflected increasing percentage of alkyl C with decreasing particle size. O‐alkyl C were lower in silt HA than in clay HA whereas aromatic C tended to peak in silt HA. These results suggested that sand‐sized separates were enriched in plant residues (primary resources) whereas clay‐sized separates were dominated by products of microbial resynthesis (secondary resources). Silt was rich in selectively preserved and microbially transformed primary resources.

13C NMR spectroscopy showed only small differences in SOM composition between land‐use regimes, except that silt and silt HA from Ar were richer in aromatic C than those from the other plots. But enrichment factors (E= content in fraction/content in whole soil) revealed differences in the distribution of C species across the size separates. Relatively highEaromatic(0.9) andEo‐alkyl(1.0) for sand from Gp indicated high amounts of plant residues, probably due to intense rhizodeposition and to occlusion of plant debris within aggregates. LowEaromatic(0.3) and Eo‐alkyl(0.3) for sand from Ar suggested depletion of primary resources, which could be attributed to disintegration of soil aggregates upon cultivation. A pronounced enrichment of alkyl C in Ar clay‐sized separates (Ealkyl= 3.1) suggested large amounts of microbial carbon. Microbial products attached to clay surfaces by a variety of physico‐chemical bondings appeared more stable against mineralization induced by cultivation than plant residues sequestered in aggregates.
A weathering score for sandy soil materials based on the intensity of etching of quartz grains
Authors: MARCELINO V. , STOOPS G.
Published: August 2005

A semi‐quantitative method is developed to assess the relative weathering of tropical sandy soils. Individual quartz sand grains are classified on the frequency and the size of dissolution etch pits in their surfaces, using scanning electron microscopy (SEM). A scale with five grain‐type classes reflecting increasing intensity of etching is defined, and the relative frequency of each of these classes in horizons of selected soil profiles is determined. To summarize the information contained in the frequency distribution a weighted score, the weathering score, is calculated. This score assesses the relative weathering of individual horizons, and it makes it easy to compare them with other horizons within the same or from other profiles. It is useful in detecting weathering trends and differences between horizons of tropical sandy soils.
The dynamics of soil water following single surface wettings
Authors: ROSE D.A.
Published: August 2005

Single rainfall events play an important agricultural and ecological role, especially in dry regions where precipitation is erratic. Infiltration, redistribution and evaporation of single quantities of water are important in this context and have been investigated in the laboratory.

Three soils of differing texture were packed at two uniform initial water potentials (– 100 MPa and –1.5 MPa) into columns, after which 12.7, 25.4 and 50.8 mm of water were applied as a single irrigation. The columns were maintained in a controlled hot and dry atmosphere (evaporativity = 16.7 mm d‐1) for up to 30 days, during which water‐content profiles were measured at intervals.

Infiltration was rapid to depths ranging between 35 and 250 mm. Thereafter redistribution was small. Evaporation caused the water profile to develop three zones: dry between the soil surface and the drying front, dry below the wetting front, and an intermediate wetter zone between the drying and wetting fronts. As evaporation continued, the drying front moved deeper into the soil and the water content in the intermediate zone decreased.

During the first few hours evaporation was rapid and constant, at the evaporativity of the atmosphere. Subsequently, evaporation was slower. Total evaporation (E) increased with time (t) as Eα tn for t > 1 d, where n = 0.24 for a loamy sand, 0.33 for a clay loam and 0.31 for a silty clay loam.

Weighted‐mean soil‐water diffusivities, averaged over the profile above the wetting front, ranged between 1000–2000 mm2 d−1 at the start of the falling‐rate stage and 200–400 mm2 d−1 near air‐dryness, in reasonable agreement with the few results in the literature.
A simple analytical transfer function approach to modelling the leaching of reactive solutes through field soil
Authors: HENG L.K. , WHITE R.E.
Published: August 2005

Results from a previous study, in which two different forms of sulphur fertilizer were applied to a grazed pasture, were re‐examined to show that the movement of sulphate to subsurface drains over three consecutive drainage seasons could be satisfactorily predicted using a simplified analytical form of the transfer function model (TFM). Two simple parametric forms: the exponential Burns and lognormal functions, were used for the probability density function for solute travel, and more realistic assumptions about the biologically mediated source‐sink terms were incorporated. Sensitivity analysis showed that the model's prediction was very sensitive to the retardation coefficient for sulphate relative to the non‐adsorbed solute chloride and the volume of soil water that is effective in solute transport. The procedures can be easily adapted for management purposes.
Changes in the amount and distribution of neutral monosaccharides of savanna soils after plantation of Pinus and Eucalyptus in the Congo
Authors: TROUVE C. , DISNAR J‐R. , MARIOTTI A. , GUILLET B.
Published: August 2005

In the Congo, near Pointe‐Noire, Pinus and Eucalyptus were planted on the savanna for 30 years. We have characterized the effects of this change on land‐use on the composition of carbohydrates in whole soil and particle‐size fractions of the soil. Carbohydrates represent variable proportions of the total soil organic carbon (TOC) of various particle size fractions. The largest proportions of sugar‐C were found in the savanna soil with as much as 250 mg g−1 TOC in the coarsest plant remains and approximately 190 mg g−1 TOC in the finest organo‐mineral fractions, whereas there was always less sugar in plantation soils. The monosaccharide xylose and mannose have different distributions: xylose appears to be the marker of the vegetal inheritance, whereas the dominance of mannose in the clay fraction bears the signature of current microbial sugar synthesis.

The quantitative and qualitative evolution of the whole soil carbohydrates was studied as a function of plantation age. Carbohydrate‐C represents 131 mg g−1 of the soil organic carbon in the savanna soil, but decreases to an average value of 75 mg g−1 in plantations more than 6 years old. This appears to be due mainly to the stimulation of the mineralization of the glucose, which represented 60% of the total sugars in savanna soil and only 45–48% in tree plantations. The ratio [arabinose + galactose + fucose]/[rhamnose + xylose], which is the largest in the oldest plantations, is significant for evaluating the replacement of carbohydrates of the original grass savanna by those of the trees.
Land use effects on the composition of organic matter in soil particle size separates. III. Analytical pyrolysis
Authors: SAIZ‐JIMENEZ C. , HERMOSIN B. , GUGGENBERGER G. , ZECH W.
Published: August 2005

Soil samples from the A horizon of an Eutrochrept under spruce forest and permanent grass were fractionated into clay‐, silt‐ and sand‐size separates. Humic acids extracted from each fraction were analysed by pyrolysis‐gas chromatography‐mass spectrometry. Protection of functional groups by simultaneous pyrolysis and methylation yielded pyrolysates in which methyl esters of fatty acids, aliphatic dicarboxylic acids, abietic acids, phenolic acids and benzenecarboxylic acids were represented. However, methylation was not complete, and unmethylated compounds were also present. Spectra showed differences in humic acid composition between size separates as well as across land use regimes. The abundance of lignin‐derived pyrolysis products increased with decreasing particle size, and was greater in soil under spruce than in soil under grass. Also, the lipid components differed, with hexadecanoic and docosanoic acid methyl esters being the dominant compounds in humic acids from soil under spruce and hexadecanoic and octadecanoic acid methyl esters in the humic acids from grassland. A good correlation was found between previous 13CNMR and wet chemical data and pyrolysis data, indicating that pyrolysis‐methylation can be used for fast detailed chemical characterization of humic acids extracted from size separates.
The chemistry of aluminium in strongly acidified sandy soil in Poland
Authors: POREBSKA G. , MULDER J.
Published: August 2005

Organically‐bound Al in acidic, base‐poor sandy soils seems to be a major contributor to dissolved Al. This hypothesis has been tested under field conditions at two intensely acidified sites. The research was conducted at the agricultural trial fields at Lyczyn, Poland, which have been fertilized continuously for 30 yr with 130 kg ha−1 a−1 N (as NH4NO3 or urea). The effect of depletion of Al on its solubility was also studied.

Field data confirmed that mobilized Al originates largely in the organically bound fraction of soil Al. Depletion of this fraction resulted in a considerable decrease in the mobility of Al such that, at pH = 3.0 and reaction times of a few weeks, small amounts of Al were mobilized. Apparently, the rate at which Al is dissolved from structural silicates, which are abundant in the soil investigated, is small. This suggests that at sites in Poland most exposed to acid deposition, where the rates of soil acidification and depletion of organically‐bound Al are greatest, both the concentration of Al and the soil solution pH are expected to decline with time.
Chemical significance of aluminium extracted from three horizons of an acid forest soil, using chloride salt solutions
Authors: PONETTE Q. , ANDRE D. , DUFEY J.E.
Published: August 2005

Aluminium is usually important in exchange reactions in acid soil, but amounts extracted are strongly affected by procedure. We examined the impact of proton release, following salt addition, on Al removal from three horizons (Oe, Oa and A12) of an acid, brown forest soil.

The Al was first extracted with either 1 M KC1, 1mNH4C1 or 0.5mCuCl2on the three horizons limed to pH 5.5. For any lime addition, both the pH drop following salt addition and the amounts of extracted Al were greater with CuCl2than with KC1 or NH4C1. For the Oa and A12horizons there was a single inverse linear relation between amount of Al extracted and the pH of final extract, independent of the extracting cation. In the Oe horizon CuCl2extracted constant amounts of Al, whereas the quantity of Al removed by KC1 or NH4C1 increased linearly as pH declined below pH 3.6.

Extra Al was mobilized following unbuffered KC1 extraction, a side effect induced by the extracting procedure itself.

Al mobilization increased with increasing H+additions. In the Oe horizon, equilibrium was established rapidly (≤ 24 h), and decreasing the pH of the soil‐KCl suspensions to 2.0 resulted in extracted Al amounts equal to or slightly greater than those obtained with CuCl2solutions. Al amounts extracted with acidified KC1 solutions from the A12were close to those obtained using CuCl2solutions at similar pH in the extract. In the samples from the A12layer, increasing the equilibration time resulted in increasing proton consumption and equivalent release of Al ions in the extract.
Phosphorus sorption in soils of the Mekong Delta (Vietnam) as described by the binary Langmuir equation
Authors: QUANG VO DINH , THAI VU CAO , LINH TRAN THI TUONG , DUFEY J.E.
Published: August 2005

Phosphate adsorption isotherms were determined for 20, mostly very acidic, soils from the Mekong Delta. The experimental data were well described by a binary Langmuir equation which considers two groups of sorption sites that differ in their P bonding energies. The maximum P‐sorption capacities of these sites were related to the soil properties by simple linear correlation and by stepwise multiple regression. Results suggest that high energy sites are on Al‐oxihydroxides or small Al‐substituted Fe‐oxides and, to a lesser extent, on poorly ordered Fe‐oxihydroxides. On the other hand, the P‐sorption capacity of low energy sites is mainly related to clay content, and it increases as pH decreases. These sites are also positively correlated with organic carbon and poorly crystalline Fe‐oxihydroxides. However, as these two variables are closely correlated with each other, organic matter is likely to be considered as an indirect factor of P fixation through its association with Fe‐oxihydroxides and not as an important source of P‐sorption sites.

The maximum sorption capacity, i.e. the sum of sorption capacities of the two groups of sites, is well described (r2= 0.88) by an equation that takes into account the four variables identified above: Al‐bearing oxihydroxides, poorly ordered Fe‐oxihydroxides, clay content and pH. Grouping the soils according to the orders of Soil Taxonomy, the P‐sorption capacity increases in the following sequence: ultisols < entisols < inceptisols.

A P concentration often considered adequate for plant nutrition is 0.2 mg P 1−1solution, and only the high energy sites are involved in sorption at that concentration. Thus an equation including only Al‐ and Fe‐oxihydroxides could be used to fix P norms in these soils.
Phosphate speciation in excessively fertilized soil: a 31P and 27Al MAS NMR spectroscopy study
Authors: LOOKMAN R. , GEERTS H. , GROBET P. , MERCKX R. , VLASSAK K.
Published: August 2005

Both P and Al MAS NMR spectra of samples of excessively fertilized sandy soil provided information about the P and Al speciation. Peak deconvolution was used to interpret reliably and quantitatively the 31P NMR spectra recorded. Most of the P was found to be associated with Al. Part of the P exhibited a chemical shift that could be attributed to octocalcium phosphate, amorphous calcium phosphate or apatite. Apatite has, however, never been reported to occur in sandy soils of temperate climates. A dithionite extraction used to remove interfering Fe from the samples also removed most of the octahedral Al‐P phase. After oxalate extraction more than 99% of the original P signal disappeared. About 7.5 to 11 % of the total oxalate extractable P of the excessively fertilized soil was present as a Ca‐P phase, even though these soils are slightly acid to acid. The estimated size of the Ca‐P phase roughly corresponds to the size of the labile P pool of these soils, as assessed in long‐term batch desorption experiments. It still remains unclear whether the labile P pool should be attributed solely to such a Ca‐P phase.
The effects of time of incubation on the relation between charge and pH of soil
Authors: MORA M.L. , BARROW N.J.
Published: August 2005

Samples of two Andisols and two Ultisols from southern Chile were incubated with acid or with lime for up to 60 d at 25°C or for 1 day at 40°C or at 60°C. The changes in positive and negative charge were measured. The Andisols reacted slowly at 25°C. They lost protons to the solution at high pH, thus increasing the negative charge on the soils and decreasing the pH of the solution. They gained protons at low pH, thus increasing the positive charge on the soils and increasing the pH of the solution. The Ultisols reacted more quickly but again charge and pH changed through time. For all samples, the rate of reaction was increased by incubating at 60°C. Brief incubation at 60°C produces a similar relation to that obtained after longer incubation at 25°C. This provides a convenient means by which measurements can be made more quickly.
The nature of smectites and associated interstratified minerals in soils of the Gharb plain of Morocco
Authors: BOUABID R. , BADRAOUI M. , BLOOM P.R. , DANIANE M.
Published: August 2005

The mineralogies of ‘Tirs’ (Typic Pelloxererts), and ‘Debs’ (Typic Haploxerolls and Typic Xerochrepts) soils of the Gharb plain in north‐western Morocco are investigated, with special attention given to the determination of the nature of the smectitic phase using the lithium test (Li test) and the alkylammonium method. The sand and silt mineralogy of Tirs soils is dominated by quartz with small amounts of feldspars and kaolinite. The sand and silt fractions of Dehs soils also contain significant amounts of mica, chlorite, and interstratified phyllosilicates. The clay minerals of Tirs soils are predominantly a high‐charge smectite. The estimated interlayer charge for this phase is 0.61 mol(c)/O10(OH)2 and the fraction of tetrahedral charge varies from 38 to 44%. Although the percentage tetrahedral charge is less than 50%, the smectitic phase behaves as beidellite with the Li test. Dehs clays are more heterogeneous, consisting of smectite, vermiculite, illite, kaolinite, chlorite, and interstratified illite/smectite and illite/vermiculite. The Li test and the alkylammonium method demonstrate that a high‐charge smectite or vermiculite is interstratified with illite. A low‐charge montmorillonite is also present both in Tirs and in Dehs soils. The high‐charge beidellitic phase is believed to be a transformation product of mica, whilst the low charge montmorillonite is thought to be inherited from the parent material.
Methanogenesis in saltmarsh soils of the North Sea coast of Germany
Authors: GIANI L. , DITTRICH K. , MARTSFELD‐HARTMANN A. , PETERS G.
Published: August 2005

Temperate saltmarshes are a potential source of atmospheric methane. We have measured the concentration and emission of methane in typical saltmarsh soils (Salic Fluvisols) and humus‐rich saltmarsh soils (Thionic Fluvisols) from the German North Sea coast. We also measured the methane production rates of the latter. The methane content of typical saltmarsh soils reached 12.0 μmol 1−1, although values of 1–4 μmol 1−1 were usual. The sulphate concentrations of the pore‐water were about 10 mm, which means sulphate reduction is not limited and methanogenesis would be suppressed. Methane concentrations were generally largest in summer. Independent of the redox potential and the degree of soil development, methane concentrations were smallest in those soils poorest in humus. Methane emission rates were almost zero. In the humus‐rich saltmarsh soils, methane concentrations were roughly a thousand times larger than those in typical saltmarsh soils, reaching values of 23 mmol 1−1 The sulphate concentrations of the pore‐water were often less than 1 mM, indicating limited sulphate reduction. Methane production was up to 80 μg cm−3 day−1 and was not inhibited when we added sulphate. Methane emission rates reached up to 190 μg m−2 day−1 in summer, with values up to 20 μg m−2 day−1 at other times. The two kinds of saltmarsh soil behave quite differently: the typical saltmarsh soils act as a sink for methane; the humus‐rich saltmarsh soils are a source.
Effect of pH and some organic anions on the solubility of soil phosphate: implications for P bioavailability
Authors: STAUNTON S. , LEPRINCE F.
Published: August 2005

The effect of pH and the addition of carboxylates (acetate, oxalate, tartrate, salicylate and citrate) on the solubility of soil phosphate has been investigated to assess the possible effects of root exudates on phosphate availability. The soil was a neutral calcic Luvisol with a large pH buffer capacity. Various concentrations of strong acid (0–20 mmol kg−1) and anion (0–2 mmol kg−1) were applied to soil in suspension (0.5 g soil cm−3). The effect of 2 mmol kg−1 oxalate on the adsorption isotherm of phosphate was also studied. The rate of isotopic exchange was largely unchanged by any of the treatments. Neither pH nor acetate had an effect on phosphate solubility. The addition of di– and trivalent anions increased phosphate solubility somewhat. The effect increased with increasing concentration of anion, and was generally independent of pH and the proportion of anion adsorbed. Oxalate was more efficient in limiting the adsorption of freshly added phosphate than in desorbing native phosphate. The results illustrate that rhizosphere acidification does not necessarily increase phosphate solubility. However, carboxylates, the conjugate bases of organic acids, may play an important role in improving the availability of soil phosphate.
Zinc sorption–desorption by two Andepts: effect of pH and support medium
Authors: PARDO M.T. , GUADALIX M.E.
Published: August 2005

Laboratory experiments were carried out to evaluate the effect of pH, ionic strength and electrolyte composition on zinc sorption–desorption by two Andepts from the Canary Islands (Spain). At the natural soil pH, the soils exhibited little net negative surface charge and small Zn sorption capacities. More than 75% of the sorbed Zn was apparently strongly bonded. The pH greatly influenced the sorption–desorption reactions. Sorption increased with increasing pH, and retention increased abruptly at pH > 6.0. Sorption also occurred at pH values below the point of zero charge (PZC) of the soils, when most of the surface sites are positively charged. Desorption decreased continuously with rising pH and became a trace at pH > 6.0. An increase in the ionic strength of the background electrolyte decreased Zn sorption and enhanced the amount of sorbed metal that could be subsequently released. In the two soils, Zn sorption diminished somewhat in the K and Ca electrolytes as compared with the Na electrolyte. However, this did not happen at small Zn loadings. Desorption was not affected by the type of electrolyte and cation used. The results are consistent with chemisorption being responsible for most of the sorption. The results also suggested a strong affinity sorption or even precipitation at high pHs.
The properties, genesis and significance of a man‐made iron pan podzol near Castletownbere, Ireland
Authors: CONRY M.J. , DE CONINCK F. , STOOPS G.
Published: August 2005

Beneath a layer of artificially deposited peat debris, an eluvial E horizon and thin iron pan (Bsm) has developed in the upper part of the Ap horizon of a brown podzolic soil (Haplorthod) near Castletownbere, Ireland. The thickness of the E horizon and the depth of the pan are directly related to the thickness of the layer of peat debris.

The original soil (Haplorthod) was strongly podzolized with a significant accumulation of organic carbon, iron and aluminium in the spodic Bs. The iron pan of the upper sequum, on the other hand, is rich in iron. This iron pan seems to have developed as a result of reduction of iron in the E horizon, transport of divalent Fe2+ cations and precipitation in the Bsm as Fe2(OH)3, after the peat layer was deposited. We conclude that podzolization and iron pan development were fundamentally different processes.

The study suggests a fundamental change in the iron pan‐blanket peat development sequence previously postulated by palaeoenvironmentalists in Ireland.
Macromolecular changes of humic substances induced by interaction with organic acids
Authors: PICCOLO A. , NARDI S. , CONCHERI G.
Published: August 2005

Two different humic materials, one from a forest soil and the other from wormcasts, were used to study the influence of mineral and organic acids on the conformational properties of humic substances. The macromolecular changes were followed by low pressure gel permeation chromatography after titrating humic material to low pHs with acids. All organic acids (mono‐, di‐, tri‐carboxylic, and oxy‐acids), added to humic solution prior to a gel permeation in an alkaline buffer, were able to shift the totality of absorbance of the humic chromatographic peak from high to low molecular sizes. Mineral acids, phenol, alcohols, dipolar aprotic solvents, could not produce the same shift and gave total absorbance at the column void volume as in the case of humic substances alone. The chromatographic peak shifted back to elution volumes proper of higher size molecules when the humic‐organic acid mixture was back‐titrated to high pHs before gel permeation. Elution in a much stronger alkaline buffer did not change the overall macromolecular behaviour.

These results suggest that humic substances behave as micelles in solution and that hydrophobic bondings play an important role in holding humic molecules together. The organic acids enter the interior of the humic micelle‐like aggregates and alter the stereochemical hydrophobic arrangement of the humic material. In alkaline conditions the negative charges developed disrupt the apparent high molecular size configuration and disperse the humic aggregates into small micelles. Such conformational properties of humic substances appear to be a function of pH and of the concentration of organic acid.
Effect of water: soil ratio on phosphate release: P, aluminium and fulvic acid associations in water extracts from Andisols and Andic soils
Authors: NEGRÍN M.A. , ESPINO‐MESA M. , HERNÁNDEZ‐MORENO J.M.
Published: August 2005

The influence of water: soil ratio (w: s) on phosphorus fractions was studied in Andisols and Andic soils. Phosphorus was fractionated in 0.45‐μm filtered water extracts, and pH, monomeric Al and dissolved organic carbon (DOC) were determined. Speciation of dialysed extracts was also studied. Increasing w: s from 2.5 to 75 resulted in a release of dissolved reactive phosphorus (DRP), while DOC decreased significantly and pH increased moderately. For the 29 samples studied desorbed DRP =K (w: s)β, where β represents a P buffering term and ranged from 1 to 1.26 for DRP (mg kg−1). On dialysis, we observed a uniform partition between dialysable (DP) and non‐dialysable P (NDP) which was explained by the corresponding difference in DOC; these results were consistent with the existence of soluble P‐Al‐fulvic complexes. Study of solubility diagrams for determining solid‐phase controls on P activities in the extracts suggested that dissolution of Al‐phosphates could explain most of P release to water as free orthophosphate. Both this mechanism and organic P partition between solid and solution soil phases may be considered to reflect the main sources of P in Andisols: Al‐phosphate and organic‐Al‐P complexes.
Comparison of soil solution chemistry assessment using zero‐tension lysimeters or centrifugation
Authors: GIESLER R. , LUNDSTRÖM U. S. , GRIP H.
Published: August 2005

The composition of soil solutions obtained from the field varies with the method of extraction. Variations in sampling methods and the difficulties in extracting representative samples from soils in space and time, can explain divergent results. In this study we compared soil solutions from a forest soil in northern Sweden obtained by a centrifuge drainage technique and by zero‐tension monolith lysimeters. Zero‐tension lysimeters were destructively sampled, and centrifuge solutions from this soil were compared with that from soil outside. In our study we found three major differences in the solute composition between the centrifugate and the lysimeter leachate: (i) larger concentrations of most solutes in the mor layer centrifugate than in the mor layer leachate, (ii) accumulation of nitrate in the lysimeters, and (iii) larger concentrations of base cations in the zero‐tension lysimeters below 0.3 m depth. Water contents within the lysimeters were up to 3.5 times greater than under natural conditions and the water yields from the lysimeters indicate that water residence time ranged from < 1 to >5 years. This study shows that differences in results from the two methods are due to inherent differences in the methods themselves and not just to the collection of different soil waters. The hydrological anomaly and disturbance induced by the zero‐tension lysimeters affects the solute chemistry and thus the applicability of the results to field conditions.
Mobilization of phosphate from phosphate rock and alumina–sorbed phosphate by the roots of ryegrass and clover as related to rhizosphere pH
Authors: HINSINGER P. , GILKES R.J.
Published: August 2005

The mobilization of phosphate by two pasture species was studied in an alumina sand with P added as phosphate rock (PR–P) and as phosphate sorbed on to alumina (A12O3–P). The experiments were performed with a cropping device which enables the collection of rhizosphere material at defined distances from the roots of ryegrass (Lolium rigidum L.) and clover (Trifolium subterraneum L.). Nitrogen was supplied as nitrate alone or in combination with ammonium, which stimulates proton excretion by plant roots. Whatever the phosphate source, rhizosphere pH decreased relative to control pots without plants when ammonium was supplied. The pH decrease was larger for ryegrass than clover and reached 1.5 pH units when phosphate was supplied as A12O3–P. When nitrogen was supplied as nitrate only, rhizosphere pH remained close to the near neutral pH of the control. Root–induced dissolution of PR ranged between 20% of applied PR–P for nitrate–fed ryegrass and 40% for ammonium–fed ryegrass. Most of the P dissolved from PR was sorbed on to alumina and accumulated as alkali soluble P in the rhizosphere of both species, whilst dissolved Ca from PR also accumulated. The excretion of protons by roots of clover and ryegrass was thus the driving force for the root–induced dissolution of PR. When P was supplied as A12O3–P, ryegrass grew more than clover and took up more phosphate whatever the nitrogen source. Depletion of alkali soluble P, that is phosphate sorbed on to alumina, occurred only for ryegrass and extended up to two millimetres from the roots. These differences between the two plant species suggest that the mobilization of A12O3–P was not simply related to rhizosphere pH. Instead, the superior ability of ryegrass to mobilize A12O3–P relative to clover might be related to the ability of its roots to cope with smaller solution P concentration (smaller external P requirement).
Defining monitoring strategies for runoff and erosion studies in agricultural catchments: a simulation approach
Authors: JETTEN V. , BOIFFIN J. , DE ROO A.
Published: August 2005

The number of runoff and erosion studies in cultivated catchments in north west Europe is increasing. Measurements are usually costly and time‐consuming, in particular of soil hydrological properties because agriculture causes considerable spatial and temporal variation. A subdivision of the catchment is necessary to optimize the choice of sampling sites and to define extrapolation procedures. Four sampling strategies were examined, consisting of subdividing the catchment according to the crops (1), or according to the surface structure (2), followed by increasing the number of sampled fields by adding fields that are presumed to have maximum runoff activity (3) or intermediate activity (4). These strategies were compared by modelling the runoff and erosion in a virtual catchment, containing 15 arable fields with various crop sequences, at three times during a winter season. Strategy 2 gave better results than 1, because of the large variability of surface structure of the bare fields, which was not correlated with the crop type. Runoff prediction benefited from extra sampling only when groups of fields with a large heterogeneity in hydrological behaviour were included. The quality of erosion prediction increased when the steeper fields were included in the sampling scheme. A monitoring strategy is proposed that consists of: (i) repeated, fast and exhaustive descriptions of the soil surface using surface structure classes, (ii) subdivision of the catchment into groups of fields with identical surface characteristics before measurements are made, and (iii) sampling and extrapolation according to these groups.
Application and sensitivity analysis of process‐based erosion model GUEST
Authors: MISRA R.K. , ROSE C.W.
Published: August 2005

As long‐term measurement of erosion is expensive, models are required to predict spatial and temporal variation in erosion accurately for various land uses and to develop suitable practices for soil conservation. Details of the steady‐state, process‐based erosion model GUEST are presented with emphasis on the behaviour and sensitivity of the model to a fundamental erodibility parameter J. The model is developed to interpret temporal fluctuation in sediment concentration from bare soil in single erosion events. It incorporates the simultaneous effects of rainfall and runoff and of deposition on sediment concentration. An example of its application to temporal variation in sediment concentration for varying slopes and soil strengths show reasonable agreement between the measured data and the limits of sediment concentration predicted by the model. Sensitivity analysis indicated the model to be most sensitive to land slope and a defined sediment characteristic called depositability, but not as sensitive to frequency and dimensions of rills.
Nature and origin of soluble lipids shed into the soil by rainwater leaching a forest cover of Pinus maritima sp.
Authors: COLINA‐TEJADA A. , AMBLÈS A. , JAMBU P.
Published: August 2005

We have studied the ability of rainwater leaching a pine (Pinus maritima sp.) forest cover to solubilize lipid components. The nature and origin of straight‐chain hydrocarbons, alcohols and monoacids shed into the soil by rainwater was determined. Rainwater falling from branches and pine needles (RF) and rainwater running down the tree trunks (RR) were collected separately. The compounds were quantified and identified by gas chromatography used alone or coupled with gas chromatography. The dominant fraction of soluble lipids was aliphatic alcohols (alkanols) in RR and monoacids in RF. The concentration in water of total hydrocarbons and of total alkanols was the greatest in winter. In contrast, the proportion of alcohols with a number of carbon atoms < C20 in total alcohols and of branched monoacids in total monoacids were the highest in summer. Hydrocarbons were almost solely of bacterial origin but alcohols and monoacids showed components both of bacterial and of higher‐plant origin, the latter being more abundant in RR than in RF.
An investigation into sources of soil crack heterogeneity using fractal geometry
Authors: PRESTON S. , GRIFFITHS B.S. , YOUNG I.M.
Published: August 2005

Fractal dimensions were used to quantify the complex geometry of soil cracking patterns. Two fractal dimensions were measured to provide an adequate description of soil cracks: the mass fractal dimension, DM (which provides a measure of crack heterogeneity) and the spectral dimension, d (which provides a measure of crack connectivity). The cracking patterns generated by four soils were compared and related to their textural characteristics. As the clay content of the soil increased (6·1%–24·3%), so did DM (1·29–1·43), and d (1·03–1·33). Correlations indicated that there was a significant (P < 0·01) linear relation between the soil's clay content and the DM and d of cracks, the correlation coefficients obtained being equal to 0·965 and 0·840, respectively. Significant correlations (P < 0·01) were also observed between structural stability indices of the soil and the fractal dimensions of its cracks, increases in the stability of the soil, particularly of the clay fraction, being associated with cracks with greater fractal dimensions.
Aggregate stability and assessment of soil crustability and erodibility: II. Application to humic loamy soils with various organic carbon contents
Authors: LE BISSONNAIS Y. , ARROUAYS D.
Published: August 2005

Surface crusting results from aggregate breakdown under raindrop impact. It reduces the infiltration rate and may induce erosion by increasing runoff. Soil crustability and erodibility generally increase as organic carbon content decreases. Samples of topsoil were collected from fields cropped continuously for maize after land clearance at various dates. Organic carbon content ranged 4–30 g kg‐1. Aggregate stability was assessed by measuring fragment size distribution after different treatments. Samples were also subjected to simulated rainfall. The size of fragments forming the seal when the rain ceased and the infiltration rate during the rainfall were measured to characterize seal structure and hydraulic properties. Development of the seal and infiltration capacity were related to aggregate stability, which was itself a function of organic carbon content. The relations between aggregate breakdown, crusting and infiltration enabled us to predict soil physical behaviour from measurements of aggregate stability which are easier to make than direct measurements under rainfall. The proposed tests offer the opportunity to evaluate actual or potential soil physical degradation and erosion risks without extensive field measurements.
Deformation of saturated clays under mechanical and osmotic stress and its relation with the arrangement of the clays
Authors: CHARPENTIER S. , BOURRIÉ G.
Published: August 2005

The relation between physical and mechanical responses and the structure of clay has been studied. Deformation in response to mechanical stress was measured on samples of Wyoming mont‐morillonite and Saint Austell kaolinite in solutions of NaCl (0·005; 0·05; 0·5 and 5 M). Salt concentration was expressed in terms of osmotic potential of water and loading in terms of envelope pressure potential of water, and deformation was measured precisely under small mechanical stress. During shrinking and swelling, deformation was small and reversible, and consolidation was related to the largest mechanical stress applied previously in its history. The arrangement of the clay at equilibrium depends foremost on the type of clay mineral and physico‐chemical properties of the liquid phase. For Na‐montmorillonite suspensions, transition between gels with tactoids and rigid media with quasicrystals is abrupt and occurs at a potential of about −2000 J kg−1, whether the stress is osmotic or mechanical. Swelling indexes obtained under osmotic and mechanical stress are of the same order of magnitude. Deformation of smectite results from modifications of interassemblage and intra‐assemblage organization.
The absorption of water into a porous solid consisting of two dissimilar layers
Authors: WILSON M.A. , HOFF W.D.
Published: August 2005

Analytical modelling of the movement of water through porous solids consisting of more than one homogeneous layer is a problem in soil science and in construction engineering. In this paper equations based on the Green and Ampt model describe the absorption of water into a two‐layer composite bar for the cases of absorption through a material of large sorptivity into one of smaller sorptivity and vice versa. These predictions are compared with experimental results. There is broad agreement between theory and experiment at all times. We conclude that the absorption of water into a two‐layer composite is ultimately controlled by the second material in the absorbing sequence irrespective of whether this material has the larger or the smaller sorptivity.
Solubility of a proto‐imogolite sol in oxalate solutions
Authors: LUMSDON D.G. , FARMER V.C.
Published: August 2005

The amounts of aluminium liberated from a proto‐imogolite (P‐I) sol by 1–3 mM oxalate at pH 4·8–7·1 are found to be in excellent agreement with those calculated on the basis of the solubility product of the P‐I sol, and the stability constants for aluminium oxalate complexes established earlier. Calculated solubility curves are presented for proto‐imogolite sols in the presence of 10−3, 10−4 and 10−5 M total oxalate over the pH range 4–8, and also for the case where oxalate concentrations are controlled by solid calcium oxalate in the presence of 0·05, 0·5 and 5 mM calcium ion. We conclude that the presence of a proto‐imogolite allophane in podzol B horizons of pH 5·0 requires oxalate concentrations less than 10−5 M. For fertile agricultural soils with typical calcium concentrations (approximately 5 mM) in the soil solution, aluminium oxalate concentrations would be limited by calcium oxalate solubility to less than 10−6 M in the presence of proto‐imogolite allophane at pH values exceeding 5·5. In leached podzolic soils calcium concentrations are generally less than 0·05 mM, and would not limit the formation of aluminium oxalate complexes.
The role of amino acids and nucleic bases in turnover of nitrogen and carbon in soil humic fractions
Authors: KUZYAKOV Y.V.
Published: August 2005

Incorporation of labelled 15N and 14C amino acids and nucleic bases into soil humus fractions as well as humus turnover was investigated under field conditions. The dynamics of 15N and 14C incorporation into organic matter was characterized by the following main steps: rapid incorporation of the labelled substance prevailing for the first 1–3 weeks, and decomposition of included labelled fragments prevailing beyond one month after substance addition. The annual turnover rates of N and C in humus fractions due to incorporation of amino acids and nucleic bases were calculated. The turnover rate of N in humus is two to three times that of C. The contribution of amino acids to organic matter generation is about twice as great as that of nucleic bases and other N‐containing organic substances. This indicates the important role of amino acids in the humification process and humus turnover. Turnovers of humic acids (0·002 year−1 for C and 0·02 for N) are the most rapid of humic fractions investigated, and humin is characterized by the slowest turnover (0·0002 year−1 for C and 0·007 for N). There are no significant differences in the turnover rates of fulvic acid fractions (0·0002 year−1 for C) with different molecular weight.
Factors affecting phosphate sorption along a Mediterranean, dolomitic soil and vegetation chronosequence
Authors: CARREIRA J.A. , LAJTHA K.
Published: August 2005

Phosphate sorption by calcareous soils has been studied mainly on heavily fertilized agricultural soils and soils with calcite as the main carbonate mineral. We examined factors affecting phosphate adsorption in the soils of a semi‐arid, mediterranean, dolomitic, soil and vegetation chrono‐sequence in southeastern Spain. The youngest soils are highly eroded, Sandy Regosols (Typic Xerorthents) under gorse‐scrubland vegetation. These have small P sorption capacities, large Mg‐Ca carbonate contents but small amounts of Fe and Al oxides. Small total P (HNO3/HClO4 digestion) concentrations (30–130 μg P g−1), of which up to 90% is Ca‐bound (HCl‐extractable), are typical of these young soils. P sorption markedly increased when Ca2+ was added to the solution. The fractionation of previously sorbed P indicates that the fate of most of this extra‐sorbed P is the labile‐P fraction sorbed on to (carbonate) surfaces and the apatite‐like fraction (NaHCO3‐extractable and HCl‐extractable fractions). At the other extreme, older more‐intensively weathered, sandy‐clay‐loam rendzinas (Entic Haploxerolls), supporting dense mature garrigue, have a much greater P adsorption capacity and larger clay and Fe and Al oxide concentrations. They have more total P (ca 400 μg P g−1), much of it in occluded form (residual fraction). These soils show no significant differences in P sorption whether or not CaCl2 was used as a background electrolyte. Considering the overall variations within the chronosequence, dithionite extractable Fe and Al are the properties best correlated with P sorption. This support the general finding that crystalline Fe‐oxides (e.g. goethite and haematite) appear to be the most important P‐sorbing component for soils in the Mediterranean region, rather than amorphous Fe‐oxides (e.g. ferrihydrite) as is reported for more mesic areas. Stepwise multiple regression and fractionation data, however, suggest that, provided the soil solution is rich in Ca2+, carbonate may also be a significant contributing factor to P sorption, especially in the youngest of these dolomitic soils.