bluuebunny/crossref_metadata_2025_split huggingface dataset:
A usable mechanistic model of nitrate leaching II. Application
Authors: BARRACLOUGH D. 
Published: July 2006

SUMMARYSimulations produced by a mechanistic model are compared with field observations of nitrate leaching from 0.35 ha hydrologically isolated experimental plots. The parameters used in the model are obtained in two ways. First by fitting the model to field observations in one year. These parameters are then used to simulate leaching in other years. Second, model parameters are obtained by fitting eluant profiles from pulse inputs of solute to undisturbed cores in the laboratory. When used with the field‐derived parameters, the model simulates total leaching losses well in other years, although the pattern of loss is only approximately reproduced. The simulation suggests that water and solute flow in drained, structured soils is complex; preferential flow in the upper horizons resulting in 20% of the water‐filled pore space carrying most of the solute flow, and by‐pass flow in the subsoil causing rapid movement of water and solute to the drains. The result is that much of the nitrate in the upper horizons appears to be protected from leaching. When used with laboratory‐derived parameters, the model was a poor predictor of both the pattern and quantity of nitrate leached.
The hydrolysis of kaolinitic soils as affected by the type of the exchangeable cation
Authors: LEVY G. J. , ALPEROVITCH N. , van der MERWE A. J. , SHAINBERG I. 
Published: July 2006

SUMMARYThe hydrolysis of four kaolinitic soils from the Transvaal (RSA), and the effects of exchangeable Na, Ca and Mg were studied by monitoring the changes in the electrical conductivity of the soil suspension with time. The rate of the hydrolysis was found to be linearly related to the square root of time. The rates of the hydrolysis of the Half‐Way House and Krugersdorp soils were appreciably lower than those of the Potchefstroom and Amsterdam soils, and rates reported in the literature for smectite soils. This was probably because of the low content of hydrolysable bases in the Half‐Way House and Krugersdorp soils, resulting from the low cation exchange capacity of their clay fraction. The extensive hydrolysis observed in the Potchefstroom and Amsterdam soils could be ascribed to the presence of significant amounts of smectite in the former and high silt content in the latter; both factors are known to enhance the rate of weathering and hydrolysis. With the exception of the Potchefstroom soil, the rate of hydrolysis was not affected by the cationic composition of the adsorbed phase. Contrary to expectations, exchangeable Mg did not inhibit hydrolysis in the soils studied, probably because of the limited isomorphic substitutions in kaolinite which lead to very small amounts of octahedral Mg in the crystal. The hydrolysis of the Potchefstroom soil was enhanced in the presence of exchangeable Mg compared to exchangeable Ca; the reason for this is not fully understood.
Selenite fixation by soil particle‐size separates
Authors: CHRISTENSEN B. T. , BERTELSEN F. , GISSEL‐NIELSEN G. 
Published: July 2006

SUMMARYThe fixation of selenite by clay‐ (< 2 μm), silt‐ (2–20 μm) and sand‐size (20–2000 μm) separates from two arable soils was examined in solutions of 75Se‐labelled sodium selenite using a Se/sample ratio of 1/106. Size separates were isolated by ultrasonic dispersion and gravity sedimentation. Selenite fixation was determined after equilibration periods ranging from 5 min to 26 h. Hydrogen peroxide‐treated samples were included to examine the effect of organic matter on selenite fixation capacity.The relative distribution of native Se, C, dithionite/citrate‐extractable Fe and Al between size separates was similar. Concentrations in clay were four to nine times higher than in whole soils, silt showing two to five times higher concentrations and sand being very low in Se, C, Fe and Al.After 1 h, clay, silt and sand fixed 64–65%, 45–61% and <5% of the selenite added, respectively. The fixation on whole soils was 14–18%. After 1 day, fixation on clay, silt, sand and whole soil increased to 78–87%, 67–79%, 3–14% and 31–39%, respectively.Hydrogen peroxide‐treatment reduced the selenite fixation capacity of whole soil, silt and sand to very low levels. Fixation on peroxide‐treated clay was in accord with values for pure clay minerals reported in the literature. Generally, the fixation capacity of peroxide‐treated natural clay and pure clay minerals was only half that observed for intact clay‐size separates, demonstrating the importance of organic matter in soil selenite fixation capacity.
A critical comparison of methods to monitor the decay rate of straw in field soils
Authors: HARPER S. H. T. 
Published: July 2006

SUMMARYStudies of straw decay in soil have concentrated primarily on three separate issues: the loss of potential to produce phytotoxins, the disappearance of surface mulches and the contribution of the products of decay to the carbon cycle. Cost and availability prohibit the use of 14C‐labelled plant materials in extensive field trials and mesh bags can be criticized for providing conditions which are potentially unlike those within the bulk soil. In consequence, a range of chemical and physical tests for estimating the extent of straw decay have been compared. Strength measurements are of direct value where mechanical impedance of machinery is involved, but otherwise are satisfactory only when straw is both harvested and returned to the soil by hand. The increase in lignin content was found to provide the most satisfactory method for the estimation of weight losses of up to 60%.
Modelling volume changes in peat under conifer plantations
Authors: PYATT D. G. , JOHN A. L. 
Published: July 2006

SUMMARYIt is proposed that the shrinkage of well‐decomposed peat beneath conifer plantations owing to the irreversible loss of water takes place in two stages. The first stage of pure subsidence is followed by a second stage of the formation of natural aggregates with a network of large cracks and a slower rate of subsidence. A plot of specific volume against gravimetric water content for natural aggregates or for excised blocks dried artificially shows a linear relationship throughout the range of water content. The assumption of equi‐dimensional shrinkage in stage two allows a prediction of the relative amounts of subsidence and crack formation. The shrinkage of non‐cracking fibrous peat is also dealt with. A preliminary attempt at verification of the predictive model from measurements of crack volume or subsidence of ground level shows that the model gives values of the right order of magnitude.
Brief and vigorous N2O production by soil at spring thaw
Authors: CHRISTENSEN S. , TIEDJE J. M. 
Published: July 2006

SUMMARYIn an acid sandy loam soil (pH 3.8), field production of N2O was two orders of magnitude higher at thaw in the spring than at any time during the rest of the year. Soil thaw in midwinter did not result in any increase in N2O flux. Soil water content remained at, or above field capacity during measurements; nitrate was added in excess. This effect could be reproduced in the laboratory: thawing soil cores at controlled temperature, nitrate and moisture yielded a large flush of N2O compared to an unfrozen control. The results indicate the importance of microbial N2O production during thaw for total annual N2O‐emission.
Alysimeter study of the effects of a ryegrass catch crop, during a winter wheat/maize rotation, on nitrate leaching and on the following crop
Authors: MARTINEZ J. , GUIRAUD G. 
Published: July 2006

SUMMARYThe effects of an intercrop catch crop (Italian ryegrass) on (i) the amounts and concentrations of nitrate leached during the autumn and winter intercrop period, and (ii) the following crop, were examined in a lysimeter experiment and compared with that from a bare fallow treatment. The catch crop was grown in a winter wheat/maize rotation, after harvest of the wheat, and incorporated into the soil before sowing the maize. A calcium and potassium nitrate fertilizer labelled with 15N (200 kg N ha−1; 9.35 atom per cent excess) was applied to the winter wheat in spring. Total N uptake by the winter wheat was 154 kg ha−1 and the recovery of fertilizer‐derived N (labelled with 15N) was 60%. The catch crop (grown without further addition of N) yielded 3.8t ha−1 herbage dry matter, containing 43 kg N ha−1, of which 4.1 % was derived from the 15N‐labelled fertilizer. Two‐hundred kg unlabelled N ha−1 was applied to the maize crop. During the intercrop period the nitrate concentration in water draining from the bare fallow lysimeters reached 68 mg N1−1, with an average of 40 mg N1−1. With the catch crop, it declined rapidly, from 41 mg N I−1 to 0.25 mg N I−1, at the end of ryegrass growth. Over this period, 110 kg N ha−1 was leached under bare fallow, compared with 40 kg N ha−1 under the catch crop. 15N‐labelled nitrate was detected in the first drainage water collected in autumn, 5 months after the spring application. The quantity of fertilizer‐N that was leached during this winter period was greater under bare fallow (18.7% of applied N) than when a catch crop was grown (7.1 %). In both treatments, labelled fertilizer‐N contributed about 34% of the total N lost during this period. With the ryegrass catch crop incorporated at the time of seedbed preparation in spring, the subsequent maize grain‐yield was lowered by an average of 13%. Total N‐uptake by the maize sown following bare fallow was 224 kg N ha−1, compared with 180 kg ha−1 with prior incorporation of ryegrass; the corresponding values for uptake of residual labelled N were 3% (bare fallow) and 2% (ryegrass) of the initial application. Following the maize harvest, where ryegrass was incorporated, 22.7% of the previous year's labelled fertilizer addition was present in an organic form on the top 30 cm of lysimeter soil. This compares with 15.7% for the bare fallow intercropping treatment. Tracer analyses showed overall recoveries of labelled N of 91.7% for the winter wheat/ ryegrass/maize rotation and 97% for the winter wheat/bare fallow/maize rotation. The study clearly demonstrated the ecological importance of a catch crop in reducing N‐leaching as well as its efficient use of fertilizer in the plant‐soil system from this particular rotation. However, the fate of the organic N in the ploughed‐down catch crop is uncertain and problems were encountered in establishing the next crop of maize.
Chemical nature of organic phosphorus in cultivated and uncultivated soils under different environmental conditions
Authors: CONDRON L. M. , FROSSARD E. , TIESSEN H. , NEWMANS R. H. , STEWART J. W. B. 
Published: July 2006

SUMMARYA sequential alkali extraction procedure followed by ultrafiltration and quantitative 31P nuclear magnetic resonance (NMR) spectroscopy was used to examine organic phosphorus in a Brown Chernozem, and an adjacent Gleysol developed under native prairie vegetation, and a Grey Luvisol formed under aspen forest in Saskatchewan, Canada. Differences in the nature of organic P in the native soils were related to moisture status and vegetation. In the grassland soils, a greater proportion of orthophosphate diester P was found in the bottom‐slope Gleysol. This difference was partly attributed to less favourable conditions for mineralization in the bottom slope soil compared with the mid‐slope Brown Chernozem. Teichoic acid P occurred only in the native Grey Luvisol (NMR δ p.p.m. 0.36‐0.95) under boreal forest and not under native grassland. At all three sites, soils under long‐term cultivation were also examined and while orthophosphate monoester P (83.4‐94.6% total Po), orthophosphate diester P (3.9–8.7% total Po) and teichoic acid P (12.7% total Po in forested Grey Luvisol) were detected in native soils, only orthophosphate monoester P was found in the corresponding soils that had been cultivated for 70–80 years. These findings suggest that orthophosphate diester P and teichoic acid P are more readily mineralized in the soil environment than orthophosphate monoester P forms.
Stability of soil aggregates in relation to organic constituents and soil water content
Authors: HAYNES R. J. , SWIFT R. S. 
Published: July 2006

SUMMARYThe effects of soil organic matter content, soil water content and duration of wet‐sieving on aggregate stability of soils with contrasting cropping histories were investigated. Long‐term pasture samples had a greater aggregate stability than long‐term arable samples. However, air‐drying aggregates before wet‐sieving increased the aggregate stability of long‐term pasture samples, but decreased that of long‐term arable samples. With increasing duration of wet‐sieving, the proportion of water‐stable aggregates declined until a near‐constant value was reached for each sample. Thus, within a sample there are aggregates possessing a wide range of stabilities; with increasing time under arable cropping there is an increase in the proportion of unstable aggregates present, and the measured aggregate stability, therefore, declines. Unstable aggregates (defined as those dispersed after wet‐sieving for 1 min) generally had lower organic matter content than stable ones (those still intact after sieving for 15 min).The aggregate stability of a regrassed site (13 years of arable plus 2 years of pasture) was markedly higher than that of a corresponding site from 15 years of arable cropping. Nonetheless, levels of organic matter (organic C, total N and hydrolysable carbohydrate) were almost identical at the two sites. However, aggregates from the regrassed site did have a higher biomass C and water‐extractable carbohydrate content than those from the 15‐year arable site. For a group of soils with varying cropping histories, aggregate stability was significantly more closely correlated with hot water‐extractable carbohydrate content than with organic C or hydrolysable carbohydrate content. It is suggested that the hot water‐extractable carbohydrate fraction may represent a pool of carbohydrate involved in the formation of stable aggregates.
The control of aluminium solubility in some acidic Australian soils
Authors: CONYERS M. 
Published: July 2006

SUMMARYThe variation in the solubility of aluminium in some acid soils of south‐eastern Australia has been investigated. The hypothesis was that leaching of silicic acid influenced the extent of clay decomposition, and hence aluminium solubility, in accord with Le Chatelier's principle. In lower rainfall areas (< 800 mm) control of Al solubility was consistent with a poorly crystalline kaolinite as the source phase, while at higher rainfall (> 1200 mm) it was consistent with gibbsite control. However, soluble silica concentrations were too high for gibbsite and kaolinite to be thermodynamically stable phases. The hypothesis was modified to take account of a quasi‐equilibrium in the open soil system, in which clay dissolution and A1 solubility appear to depend on the relative rates of H+ input and of silica leaching. These may result in a supersaturated, or possibly even undersaturated system, at any point in time. The relative sizes of the organic sink and mineral Al source determine the influence of organic matter on Al solubility.
Seasonal variation in field‐saturated hydraulic conductivity in two swelling clay soils in Sweden
Authors: MESSING I. , JARVIS N. J. 
Published: July 2006

SUMMARYA simple method (the inversed auger hole method) for measuring field‐saturated hydraulic conductivity (Kfs) was investigated. Measurements were carried out in the spring, summer and autumn at three depths in two Swedish clay soils (Ultuna and Limsta, with clay contents of 45–60%0 and 65–80%, under barley and grass/clover ley respectively). Seasonal fluctuations in Kfs at Limsta were more pronounced, and were observed deeper in the profile. This was attributed primarily to larger structural changes due to a higher capacity for swell/shrink (normal shrinkage over the available water range) and an earlier drying up of the soil under grass/clover ley.It was shown that the measured Kfs values were strongly correlated with the total inter aggregate (macro‐) porosity (et), estimated from a simple model of soil shrinkage. Combining the data from both soils, a single power‐law relation was adequate (r= 0.73) to describe the variations in Kfs with et
Soil and soil solution chemical composition at three sites within the Loch Dee catchment, SW Scotland
Authors: GRIEVE I.C. 
Published: July 2006

SUMMARYSoil solution chemical composition was monitored over a 2–year period at three plots near the edge of an igneous intrusion in SW Scotland. Soil solutions were all strongly acidic with mean pH of approximately 4.0 at 0.20 m depth and 4.5 at 0.60 m. Chemical composition of the deeper soil solutions varied with geology. Greatest acidity and aluminium concentrations were found over metamorphosed Ordovician sedimentary rocks, with mean pH of 4.3 and A1 concentration of 22 mmol m−3 at 0.60 m. At a site on similar rocks nearer the intrusion, mean pH at 0.60 m was 4.8, with very low A1 (3 mmol m−3) and correspondingly greater Ca. At a peaty site on igneous rocks, increased pH with depth was associated with no change in base cations and A1 but a reduction in mean SO, from 41 mmolm−3 at 0.20m to 10mmol m−3 at 0.60m. Ion‐exchange selectivity coefficients calculated from soil and solution chemistry differed slightly from currently used values for peaty upland soils, with larger coefficients for A1/K exchange and smaller coefficients for A1/Ca and A1/Mg exchange. Relations between A1 and H ion concentrations confirmed under‐saturation of solutions with respect to gibbsite at 0.20 m. The differences between plots were consistent in different seasons, despite significant seasonal variability in Na, C1, dissolved organic carbon and Fe.
Extractability and adsorption of sulphate in soils
Authors: CURTIN D. , SYERS J. K. 
Published: July 2006

SUMMARYVirtually all of the indigenous sulphate (SO4) in a range of UK soils with moderately high pH values (> 6) was found to be present in the soil solution and, as a consequence, was highly susceptible to leaching. For acid soils containing adsorbed SO4, the extractability of SO4in NaCl and CaCl2solutions was dependent on both the ionic strength and cation species. Addition of small amounts (<∼ 10−2M) of either NaCl or CaCl2actually decreased the amount of SO4extracted, but SO4extractability increased sharply with concentrations of NaCl or CaCl2higher than about 0.1 M. At a similar ionic strength, more SO4was extracted by NaCl than CaCl2. Sequential extraction with 1 M NaCl removed essentially all of the absorbed SO4. The release characteristics of SO4were very different to those of phosphate and this difference in behaviour is not easily reconciled with the view that SO4is chemisorbed, as is phosphate. Except for a few acid soils with high oxide contents, the capacity of the soils to adsorb added SO4was quite small. None of the soils with pH values higher than 6 adsorbed a significant amount SO4. The results raise questions regarding the efficiency of SO4‐containing fertilizers in correcting and preventing S deficiency in situations where leaching is important.
Modelling water and solute transport in macroporous soil. II. Chloride breakthrough under non‐steady flow
Authors: JARVIS N. J. , BERGSTRÖM L. , DIK P. E. 
Published: July 2006

SUMMARYA model of water and solute transport in macroporous soils (Jarvis et al., 1991) has been evaluated in column breakthrough experiments under field conditions. Hydraulic properties were first measured in replicate soil monolith lysimeters sampled from grass ley and continuous barley treatments in a clay soil. A pulse input of 0.05 M KCl was then supplied by drip irrigation and measurements made of the water discharge and chloride leaching resulting from the natural rainfall over a 1‐month period.The results showed that the macropores constituted the dominant flow pathway (accounting for 80% of the total water outflow) and that diffusive exchange of chloride between the two flow domains was the main factor causing variability in leaching. Larger hydraulic conductivities and macroporosities in the lower topsoil and at plough depth in the grass ley monoliths were taken as evidence of structural amelioration. Less of the applied chloride was leached in the grass monoliths than in the barley (means of 20% and 31% respectively). This was mainly due to a smaller effective aggregate size and thus a more efficient diffusion‐controlled retention.
A statistical approach for predicting accuracies of soil properties measured by single, double and dual gamma beams
Authors: MIYAZAKI T. , KASUBUCHI T. , HASEGAWA S. 
Published: July 2006

SUMMARYA statistical analysis is developed to describe the accuracies of soil properties measured through single, double and dual gamma‐beam methods. The gamma sources are 137Cs for the high‐energy beam (662 keV) and 241Am for the low‐energy beam (60 keV). The theoretical confidence intervals of the thicknesses or water contents of samples are determined using a modification of the method of Gardner el al. (1972), who derived theoretical variances using standard error propagation formulae. The finding of Gardner et al. that, in the double‐beam method, errors in the soil bulk density and water content measurements are primarily caused by random emission from the sources, is confirmed by the theoretical confidence intervals and is applied to the single‐beam and dual‐beam methods. The effects of interferences between gamma rays in the dual‐beam method on the accuracies of estimations are examined. The minimum measurement time to attain a required accuracy can be predicted from the theoretical confidence intervals.
A basis for soil mesomorphological analysis
Authors: KOPPI A. J. , McBRATNEY A. B. 
Published: July 2006

SUMMARYTo bridge the gap between description of soil in the field, i.e. macromorphology (which is usually qualitative), and micromorphology (which may be quantitative), a technique for mesomorphological description is presented. Undisturbed vertical soil monoliths are impregnated with epoxy resin and features of interest are quantified using image analysis. Resin is applied, in the field, to the soil profile. A monolith is excavated and resin is applied to the reverse side which is then ground flat with a disc grinder. The prepared surface is then presented to a video camera for image capture. In general, the resolution is > 0.2 mm and the field of view is 100 × 500 mm. This scale permits details of horizons to be quantified and the relationship between features of interest in different horizons to be discerned. The technique is illustrated by quantifying the pore structure and distribution of manganese/ iron‐rich material (nodules and some rock fragments) in the first three horizons of a Paleustalf. The attributes measured on each of these features are the volume proportion and surface area. The three horizons may be distinguished by the quantity and two‐dimensional pattern of the pore structure. The macroporosity (volume proportion of pore space) and surface area are greatest in the surface horizon, smallest in the second horizon and intermediate in the third horizon. The concentration of manganese/iron‐rich nodules is at a maximum in the second horizon which corresponds to the region of least macroporosity.