bluuebunny/crossref_metadata_2025_split huggingface dataset:
Effect of natural organic soil solutes on weathering rates of soil minerals
Authors: RAULUND‐RASMUSSEN K. , BORGGAARD O. K. , HANSEN H. C. B. , OLSSON M. 
Published: March 2003

SummaryThe rate at which minerals in the soil weather is affected by pH and concentration of organic solutes (DOC). The rates of release of Al, Ca, Fe, K, Mg, Na, P and Si from a mineral soil sample to solutions of natural organic solutes and HCI (control) were determined at pH 3 and 4 for up to 17 weeks. Soil solutions were collected by centrifuging materials of O horizons from various soil types under four tree species (spruce, birch, oak, beech) and passed through a cation‐exchange resin to yield H+‐saturated organic solutes. The acceleration of the elements' release by the organic solutes was shown directly by the relative ligand effect (RLE), that is, the release rate in the organic solute solution divided by the release rate in the HCI solution (control) at the same pH. The RLE was greater at pH 4 than at pH 3, and it decreased for the elements in the order Fe > Ca > Mg > Al ≈ Si > K ≈ Na. This indicates that natural organic solutes are more important weathering agents at higher than lower pH and for weathering of mafic minerals rich in Ca, Fe and Mg than of felsic minerals such as K‐ and Na‐feldspars. For all elements and at both pHs, RLE was strongly correlated with the concentration of DOC, which was also closely correlated with titratable acidity of the organic solutes. The important effect of soil type and tree species in producing weathering‐promoting organic solutes therefore seems to be expressed through the concentration and not the composition of the organic soil solutes.
Equilibria of weak acids and organic Al complexes explain activity of H+and Al3+in a salt extract of exchangeable cations
Authors: Nissinen A. , Ilvesniemi H. , Tanskanen N. 
Published: March 2003

SummaryTwo sequential extractions with unbuffered 0.1 mBaCl2were done to study the release of salt‐exchangeable H+and Al from mineral horizons of five Podzols and a Cambisol. Released Al was found to have a charge close to 3+ in all horizons and in both extractions. This finding was supported by the near‐equality of the titrated exchangeable acidity (EAT) and the sum of exchangeable acids (EA = He + 3Ale, calculated from the pH and Al concentration of the extract). The ratio between EA of the second and the first extraction was over 0.50 in the Bs2 and C horizons and smaller in the other horizons. H+was assumed to be in equilibrium with weak acid groups, and the modified Henderson–Hasselbach equation, pKHH = pH − n log (α/(1 − α)), was used to explain pH of the extract. The degree of dissociation (α) was calculated as the ratio between effective and potential cation exchange capacity. Value of the empirical constantnwas found to be near unity in most horizons. When the monoprotic acid dissociation was assumed in all horizons, pKHHhad the same value in both extractions. For Al3+, two equilibrium models were evaluated, describing (i) complexation reactions of Al3+with soil organic matter, and (ii) equilibrium with Al(OH)3. Apparent equilibrium constants were written as (i) pKo = xpH − pAl3+, and (ii) log Qgibbs= log Al3+ − 3log H+.The two extractions gave an average reaction stoichiometryxclose to 2 in all horizons. Results suggest that an equilibrium with organic Al complexes can be used to express dissolved Al3+, aluminium being apparently bound to bidentate sites. The value of log Qgibbswas below the solubility of gibbsite (log Kgibbs = 8.04) in many horizons. In addition, log Qgibbsof the second extraction was greater than that of the first extraction in all horizons except the C horizon. This indicates that equilibrium with Al(OH)3cannot explain dissolved Al3+in the soils. We propose that the models of pKHHand pKocan be used to simulate exchangeable H+and Al3+in soil acidification models.
Effects of mineral and monocarboxylic acids on the molecular association of dissolved humic substances
Authors: Piccolo A. , Conte P. , Cozzolino A. 
Published: March 2003

SummaryThe conformational structure of dissolved humic substances is an important property that controls the reactivity of humus in the soil solution. High performance size‐exclusion chromatography was used here to study the changes in molecular size of different humic substances brought about by addition of mineral (HCl) and monocarboxylic (formic, acetic, propionic, and butyric) acids. The CPMAS‐NMR spectra showed that humic substances had varying chemical composition and that the ratio of hydrophilic to hydrophobic carbon (HI/HB) was greater for a humic acid from soil than for ones from oxidized coal and lignite. All humic substances showed a decrease in UV absorbance of chromatographic peaks when treated with either HCl or monocarboxylic acids. This was due to the hypochromic effect by which the absorptivity of associated molecules is decreased when they are separated. We attributed the molecular separation upon acid treatment to the formation of intermolecular hydrogen bonding that alters the original conformation stabilized mainly by weaker hydrophobic interactions. Addition of organic acids not only further decreased peak absorbances of humic acids but also caused their shift to larger elution volumes, indicating a larger conformational disruption than with HCl. The extent of the molecular size changes showed a relation to the number of carbons of monocarboxylic acids and to the HI/HB ratios of humic materials. The larger the carbon content of organic acids and the smaller the HI/HB ratio of humic materials, the larger was the decrease of the average molecular size of humic acids. These results suggest that dissolved humic substances associate predominantly by hydrophobic forces and that the apolar components of humic substances largely control their aggregation and reactivity in the environment.
Application of two methods to determine hydraulic conductivity with disc permeameters on sloping land
Authors: Joel A. , Messing I. 
Published: March 2003

SummaryThe purpose of the present study was to compare two methods for estimating the hydraulic conductivity near saturation with disc permeameters, and to identify their merits when studying runoff on sloping land. The soil's hydraulic conductivity (K) was measured with disc permeameters at a sequence of nominal pressures (ψ) in three blocks with average slope gradients of 11.0% (two occasions), 21.5% (two occasions) and 29.3% (one occasion), respectively, within a sloping area, 40 m wide and 100 m long. Two different methods were used. In the first, the split‐location method, the permeameter was moved to an adjacent spot after measurement at each applied ψ. The estimate of K(ψ) was based on measured sorptivity, steady‐state volumetric flow, initial volumetric water content and the volumetric water content at the applied ψ. In the second method, the one‐location method, the permeameter was not moved during the measurements at each sequence of applied ψ and the estimate of K(ψ) was based only on steady‐state volumetric flow and piece‐wise application of the exponential relation between K and ψ. The latter method generally gave smaller estimates of K than the former on the gentle slopes. These differences were smaller or negligible on steeper slopes. The slope gradient and the conditions in the uppermost soil layers had a definite influence on the values of K obtained. The one‐location method is recommended in studies in which disturbance of the soil surface must be kept to a minimum, as is the case in experimental plots.
Temporal changes in chemical properties of air‐dried stored soils and their interpretation for long‐term experiments
Authors: Blake L. , Goulding K. W. T. , Mott C. J. B. , Poulton P. R. 
Published: March 2003

SummaryThe usefulness of stored soils from long‐term experiments is often questioned because of changes that might occur during storage. We examined changes during long‐term storage (8–69 years) in the chemical properties of soils with a range of pH values (3.4–8.1 in water) from woodland and grassland experiments at Rothamsted Experimental Station in the UK. No significant changes during storage were measured for total C and N. Large but erratic changes in exchangeable Na+ content between 1959 and 1991 were probably caused by contamination of the 1959 samples by perspiration and from sodium‐based glassware. Exchangeable K+ increased during storage but only by a small amount. Small changes in exchangeable Ca2+ and Mg2+ were measured in some samples but not in others. Generally the amount of exchangeable cations increased slightly during storage. This is probably linked to the decreases of  0.4 units in the pH of acid soils, which we attribute to the hydrolysis of approximately 0.25% of the exchangeable Al3+. A doubling of the amount of exchangeable Mn2+ during storage for 32 years was probably caused by re‐equilibration of Mn species. The most practicable way to prepare soil samples for long‐term storage is to dry them in air. However, those who study changes in soil by re‐analysing samples of the soil stored for a long time must (i) use the same methods of analysis, or (ii) demonstrate that different methods lead to the same results, and (iii) know what changes can arise during storage.
Effects of lanthanum and mixtures of rare earths on ammonium oxidation and mineralization of nitrogen in soil
Authors: Xu Xingkai , Wang Zijian 
Published: March 2003

SummaryRare earths are applied widely in Chinese agriculture to improve crop nutrition and incidentally in fertilizers, yet little is known of their effect on the biological functioning of the soil. We have studied the effects of lanthanum and of mixtures of rare earths on the potential ammonium oxidation and nitrogen mineralization in soil by incubation experiments in the laboratory. The no‐observed‐effect and median effective concentrations of the rare earths on these two processes are reported, and their corresponding mechanisms are discussed. For mixtures, the no‐observed‐effect concentrations relating to potential ammonium oxidation and N mineralization were at 393 and 373 mg rare earths kg−1 soil, respectively, and their median effective concentrations were at 1576 and 1108 mg kg−1 soil, respectively. When lanthanum was applied alone, the no‐observed‐effect concentrations relating to potential ammonium oxidation and N mineralization were at 432 and 443 mg La kg−1 soil, respectively, and their median effective concentrations were at 18 212 and 1237 mg kg−1 soil, respectively. Therefore, the influence of mixtures of rare earths on potential ammonium oxidation or on N mineralization was slightly stronger in comparison with that of lanthanum. Mineralization of nitrogen is apparently more sensitive to the stress caused by rare earths than ammonium oxidation. We conclude that the influence of individual rare earths in the mixtures on the above two processes can be additive and that the present dosage of mixed rare earths (< 230 g ha−1 year−1 or 0.15 mg kg−1 soil year−1) currently applied in China can hardly affect the potential ammonium oxidation and N mineralization in the soil even over a long period.
Seasonal differences in the soil water balance under perennial and annual pastures on an acid Sodosol in southeastern Australia
Authors: Heng L. K. , White R. E. , Helyar K. R. , Fisher R. , Chen D. 
Published: March 2003

SummaryReplacement of native deep‐rooted grasses by shallow‐rooted ones has resulted in greater losses of water and nitrogen by drainage. To counter this effect we have tested the hypothesis that liming, and the conversion of annual grass pastures to perennial grass pastures, could improve the sustainability of grazing systems in the high rainfall zone (> 600 mm per annum) in southeastern Australia, through better use of water and nitrogen. A field experiment consisting of sixteen 0.135 ha (30 m × 45 m) grazed paddocks representing four pasture combinations (annual pasture (mainly Lolium rigidum) without lime (AP–); annual pasture with lime (AP+); perennial pasture (mainly Phalaris aquatica) without lime (PP–), and perennial pasture with lime (PP+)) was carried out from 1994 to 1997 on an acid Sodosol (Aquic Hapludalf) in southern New South Wales, Australia. Measurements were made of surface runoff, subsurface flow (on top of the B horizon) and soil water content. The results showed that perennial grass pastures, especially PP+, extracted approximately 40 mm more soil water each year than the annual grass pastures. As a result, surface runoff, subsurface flow and deep drainage were at least 40 mm less from the perennial pastures. These measurements were further supported by a simulation of soil water deficit and deep drainage for AP– and PP+ paddocks, using 10 years' past meteorological records. Overall, the results suggested that well‐grown, phalaris‐based pastures could reduce recharge to groundwater and make pastoral systems more sustainable in the high rainfall zone.
The origin and nature of organic nitrogen in soil as assessed by acidic and alkaline hydrolysis
Authors: Greenfield L. G. 
Published: March 2003

SummaryThe origin and nature of much of the organic nitrogen in soil is unknown or speculative. Alkaline hydrolysis was used to fractionate soils into alkali‐soluble and insoluble fractions. Alkali‐insoluble residues from a wide variety of soils contained largely fixed ammonium or chitin or both. Acid hydrolysis of alkali‐insoluble residues from soils, microbes, insects and well‐rotted plant litter confirmed the presence of hexosamine‐N.The hexosamine‐N derived from estimates of soil biomass accounted for less than 10% of the chemically determined hexosamine‐N values. It is suggested that most of the hexosamines are held in complexes formed between chitin complexed with other materials, e.g. pigments, that occur predominantly in the cell walls of microbes and insects. These complexes show resistance to enzymes during the life of soil organisms that persists after their death.
Ion transport through unsaturated soils: field experiments and regional simulations
Authors: Kumke T. , Streck T. , Richter J. 
Published: March 2003

SummaryThis paper describes the movement of anions and cations through soils at the regional scale using block‐scale and regional simulations of one‐dimensional ion transport through cultivated soils. The simulations were based on field experiments in a region of about 10 km2in Lower Saxony, Germany. Transport was modelled with the convection–dispersion equation, and the cation exchange was described using the Gapon equation. We evaluated the spatial variation of cation exchange parameters, obtained estimates valid at the block scale, and simulated the one‐dimensional transport of anions and cations. The movement of anions and cations was simulated over blocks using effective transport parameters calculated from local transport parameters. The approach led to a good agreement between measured and predicted concentrations of Br–, Na+, K+, Ca2+and Mg2+on four different 1 ha blocks. However, the mean concentrations of K+in the soil solution in the uppermost horizons could not be described satisfactorily by the model. For the regional simulations, transport and exchange parameters were estimated by block kriging. All variograms of the exchange parameters were spatially structured with correlation lengths varying from 100 m to 300 m. Results of the regional simulations imply that Cl–and K+were transported substantially deeper in the southern part than in the northern part of the area. The transport depth of the ions strongly depended on the pore water velocities. The simulation of solute transport to the water table showed the influence of the depth of water table on the estimated travel times, superimposing the influence of the transport parameters in the region. The results of the regional simulations also emphasize the importance of careful fertilization, especially in regions with shallow water tables such as in the north of the area.
Effect of reduced air pressure on soil thermal conductivity over a wide range of water content and temperature
Authors: Momose T. , Kasubuchi T. 
Published: March 2003

SummaryTo clarify the role of air molecules in coupled heat and mass transfer in soil, we measured the thermal conductivity of three kinds of soil (Ando soil, Red Yellow soil, and Toyoura sand) under reduced air pressure over a wide range of water content and temperature (10–75°C). The thermal conductivity increased sharply under reduced air pressure above a critical water content of the soil, becoming several times larger than that under normal pressure (101 kPa). The maximum thermal conductivity for each soil was obtained below 75°C and was similar to the thermal conductivity of some metals such as Mn, Hg and stainless steel. When the soil was drier than its critical water content, the thermal conductivity did not increase under reduced air pressure. The hydraulic diffusivity at the critical water content for each soil was of the order of 10−8 m2 s−1. This suggests that the latent heat transfer is enhanced by the circulation of the condensed water. However, very little is known about the effect of circulating water on the latent heat transfer under reduced air pressure. To make this clear, the thermal conductivity would need to be measured in the steady state under reduced air pressure.
Laboratory experiments on drought and runoff in blanket peat
Authors: Holden J. , Burt T. P. 
Published: March 2003

SummaryGlobal warming might change the hydrology of upland blanket peats in Britain. We have therefore studied in laboratory experiments the impact of drought on peat from the North Pennines of the UK. Runoff was dominated by surface and near‐surface flow; flow decreased rapidly with depth and differed from one type of cover to another. Infiltration depended on the intensity of rain, and runoff responded rapidly to rain, with around 50% of rainwater emerging as overland flow. Drought changed the structure of the peat and the subsequent behaviour of the peat in response to rain. Surface runoff was reduced, infiltration increased and flow increased within the deeper peat layers. Old and new water produced from the peat during simulated storms was identified by bromide tracing; the amount of old mobile water flushed out of the top few centimetres was small and there was less from deeper peat layers. No significant difference in the old and new water mixing processes could be identified between the control plots and the drought treatment plots. Lissamine staining showed preferential bypass flow through macropores in the peat, though only in the top 5 cm. Following drought, however, macroporosity increased within the upper peat layers, and preferential flow extended deeper than in controls. Peat structure recovered somewhat after drought, but the effects of the drought were long‐lasting. If these effects extend to the field during drier summers then we can expect changes to the hydrology and associated chemistry of blanket peat catchments in the British uplands.
Modelling cation exchange in columns of disturbed and undisturbed subsoil
Authors: Ludwig B. , Kölbl A. 
Published: March 2003

SummaryCation exchange is often studied with disturbed and dried soils, but the applicability of the results to undisturbed soils is not straightforward. We investigated the value of exchange coefficients obtained from standard procedures for predicting cation exchange in soil. Columns of undisturbed and disturbed subsoil of a Luvisol (SBt horizon) were leached under saturated conditions with 0.4, 4, 20, 41, 102 and 205 mm BaCl2 at a Darcy velocity of 1400 mm day−1. The model PHREEQC was used to calculate one‐dimensional transport, inorganic complexation and multiple cation exchange. Two model variants were tested: m1 (exchangeable cations obtained by percolation with NH4Cl) and m2 (exchangeable cations obtained by shaking the soil with BaCl2). The exchange coefficients (Gaines–Thomas formalism) were calculated from the ion activities in solution and exchangeable cations obtained by NH4Cl percolation (m1) or shaking with BaCl2 (m2). Variant m1 predicted cation exchange of the disturbed (homogenized) soil for the entire BaCl2 concentration range, whereas variant m2 resulted in a two‐fold overestimation of desorbed K for all experiments, which was related to large amounts of K released from the soil by shaking with BaCl2. In experiments with undisturbed soil, variant m1 predicted the concentrations of Mg, Ca, K, and Na in the solution phase and the sum of cations released from exchange sites. However, variant m2 predicted changes in ion concentrations and exchangeable cations somewhat less well. This study suggests that the amounts of exchangeable cations and exchange coefficients obtained from experiments with homogenized soil by percolation are useful to predict cation concentrations in column experiments with undisturbed soils.
Effect of dissolved organic matter on the precipitation and mobility of the lead compound chloropyromorphite in solution
Authors: Lang F. , Kaupenjohann M. 
Published: March 2003

SummaryChloropyromorphite, CPM, Pb5(PO4)3Cl, is one of the most insoluble lead minerals. Inducing the formation of CPM by application of phosphate to soil has been suggested for immobilizing Pb at contaminated sites. We have examined the effect of organic matter on the completeness and the rate of CPM precipitation and on the particle size and the mobility of CPM crystals. We did experiments at pH 3–7 and with varying content of dissolved organic C, 0–72 mg C l−1, mixing Pb(NO3)2 (0.5 mmol l−1) and phosphate (2 mmol l−1) solutions. The organic matter was extracted from samples of a forest floor. The precipitates were identified by X‐ray diffraction, and their size and shape were analysed by scanning electron microscopy and by photon correlation spectroscopy. The presence of organic matter in the solutions did not affect the mass of CPM that precipitated within 30 minutes at pH 5, 6 and 7. At pH 3 and 4, however, organic matter strongly inhibited the precipitation. The particles were markedly smaller in solutions containing organic matter than without at all pHs and passed through water‐saturated columns filled with calcareous sand, whereas the precipitates from the carbon‐free solutions did not. We suggest that the organic matter blocked the surfaces of crystal seeds and impaired crystal growth. At high pH, organic matter may additionally decrease the crystal size of the individual crystals by increasing the number of crystal seeds. We conclude that organic matter in the solution might limit the potential of phosphate to immobilize Pb in soil because it favours the formation of mobile colloids.