bluuebunny/crossref_metadata_2025_split huggingface dataset:
Sulphur speciation and biogeochemical cycling in long‐term arable cropping of subtropical soils: evidence from wet‐chemical reduction and S K‐edge XANES spectroscopy
Authors: Solomon D. , Lehmann J. , Lobe I. , Martinez C. E. , Tveitnes S. , Du Preez C. C. , Amelung W. 
Published: September 2005

SummaryAgriculture has claimed a large share of terrestrial environments in the tropics and subtropics through cultivation of native grasslands or forests. The impact of this anthropogenic change on speciation, dynamics, and ecological significance of sulphur (S) compounds is still poorly understood. We combined degradative wet‐chemical reduction and S K‐edge X‐ray absorption near edge structure (XANES) spectroscopy techniques to evaluate the impact of long‐term agricultural management of native grassland soils in South African Highveld on the amount, form and dynamics of S species. Sulphur XANES in the humic substances extracted by 0.1 m NaOH/0.4 m NaF solution showed the presence of strongly reduced (polysulphides, disulphides, thiols, monosulphides and thiophenes), intermediate (sulphoxides and sulphonates) and strongly oxidized (ester sulphates) organic‐S. It showed that strongly oxidized‐S is the predominant form (39–54%) of the total organic‐S in the humic substances, and organic‐S in the intermediate oxidation state represented 30–37% (78–93% of which was attributed to sulphonates). The strongly reduced organic‐S comprised only 17–24% of the total organic‐S. We did not find a close correlation between the results of a degradative wet‐chemical procedure and XANES spectroscopy conducted in both the bulk soils (ester SO4‐S from XANES versus HI‐fractionation, r = 0.27; P < 0.05) and the humic substance extracts (ester SO4‐S from XANES versus HI‐fractionation, r= 0.39; P < 0.05). The ratio of reduced‐S to strongly oxidized‐S (R‐S/O‐S) in the humic substances decreased from 0.61 to 0.21, while the ratio of intermediate‐S to strongly oxidized‐S (I‐S/O‐S) declined from 0.93 to 0.61 after 90 years of arable cropping of the native grassland soils. Hence, there was a shift in oxidation state towards strongly oxidized‐S (+6) and, thereby, a change in the relative proportion of the organic‐S moieties associated with each oxidation state following cultivation of the native grassland soils. Therefore, we conclude that changes in land‐use practice brought about not only quantitative change but also altered the composition of organic‐S functional groups in these native subtropical grassland soils.
Modelling cation transport and pH buffering during unsaturated flow through intact subsoils
Authors: Ludwig B. , Beese F. , Michel K. 
Published: September 2005

SummaryPrediction of cation transport during unsaturated flow through soils is complex, and chemical models are necessary for the purpose. Our objectives were to determine whether the reactions considered in the model PHREEQC2 were appropriate to predict cation transport in soils when small amounts of CaCl2 or acid are added and to investigate the usefulness of different parameterization approaches. Undisturbed columns of three subsoils were irrigated for 2 years with 4 mm day−1 at 8°C. The experiments were (A) irrigation with 1 mm and 5 mm CaCl2 and (B) irrigation with 1 mm HCl. The model PHREEQC2 was used to calculate one‐dimensional transport, inorganic complexation, dissolution or precipitation of Al(OH)3, AlOHSO4 and CaCO3 and multiple cation exchange. We compared three models: one with no adjustable parameters, a second with optimized solubilities within a reported range, pCO2 and cation exchange capacity (CEC), and a third which also included an optimization of the exchange coefficients. The first was of little use to predict cation transport in soils. The second was appropriate for one of the soils, but an optimization of the exchange coefficients was required for an accurate description and prediction of cation transport for the others. The CEC accessible for seepage water varied between 50% and 100% of the total CEC. Our results indicate that for a reliable assessment of cation transport in soils during unsaturated flow, calibration experiments are required. The parameter optimization program PEST in combination with PHREEQC2 gave a better understanding of cation transport in undisturbed soil.
Composition of organic matter in a subtropical Acrisol as influenced by land use, cropping and N fertilization, assessed by CPMAS 13C NMR spectroscopy
Authors: Dieckow J. , Mielniczuk J. , Knicker H. , Bayer C. , Dick D. P. , Kögel‐Knabner I. 
Published: September 2005

SummaryWe know much about the influence of management on stocks of organic matter in subtropical soils, yet little about the influence on the chemical composition. We therefore studied by CPMAS 13C NMR spectroscopy the composition of the above‐ground plant tissue, of the organic matter of the whole soil and of silt‐ and clay‐size fractions of the topsoil and subsoil of a subtropical Acrisol under grass and arable crops. Soil samples were collected from three no‐till cropping systems (bare soil; oats−maize; pigeon pea + maize), each receiving 0 and 180 kg N ha−1 year−1, in a long‐term field experiment. Soil under the original native grass was also sampled. The kind of arable crops and grass affected the composition of the particulate organic matter. There were no differences in the composition of the organic matter in silt‐ and clay‐size fractions, or of the whole soil, among the arable systems. Changes were observed between land use: the soil of the grassland had larger alkyl and smaller aromatic C contents than did the arable soil. The small size fractions contain microbial products, and we think that the compositional difference in silt‐ and clay‐size fractions between grassland and the arable land was induced by changes in the soil's microbial community and therefore in the quality of its biochemical products. The application of N did not affect the composition of the above‐ground plant tissue nor of the particulate organic matter and silt‐size fractions, but it did increase the alkyl C content in the clay‐size fraction. In the subsoil, the silt‐size fraction of all treatments contained large contents of aromatic C. Microscopic investigation confirmed that this derived from particles of charred material. The composition of organic matter in this soil is affected by land use, but not by variations in the arable crops grown.
Poorly crystalline mineral phases protect organic matter in acid subsoil horizons
Authors: Kleber M. , Mikutta R. , Torn M. S. , Jahn R. 
Published: September 2005

SummarySoil minerals are known to influence the biological stability of soil organic matter (SOM). Our study aimed to relate properties of the mineral matrix to its ability to protect organic C against decomposition in acid soils. We used the amount of hydroxyl ions released after exposure to NaF solution to establish a reactivity gradient spanning 12 subsoil horizons collected from 10 different locations. The subsoil horizons represent six soil orders and diverse geological parent materials. Phyllosilicates were characterized by X‐ray diffraction and pedogenic oxides by selective dissolution procedures. The organic carbon (C) remaining after chemical removal of an oxidizable fraction of SOM with NaOCl solution was taken to represent a stable organic carbon pool. Stable organic carbon was confirmed as older than bulk organic carbon by a smaller radiocarbon (14C) content after oxidation in all 12 soils. The amount of stable organic C did not depend on clay content or the content of dithionite–citrate‐extractable Fe. The combination of oxalate‐extractable Fe and Al explained the greatest amount of variation in stable organic C (R2 = 0.78). Our results suggest that in acid soils, organic matter is preferentially protected by interaction with poorly crystalline minerals represented by the oxalate‐soluble Fe and Al fraction. This evidence suggests that ligand exchange between mineral surface hydroxyl groups and negatively charged organic functional groups is a quantitatively important mechanism in the stabilization of SOM in acid soils. The results imply a finite stabilization capacity of soil minerals for organic matter, limited by the area density of reactive surface sites.
Trace magnetic minerals to detect redox boundaries and drainage effects in a marshland soil in western France
Authors: Mathé V. , Lévêque F. 
Published: September 2005

SummaryThe marshlands of western France provide the opportunity to relate the magnetic properties of a recent sedimentary clay soil to pedogenesis, including the impact of agriculture and drainage on magnetic mineralogy. We studied a plot of drained land that had been ploughed up to 1998 and under grass since. A new thermomagnetic method was used to identify and to quantify roughly the magnetic minerals, which represent less than 1 g kg−1. The most abundant Fe oxides are haematite (45%) and goethite (45%). However, trace amounts of the ferrimagnetic minerals maghemite (4%), magnetite (3%) and, to a lesser extent, iron sulphides (1%) provide most of the magnetic signal. This signal allowed us to identify magnetic horizons that relate to the soil horizons. The topsoil is characterized by a strong magnetic enhancement (4‐fold), and the thickness of this layer increases close to the drains. Relative contents of ferrimagnetic phase also increase laterally with decreasing distance to the drains. Magnetic enhancement coincides with the most aerated and developed layer, in which water circulation, root colonization and hence soil fertility are greater. Ferromagnetic minerals sensu lato, especially maghemite and magnetite, can be considered as mineralogical tracers partly and indirectly reflecting soil fertility of these clay‐rich marshland soils.
Effect of soil depth on acid properties of humic substances extracted from an ombrotrophic peat bog in northwest Spain
Authors: Gondar D. , López R. , Fiol S. , Antelo J. M. , Arce F. 
Published: September 2005

SummaryThe most southerly ombrotrophic peat bogs in Europe are in Galicia (northwest Spain). The humic matter in them originates from chemical processes in anaerobic conditions. We studied the acid properties of fulvic acids and humic acids isolated from two peat horizons of an ombrotrophic peat bog by potentiometric titration. Solutions containing 25, 50 and 100 mg l−1 of each humic substance were titrated at ionic strengths 0.005 m, 0.01 m and 0.1 m (with KNO3 as the inert electrolyte). Charge curves were analysed with a Donnan model to determine the intrinsic proton binding parameters. The concentration of the humic substance affected the charge curves more significantly at pH exceeding 6, and tended to disappear at greater concentrations. The proton binding conditional constants decreased with increasing ionic strength, this effect being more significant in the carboxylic groups with less affinity for protons. The proton binding constant of the carboxyl groups in a fulvic acid was one order of magnitude less than the value for the corresponding humic acid, whereas for the phenolic groups the values for both fractions were similar. The total content of acid groups was approximately 2 mol kg−1 greater in the fulvic fraction than in the humic fraction. Both humic fractions from the lower horizon contained more acid groups than those from the upper horizon, mainly because the content of carboxyl groups increases with soil depth. Therefore, the humic substances in the lower horizon of the peat will be more negatively charged, which will affect their solubility and the binding of metal ions.
A simple equation for simulating C decomposition in a multi‐component pool of soil organic matter
Authors: Fang C. , Smith P. , Smith J. U. 
Published: September 2005

SummaryPartitioning carbon (C) pools presents theoretical and practical challenges in soil modelling. In most current models, organic matter (OM) is commonly divided into a number of pools using data‐fitting techniques. It is often not possible to measure the properties of each pool of soil C simulated in this way. Furthermore, there is no effective way in current soil models to properly simulate inputs of OM with variable qualities in terms of decomposability. Here, we describe a simple model, taking the OM as a whole and assuming a rate constant function changing exponentially with time, to simulate C decomposition in a multi‐component OM pool. The model requires fewer parameters to be estimated than current multi‐component models, and the simulated properties of OM are measurable. When changing the quantity and the quality of OM input, the model produces results similar to multi‐component models, but avoids the difficulty and uncertainty of OM partitioning.
Determination of labile Cu in soils and isotopic exchangeability of colloidal Cu complexes
Authors: Ma Y. B. , Lombi E. , Nolan A. L. , McLaughlin M. J. 
Published: September 2005

SummaryMeasurement of labile (isotopically exchangeable) pools of metals (E values) in soil is required to assess the size of metal pools potentially available to soil organisms, from both a micronutrient deficiency and metal toxicity viewpoint. In this paper, E values of soil Cu were measured by an isotope dilution technique using different solution extracts – water with and without resin purification, water coupled with Donnan dialysis (free ion determination) and 0.01 m CaCl2. Using these techniques, the isotopic exchangeability of Cu species in water extracts was investigated. The results showed that the specific activity of 64Cu in the water‐soluble fraction was less than in the free metal ion fraction or in the fraction that adsorbed to resin. The isotopically non‐exchangeable Cu in water extracts ranged between 4% and 40% of water‐soluble Cu (16% on average), and appeared to be associated with dispersed colloids. The existence of isotopically non‐exchangeable Cu in water extracts led to overestimation (17.7% on average) of isotopically exchangeable Cu in soils when based on the specific activity in water extracts. The method of isotope dilution coupled with resin extraction is recommended for the determination of isotopically exchangeable Cu in soils when water extracts are used.
Temporal variation in the surface‐water chemistry of a blanket bog on Dartmoor, southwest England: analysis of 5 years' data
Authors: Proctor M. C. F. 
Published: September 2005

SummaryBlanket‐bog peats, mapped as the Winter Hill and Crowdy associations by the Soil Survey of England and Wales, are an oceanic manifestation of the ombrotrophic ‘raised‐bog’ (Hochmoor) peats that cover large tracts in the boreal zone of the northern hemisphere. This paper examines monthly analyses from 1992 to 1997 of major ions and other variables from an upland blanket bog in southwest England in relation to seasonality, rainfall, and the chemical composition of rainwater. Average ionic composition of surface water (and peat) integrates variable atmospheric solute inputs over the years. The dominant ions in the surface water, Na+ and Cl–, showed only weak seasonality, but divalent cations a stronger seasonal pattern with a summer maximum. Mean pH ranged from c. 4.4 in February to c. 4.2 in August. Changes in concentration of different cations were closely interlinked by cation exchange. The anion deficit, accounted for by anionic groups on the dissolved organic matter, was strongly seasonal with a summer maximum, as was optical absorbance at 320 nm. Nitrate and NH4+ were both at much smaller concentrations than in rain. Nitrate exceeded 1 μmol l−1 only during cold periods in winter, mainly following drought in the summer of 1995; NH4+ reached a few μmol l−1 only in summer. There was evidence of net retention of S by the peat in wet sites and during wet periods, and of net release of SO42– (and acidity) under dry conditions. The 1995 summer drought and ensuing dry year in 1996 had marked and persistent effects on pH, apparent ion deficit (DEF), SO42–, the divalent cations and Fe.
Microscale Zn and Pb distribution patterns in subsurface soil horizons: an indication for metal transport dynamics
Authors: Van Oort F. , Jongmans A. G. , Citeau L. , Lamy I. , Chevallier P. 
Published: September 2005

SummaryIn many studies on soil pollution, authors conclude that there is no downward migration of metal elements if no evidence for enrichment can be inferred from profiles of total metal contents. We assessed possible transfer of mobile and less mobile metal pollutants to depth in subsurface horizons of a heavy metal contaminated soil, by a study of specific pedofeatures in thin sections by optical microscopy, and their corresponding Zn and Pb distribution patterns by synchrotron‐based X‐ray microfluorescence. In the B horizon (70 cm depth), Zn accumulation was predominantly associated with clay–iron coatings. Strong correlation was found between Zn and Fe (r = 0.94), Zn and Mn (r = 0.75), Zn and Ti (r = 0.84), and Zn and K (r = 0.88), but significant correlation was absent between Zn and Pb. In the C horizon (100 cm depth), clear Pb accumulation was observed in distinct iron coatings, with large correlation coefficients found between Pb and Fe (r = 0.94–0.75), whereas correlation between Zn and Fe was absent. Detected Zn concentrations were small and attributed to the local natural geochemical background. These results were then compared with data of the composition of gravitational soil water collected in other soils from the same study area. Thus, Zn accumulation in the B horizon was ascribed to interception of dissolved Zn2+by negatively charged constituents of clay–iron coatings. In contrast, Pb accumulation in C horizons was related to precipitation of Pb‐bearing iron colloids leading to neoformed, optically pure iron oxyhydroxide crystals and coatings. We demonstrate very localized accumulation of almost immobile Pb which occurs at greater depth than the more mobile Zn. The common, but strongly localized, occurrence of Pb‐bearing iron coatings in the soil groundmass explained the absence of changes in the total Pb concentrations of the C horizon compared with the concentrations in the B horizon.
Phosphorus solubility and sorption in frozen, air‐dried and field‐moist soil
Authors: Peltovuori T. , Soinne H. 
Published: September 2005

SummaryThe effects of freezing on soil phosphorus (P) chemistry are poorly understood, although freezing is habitual for many soils at middle and high latitudes. We studied the effects of various freezing treatments on the solubility and sorption of P in an incubation experiment on two coarse and two fine‐textured cultivated surface soils in Finland. Air‐drying was included in the experimental arrangement because freezing and drying have similar features. Compared with field‐moist soils stored at +5°C in the dark, freezing had few effects on P extractability by water or on sorption properties of P studied with a Q/I plot technique. Air‐drying, by contrast, increased almost systematically the equilibrium concentration of P estimated with Q/I plots, water‐soluble organic carbon, and the extractability of P, aluminium, iron and manganese in the soils. The results imply that drying destroys organomineral complexes. The breakdown of these complexes releases P, while simultaneously exposing new surfaces on which P could sorb. Because of the considerable impact of drying on the behaviour of P, air‐drying of soil samples should be avoided in studies of the chemistry of P in soil. Freezing seems to be a safe way of storing mineral soil for such studies, but it may significantly alter the P conditions of soils rich in organic matter.
Production of gaseous mercury in tropical hydromorphic soils in the presence of ferrous iron: a laboratory study
Authors: Peretyazhko T. , Charlet L. , Grimaldi M. 
Published: September 2005

SummaryIn this paper, we demonstrate that reduction of Hg(II) to Hg0 under anaerobic conditions, followed by volatilization of Hg0 to the atmosphere, might be responsible for the removal of Hg from tropical hydromorphic soils. We conducted a series of kinetic batch experiments in which we added Hg(II) to anoxic suspensions of a soil clay fraction and haematite nanoparticles. The clay fraction came from three depths in a hydromorphic soil by the Leblond River in French Guiana, South America: close to the upper and lower boundaries (30–40 cm and 90–100 cm) and within the central part (60–70 cm) of the frequently water‐saturated clay horizon. We prepared a second set of soil clay fraction suspensions with Fe(III) citrate, whose reduction acted as a source of dissolved Fe(II) to investigate the influence of Fe(II) on the production of Hg0. Reduction of Hg(II) to Hg0 occurred with all samples amended with Fe(III) citrate. Laboratory experiments with haematite suspensions demonstrated that adsorption of Fe(II) to the haematite surface created very reactive sites for the reduction of Hg(II), while in the absence of haematite particles, no production of Hg0 occurred. The greatest production of Hg0 was found for the depth intervals 30–40 cm and 90–100 cm, where the total mercury concentration exhibits a local minimum. The observed pseudo‐first order rate constants for the 90–100 cm depth sample were close to rate constants reported for abiotic reduction of Hg(II) by Fe(II) adsorbed on mineral surfaces. Significant production of Hg0 was found for the 90–100 cm depth interval sample, both with and without Fe(III) citrate. A biotic pathway as well as abiotic reduction by Fe(II) might be involved in the reduction of Hg(II) to Hg0.
Effect of water composition on phosphorus concentration in runoff and water‐soluble phosphate in two grassland soils
Authors: Schärer M. , Vollmer T. , Frossard E. , Stamm C. , Flühler H. , Sinaj S. 
Published: September 2005

SummaryMany irrigation experiments determine phosphorus (P) losses from soil. Often, these studies cannot be compared, because the irrigation water was not characterized. We used calcium‐rich tap water and deionized water to investigate the influence of water composition on P concentrations in induced runoff. We irrigated two grassland sites: one acid and one calcareous. Less P was measured in runoff from tap water irrigation than from deionized water, especially for the acid soil. Batch experiments confirmed the findings of the field experiments. Tap water decreased water‐soluble phosphate and increased calcium in the solid phase. This interaction increased with decreasing soil:water ratio. Water of low ionic strength gave results comparable to rainwater.Our findings demonstrate that solution chemistry and the soil:water ratio can strongly influence the availability of P for transport. We recommend that P tests or irrigation experiments should use water resembling that of the system of interest. Irrigation experiments aiming to simulate P losses by surface runoff should be carried out with water having a composition comparable to rainwater.