Home About us Contact
|
Home About us Contact | ||
|
|
||
Particle-size Fractions (particle-size + fraction)
Selected AbstractsThe turnover of carbohydrate carbon in a cultivated soil estimated by 13C natural abundancesEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 4 2006D. Derrien Summary Understanding the chemical composition of soil organic matter (SOM) requires the determination of the dynamics of each class of compounds. We measured the dynamics of carbon in neutral carbohydrates by use of natural 13C labelling in an experimental wheat and maize sequence extending over 23 years. The isotopic composition of individual neutral monosaccharides was determined in hydrolysed particle-size fractions by gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) of trimethylsilyl (TMS) derivatives. The sensitivity in terms of 13C/12C ratios ranged between 1 and 2, depending on the monosaccharide. The age distribution of neutral sugar carbon was very similar to that of total soil carbon. Particulate organic matter (POM) was characterized by the predominance of glucose and xylose of vegetal origin. In POM >,200 µm, the mean age of sugar-C (5 years) was slightly less than that of total carbon (7 years). Xylose was younger than glucose. The fine fraction 0,50 µm contained mainly glucose, arabinose, galactose, xylose, fucose and mannose, which had predominantly microbial origins. The mean age of carbohydrate carbon in the fraction 0,50 µm was between 60 and 100 years and was similar to that of total organic carbon (OC). No difference in the age of carbon between the individual monosaccharides was found. The POM fraction 50,200 µm had an intermediate signature and turnover. Considering the typical lability of carbohydrates, the relatively great age of carbohydrate carbon may be explained by physical or chemical protection from degradation, as well as by recycling of soil organic matter carbon by soil microbes. [source] Carbon and nitrogen isotope composition of bulk soils, particle-size fractions and organic material after treatment with hydrofluoric acidEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 3 2005M. W. I. Schmidt Summary Soils and sediments contain only small amounts of organic matter, and large concentrations of paramagnetic metals can give poor solid-state nuclear magnetic resonance (NMR) spectra of organic matter. Pretreatment of samples with hydrofluoric acid (HF) dissolves significant proportions of the mineral matrix and extracts paramagnetic elements. We investigated the effects of 10% HF treatment on the stable isotope content of carbon (C) and nitrogen (N) of organic matter from soils, composts and shales. Additionally we inferred molecular and isotopic characteristics of lost materials from calculations of isotope mass balances. Treatment with HF enriched C and N in mineral samples substantially (factors 2.5,42.4), except for Podzol B horizons (1.1,1.7) and organic material (1.0,1.3). After treatment most of the C (59.7,91.7%) and N (53.7,86.6%) was recovered, although changing C/N ratios often indicated a preferential loss of N-rich material. Isotope ratios of C and N in the remaining material became more negative when net alterations exceeded 0.3,. The isotope ratios of the lost material contained more 13C (1,2,) and 15N (1,4,) than the initial organic matter. Acid hydrolysis typically removes proteins, amino acids and polysaccharides, all of which are enriched in 13C, and in the case of proteins and amino acids, enriched in 15N as well. We conclude that HF treatment released fresh, soluble, probably microbial, biomass in addition to carbohydrates. Net changes of the bulk chemical composition of organic matter were small for most soils, size fractions and plant material, but not for samples containing little organic matter, or those rich in easily soluble organic matter associated with iron oxides, such as Podzol B horizons. [source] The concentrations of fatty acids in organo-mineral particle-size fractions of a ChernozemEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 3 2004G. Jandl Summary Fatty acids, the most abundant class of soil lipids, indicate pedogenetic processes and soil management. However, their quantitative distribution in organo-mineral particle-size fractions is unknown. The concentrations of n -C10:0 to n -C34:0 fatty acids both in whole soil samples and in the organo-mineral particle-size fractions of the Ap horizon of a Chernozem were determined (i) to evaluate the effects of long-term fertilization and (ii) to investigate their influence on the aggregation of organo-mineral primary particles. Quantification by gas chromatography/mass spectrometry (GC/MS) showed that long-term fertilization with nitrogen, phosphorus and potassium (NPK) and farmyard manure (FYM) led to larger concentrations (25.8 µg g,1) of fatty acids than in the unfertilized sample (22.0 µg g,1). For particle-size fractions of the unfertilized soil, the fatty acid concentrations increased from the coarse silt to the clay fractions (except for fine silt). Fertilization with NPK and FYM resulted in absolute enrichments of n -C21:0 to n -C34:0 fatty acids with a maximum at n -C28:0 in clay (×2.2), medium silt (×2.0), coarse silt (×1.8) and sand (×2.9) compared with the unfertilized treatment (the factors of enrichment are given in parentheses). New evidence for the aggregate stabilizing function of n -C21:0 to n -C34:0 fatty acids was shown by the characteristic pattern in size-fractionated, disaggregated and aggregated samples. Highly significant correlations of fatty acid concentrations with organic C concentrations and specific surface areas are interpreted as indicators of (i) trapping of fatty acids in organic matter macromolecules and (ii) direct bonding to mineral surfaces. This interpretation was supported by the thermal volatilization and determination of fatty acids by pyrolysis-field ionization mass spectrometry (Py-FIMS). [source] Quantifying the effects of aggregation, particle size and components on the colour of Mediterranean soilsEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 3 2004M. SÁnchez-Marañón Summary Aggregation, particle size, and chemical composition affect the colour of the soil. We have attempted to quantify and understand these effects in 12 Mediterranean soils. We measured the CIELAB colour variables hab, L*, and C*ab in aggregated and dispersed soil samples, and also in coarse sand, fine sand, silt, and clay samples before and after sequential removal of organic matter, carbonates, and Fe oxides. Grassmann's colour-mixing equations adjusted by regression analysis described the colour of the dispersed soil from its particle-size fractions with an error of 1% for hab, 4% for L*, and 9% for C*ab. This suggests that the contribution of each fraction to the colour of the dispersed soil can be accurately calculated by its colorimetric data weighted by its content and a regression coefficient, which was greatest for clay. We inferred the influence of a component within each fraction by measuring the colour changes after its removal. Iron oxides reduced hab of the silicated substrate by 19%, reduced L* by 12%, and increased C*ab by 64% in all particle-size fractions. Carbonates and organic matter had little influence: the former because they impart little colour to the silicates and the latter because there was little of it. The CIELAB colour-difference between dispersed and aggregated soil (mean ,E*ab = 15.3) was due mainly to ,L* (,14.7). Aggregation contributed to diminishing L* of dispersed soil by 34%. Scanning electron microscopy showed that Fe oxides and organic coatings cover the surface of aggregates thereby influencing soil colour. [source] Fate of microbial residues in sandy soils of the South African Highveld as influenced by prolonged arable croppingEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 1 2002W. Amelung Summary Long-term cultivation of former grassland soils results in a significant decline of both living and dead microbial biomass. We evaluated the effect of duration of cropping on the preservation of fungal and bacterial residues in the coarse-textured soils of the South African Highveld. Composite samples were taken from the top 20 cm of soils (Plinthustalfs) that have been cropped for periods varying from 0 to 98 years in each of three different agro-ecosystems in the Free State Province. Amino sugars were determined as markers for the microbial residues in bulk soil and its particle-size fractions. Long-term cultivation reduced N in the soil by 55% and the contents of amino sugars by 60%. Loss rates of amino sugars followed bi-exponential functions, suggesting that they comprised both labile and stable fractions. With increased duration of cropping the amino sugars attached to silt dissipated faster than those associated with the clay. This dissipation was in part because silt was preferentially lost through erosion, while clay particles (and their associated microbial residues) remained. Erosion was not solely responsible for the reduction in amino sugar concentrations, however. Bacterial amino sugars were lost in preference to fungal ones as a result of cultivation, and this effect was evident in both silt- and clay-sized separates. This shift from fungal to bacterial residues was most pronounced within the first 20 years after converting the native grassland to arable cropland, but continued after 98 years of cultivation. [source] Compound-specific stable-isotope (,13C) analysis in soil scienceJOURNAL OF PLANT NUTRITION AND SOIL SCIENCE, Issue 5 2005Bruno Glaser Abstract This review provides current state of the art of compound-specific stable-isotope-ratio mass spectrometry (,13C) and gives an overview on innovative applications in soil science. After a short introduction on the background of stable C isotopes and their ecological significance, different techniques for compound-specific stable-isotope analysis are compared. Analogous to the ,13C analysis in bulk samples, by means of elemental analyzer,isotope-ratio mass spectrometry, physical fractions such as particle-size fractions, soil microbial biomass, and water-soluble organic C can be analyzed. The main focus of this review is, however, to discuss the isotope composition of chemical fractions (so-called molecular markers) indicating plant- (pentoses, long-chain n-alkanes, lignin phenols) and microbial-derived residues (phospholipid fatty acids, hexoses, amino sugars, and short-chain n-alkanes) as well as other interesting soil constituents such as "black carbon" and polycyclic aromatic hydrocarbons. For this purpose, innovative techniques such as pyrolysis,gas chromatography,combustion,isotope-ratio mass spectrometry, gas chromatography,combustion,isotope-ratio mass spectrometry, or liquid chromatography,combustion,isotope-ratio mass spectrometry were compared. These techniques can be used in general for two purposes, (1) to quantify sequestration and turnover of specific organic compounds in the environment and (2) to trace the origin of organic substances. Turnover times of physical (sand < silt < clay) and chemical fractions (lignin < phospholipid fatty acids < amino sugars , sugars) are generally shorter compared to bulk soil and increase in the order given in brackets. Tracing the origin of organic compounds such as polycyclic aromatic hydrocarbons is difficult when more than two sources are involved and isotope difference of different sources is small. Therefore, this application is preferentially used when natural (e.g., C3-to-C4 plant conversion) or artificial (positive or negative) 13C labeling is used. Substanzspezifische Stabilisotopenanalyse (,13C) in der Bodenforschung Dieser Artikel fasst den Stand der Forschung bezüglich der substanzspezifischen Stabilisotopenanalyse (,13C) zusammen. Innovative Anwendungen und ein Ausblick für künftige Forschungsaktivitäten werden anhand von Fallbeispielen gegeben. Zunächst wird die ökologische Bedeutung von stabilen C-Isotopen kurz erläutert. Daran schließt sich ein methodischer Teil an, in welchem die verschiedenen Techniken gegenüber gestellt werden. Analog zu ,13C-Messungen der Feinerde mittels Elementaranalysator-Isotopenverhältnis-Massenspektrometrie können physikalisch isolierte Fraktionen (z.,B. Korngrößenfraktionen, mikrobielle Biomasse, DOC) analysiert werden. Der Schwerpunkt dieses Übersichtsartikels liegt jedoch in der Diskussion der C-Isotopensignatur chemischer Fraktionen (sog. Biomarker), welche Rückschlüsse auf Herkunft und Dynamik pflanzlicher (Pentosen, langkettige n-Alkane, Ligninphenole) und mikrobieller Rückstände (Phospholipidfettsäuren, Hexosen, Aminozucker und kurzkettige n-Alkane) sowie anderer interessanter Substanzen im Boden erlaubt wie z.,B. ,Black Carbon" und polyzyklische aromatische Kohlenwasserstoffe. Zu diesem Zweck kommen innovative Techniken zum Einsatz wie z.,B. Pyrolyse-Gaschromatographie-Isotopenverhältnismassenspektrometrie, Gaschromatographie-Verbrennungs-Isotopenverhältnismassenspektrometrie und Flüssigkeitschromatographie-Oxidations-Isotopenverhältnismassenspektrometrie. Innovative ökologische Anwendungen werden erläutert, welche sich prinzipiell in zwei Kategorien einteilen lassen: (1) Quantifizierung der Sequestrierung und des Umsatzes dieser Verbindungen in der Umwelt; (2) Untersuchung der Herkunft spezifischer organischer Substanzen. Umsatzzeiten physikalischer (Sand < Schluff < Ton) und chemischer Fraktionen (Lignin < Phospholipidfettsäuren < Aminozucker , Zucker) sind generall kleiner als jene der gesamten organischen Substanz in der Feinerde und nehmen in der in Klammern angegebenen Reihenfolge zu. Die Untersuchung der Herkunft organischer Substanzen (z.,B. polyzyklischer aromatischer Kohlenwasserstoffe) ist problematisch, weil die Unterschiede der Isotopensignatur verschiedener Quellen gering sind und meist mehr als zwei Quellen zur Isotopensignatur des untersuchten Biomarkers beitragen. Deswegen sollte die Untersuchung der Herkunft organischer Substanzen auf Tracer-Experimente beschränkt werden, wie z.,B. nach natürlicher (C3-C4-Pflanzenwechsel) bzw. künstlicher (13C-An- oder -Abreicherung) Markierung. [source] A method for improving predictions of bed-load discharges to reservoirsLAKES & RESERVOIRS: RESEARCH AND MANAGEMENT, Issue 2 2007Vicente L. Lopes Abstract Effective management options for mitigating the loss of reservoir water storage capacity to sedimentation depend on improved predictions of bed-load discharges into the reservoirs. Most predictions of bed-load discharges, however, are based on the assumption that the rates of bed-load sediment availability equal the transport capacity of the flow, ignoring the spatio-temporal variability of the sediment supply. This paper develops a semiquantitative method to characterize bed-load sediment transport in alluvial channels, assuming a channel reach is non-supply limited when the bed-load discharge of a given sediment particle-size class is functionally related to the energy that is available to transport that fraction of the total bed-load. The method was applied to 22 alluvial stream channels in the USA to determine whether a channel reach had a supply-limited or non-supply-limited bed-load transport regime. The non-supply-limited transport regime was further subdivided into two groups on the basis of statistical tests. The results indicated the pattern of bed-load sediment transport in alluvial channels depends on the complete spectrum of sediment particle sizes available for transport rather than individual particle-size fractions represented by one characteristic particle size. The application of the method developed in this paper should assist reservoir managers in selecting bed-load sediment transport equations to improve predictions of bed-load discharge in alluvial streams, thereby significantly increasing the efficiency of management options for maintaining the storage capacity of waterbodies. [source] | ||||||||||