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  • 1
    Language: English
    In: Geoderma, February 2015, Vol.239-240, pp.168-178
    Description: Radiocarbon measurements are frequently used to model the turnover of soil organic carbon (OC) fractions. The assumption of homogeneous turnover in these fractions is typically stated, but consequences of its violation have not been tested. We used samples of a repeated soil inventory of a German beech forest from the litter layer to 50 cm depth in 2004 and 2009 to determine the suitability of short-term repeated radiocarbon inventories for estimating the turnover times of OC in soil fractions. Samples from 0–5 and 10–20 cm soil depth were density separated into a free light fraction (fLF), an occluded light fraction (oLF), and a heavy, mineral-associated fraction (HF). Samples were analysed for radiocarbon (∆ C), OC, and total nitrogen (TN) contents. Similar portions of OC were stored in the HF, but contributions of fLF and oLF varied between study years, probably due to interannual variations or methodological constraints. Following declining atmospheric CO – C, also ∆ C values at 0–5 cm depth declined significantly between 2004 and 2009. Exchange of old for new OC was largest in the fLF and smallest in the HF, which confirms slow turnover of OC associated with minerals. Model results revealed that turnover time estimates based on single-pool models were not in agreement with observed changes in any of the fractions, suggesting all of them to be mixtures, to varying degrees, of fast and slow cycling pools. While single-pool models suggest average turnover times of 115 years for HF-OC at 0–5 cm depth, thus being a stable fraction, fitting a two-pool model to the two-point measurements of radiocarbon suggested the presence of a fast cycling pool of 15–25 years of turnover time. It was however only possible to constrain the portion of this fast pool as being between 50 and 85% of total HF-OC. Increasing ∆ C in bulk soil and density fractions between study years at 10–20 cm depth suggest that OC enters deeper soil layers with a longer time lag than topsoil layers, e.g., by slow transport, and 5 years was not enough to induce significant changes. Even 40 years after the bomb peak, radiocarbon time series are still suitable to detect OC fractions of decadal turnover and hint at time lags and translocation processes. Nevertheless, they do not allow for fixing the portions of fast and slow cycling OC in two-pool models and their turnover times. Repeated radiocarbon inventory in a German beech forest.
    Keywords: Organic Carbon Turnover ; Modelling ; Density Fractionation ; Radiocarbon ; Agriculture
    ISSN: 0016-7061
    E-ISSN: 1872-6259
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  • 2
    Language: English
    In: Geoderma, 2015, Vol.239-240, p.168(11)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.geoderma.2014.09.025 Byline: Marion Schrumpf, Klaus Kaiser Abstract: Radiocarbon measurements are frequently used to model the turnover of soil organic carbon (OC) fractions. The assumption of homogeneous turnover in these fractions is typically stated, but consequences of its violation have not been tested. We used samples of a repeated soil inventory of a German beech forest from the litter layer to 50cm depth in 2004 and 2009 to determine the suitability of short-term repeated radiocarbon inventories for estimating the turnover times of OC in soil fractions. Samples from 0-5 and 10-20cm soil depth were density separated into a free light fraction (fLF), an occluded light fraction (oLF), and a heavy, mineral-associated fraction (HF). Samples were analysed for radiocarbon (a.sup.14C), OC, and total nitrogen (TN) contents. Similar portions of OC were stored in the HF, but contributions of fLF and oLF varied between study years, probably due to interannual variations or methodological constraints. Following declining atmospheric CO.sub.2-.sup.14C, also a.sup.14C values at 0-5cm depth declined significantly between 2004 and 2009. Exchange of old for new OC was largest in the fLF and smallest in the HF, which confirms slow turnover of OC associated with minerals. Model results revealed that turnover time estimates based on single-pool models were not in agreement with observed changes in any of the fractions, suggesting all of them to be mixtures, to varying degrees, of fast and slow cycling pools. While single-pool models suggest average turnover times of 115years for HF-OC at 0-5cm depth, thus being a stable fraction, fitting a two-pool model to the two-point measurements of radiocarbon suggested the presence of a fast cycling pool of 15-25years of turnover time. It was however only possible to constrain the portion of this fast pool as being between 50 and 85% of total HF-OC. Increasing a.sup.14C in bulk soil and density fractions between study years at 10-20cm depth suggest that OC enters deeper soil layers with a longer time lag than topsoil layers, e.g., by slow transport, and 5years was not enough to induce significant changes. Even 40years after the bomb peak, radiocarbon time series are still suitable to detect OC fractions of decadal turnover and hint at time lags and translocation processes. Nevertheless, they do not allow for fixing the portions of fast and slow cycling OC in two-pool models and their turnover times. Article History: Received 16 April 2014; Revised 26 September 2014; Accepted 29 September 2014
    Keywords: Soil Structure – Analysis ; Soil Carbon – Analysis
    ISSN: 0016-7061
    Source: Cengage Learning, Inc.
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  • 3
    Language: English
    In: Geoderma, 2011, Vol.162(1), pp.207-214
    Description: In a long-term tillage experiment comparing mouldboard plough and rotary harrow since 1967, litterbags with green maize residues and wheat straw were buried, recovered from soil and analysed for biochemical degradation indices. Our objective was to investigate whether lignin units and neutral and amino sugars give information on microbial degradation after burial periods of 6 and 12 months at two different depths (0–5 and 15–20 cm). Tillage and burial depth had no effects on the content and composition of lignin. In contrast, the extended burial period resulted in higher acid/aldehyde ratios of vanillyl units, due to increasing microbial oxidation, and in higher ratios of cinnamyl/vanillyl units, due to the higher resistance of vanillyl units against microbial degradation. The contents of mannose, bacterial muramic acid, and fungal glucosamine were significantly higher in the plough than in the harrow treatment, due to a higher microbial colonisation. For the same reason, the extended burial period led to significant increases in the contents of mannose, glucosamine, muramic acid, and galactosamine as well as in the GM/AX ((galactose + mannose)/(arabinose + xylose)) ratio. The decline in the fungal C/bacterial C ratio indicated that bacterial colonisation of litter followed fungal colonisation with delay. A greater burial depth led to a lower microbial colonisation and consequently had contrasting effects to those of a longer burial period. Treatment effects on maize residues and wheat straw were generally similar, despite the strong differences in composition. The combination of litterbags and biochemical degradation indices gave further evidence that lower tillage intensity reduces microbial turnover and decomposition activity. ► The harrow exceeded the plough treatment in microbial colonisation of the litter. ► Treatment effects on maize residues and wheat straw were generally similar. ► Biochemical degradation indices reflected burial period and burial depth.
    Keywords: Amino Sugars ; Lignin Units ; Litterbags ; Mouldboard Ploughing ; Neutral Sugars ; Rotary Harrow ; Agriculture
    ISSN: 0016-7061
    E-ISSN: 1872-6259
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  • 4
    Language: English
    In: Geoderma, 2005, Vol.127(3), pp.177-187
    Description: Dissolved organic matter increases typically in streams draining forested catchments during heavy rainstorms and snowmelt. Tracer methods and model calculations suggest that the storm flow flushing of dissolved organic matter is either due to lateral near-surface flow, i.e. within the organic forest floor, or preferential flow (funnelling) through the mineral soil. Both pathways should deliver forest floor-derived dissolved organic matter to streams that is hardly changed because of little to no interaction with mineral soil material and microorganisms. Here, we investigated the effect of rain storm induced vertical flushing through the mineral soil on the composition of dissolved organic matter in a structured Rendzic Leptosols under 90-year-old European beech ( L.). During two rainstorm periods in autumn 1998 with elevated transport of organic C, N, P and S from the forest floor into the subsoil, we sampled dissolved organic matter in forest floor leachates (sampled by zero-tension plate lysimeters), subsoil solutions (sampled by suction cups at 90 cm depth) and subsoil seepage (sampled by zero-tension plate lysimeters at 90 cm depth). The chemical composition of dissolved organic matter was characterised by fractionation with XAD-8 macroreticular resin, wet-chemical analyses of carbohydrates and lignin-derived phenols, and determination of the δ C. During both rainstorm periods, all tested chemical features of dissolved organic matter in forest floor leachate and subsoil seepage matched each other greatly. In contrast, dissolved organic matter in soil solution contained smaller portions of XAD-8-adsorbable organic C, less lignin-derived phenols, more carbohydrates and showed smaller δ C values than that in forest floor leachates and subsoil seepage. These results suggest a rather direct transfer of organic solutes from the forest floor into the subsoil and probably further to ground and surface waters during heavy rainstorms. Dissolved organic matter leaving the soil in heavy rainstorms by rapid water flow through macropores is likely less biodegradable, more UV-digestible and more reactive towards metals and organic pollutants than that released from soil at low rainfall intensity by matric flow.
    Keywords: Dissolved Organic Carbon ; Dissolved Organic Nitrogen ; Dissolved Organic Phosphorus ; Dissolved Organic Sulphur ; Carbohydrates ; Lignin-Derived Phenols ; Δ 13c ; Xad-8 Fractionation ; Agriculture
    ISSN: 0016-7061
    E-ISSN: 1872-6259
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  • 5
    Language: English
    In: Geoderma, 01 December 2018, Vol.331, pp.15-17
    Description: Silicon (Si) is a beneficial nutrient for many plants, including major crop species. Yet, the impacts of agricultural practices on Si cycling have been hardly studied. We investigated the effects of long-term fertilizer (farmyard manure, NPK) and/or lime applications on concentrations of acetate-extractable Si (Si ; i.e., potentially mobile and plant-available Si) in a Chernozem topsoil (Bad Lauchstädt, Germany). The Si concentrations were between 122 and 292 mg Si kg , and thus, larger than `critical values` considered to trigger Si limitation of plant growth. We found positive relationships between Si concentrations and soil pH, which might be explained by pH-dependence of the phytolith solubility as well as of the sorption of Si to mineral surfaces. Our data suggest that differing agricultural practices affects Si fluxes and availability in soil by affecting the soil pH.
    Keywords: Silicon Cycling ; Fertilization ; Liming ; Chernozem ; Agriculture
    ISSN: 0016-7061
    E-ISSN: 1872-6259
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  • 6
    Language: English
    In: Geoderma, 15 June 2017, Vol.296, pp.86-97
    Description: Organic soils are the most important source of dissolved organic carbon (DOC) in surface water. To date, most studies have focused on natural and re-wetted peatlands, but in Central Europe a large proportion of organic soils are drained and under agricultural use. Furthermore, measures such as deep ploughing or sand addition have been conducted to improve trafficability and have resulted in topsoil horizons consisting of a peat-sand mixture. Very little is known about DOC losses from such soils. Moreover, peat soils frequently feature both mobile zones, characterised by active water and solute transport, and immobile zones, which exchange solutes with the mobile zone by diffusion. Surprisingly, however, the effects of this dual porosity on DOC transport have not yet been explored. This study investigated the physicochemical controls on DOC concentrations in a peat-sand mixture by means of a saturated column experiment with undisturbed columns. The soil came from a former bog in northern Germany where peat layers remaining after peat extraction were mixed with the underlying mineral soil by ploughing. Three pumping rates and two levels of electrical conductivity (EC) were applied. The transport properties of the soil were obtained by analysing breakthrough curves of potassium bromide using the transport model STANMOD, which is based on the two-region non-equilibrium concept. The results of the column study were compared to DOC concentrations measured bi-weekly for two years at the field site from where the columns were taken. Despite a similar texture and soil organic carbon (SOC) content, the fraction of the mobile zone in the columns varied between 51% and 100% of total porosity. Thus even heavily degraded organic soils mixed with sand still showed a dual porosity comparable to degraded peat soils. Percolating the columns with the high EC solution caused low pH values, probably due to ion exchange and cation bridging. The combination of high EC and low pH greatly decreased DOC concentrations at the outlet of the columns. DOC concentrations decreased and fluxes increased as the pumping rates increased. Taking pore water velocity in the mobile zone into account could help to explain the differences between the columns. Overall, transport of DOC did not seem to be limited by production of DOC, but by rate-limited exchange processes. In contrast to the column experiment, field concentrations of DOC were much higher and were not related to pH, but increased with higher electrical conductivity. These higher concentrations could be explained by low pore water velocities and the slightly higher SOC content in the field. This first experiment on DOC transport in peat-sand mixtures taking the dual-porosity nature of organic soils into account clearly demonstrated the importance of pore water velocity and thus the residence time for DOC concentrations. While hydrochemical conditions are frequently addressed in laboratory studies, there is a need for improved understanding of their interaction with hydrology and soil-physical properties, especially when attempting to interpret DOC data on different spatial and temporal scales.
    Keywords: Dissolved Organic Carbon ; Dual Porosity ; Tracer Experiment ; Bog ; Water Quality ; Agriculture
    ISSN: 0016-7061
    E-ISSN: 1872-6259
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  • 7
    Language: English
    In: Geoderma, 2008, Vol.147(3), pp.141-150
    Description: Hydrotalcite, a Mg–Al layered double hydroxide [Mg Al (OH) ] [CO ] · 4H O, occurs in alkaline soils, especially when affected by industrial waste input. It strongly interacts with anionic compounds, thus can contribute to the accumulation and storage of organic matter in such soils. We studied the sorption of organic matter (OM) to hydrotalcite with different nitrate to carbonate ratios in the interlayer. A range of solutions with different concentrations of organic carbon (OC) was prepared to test the hydrotalcites' affinity for OM. In order to identify sorption mechanisms, we analyzed hydrotalcites before and after sorption of OM by X-ray diffraction and determined specific surface area (SSA) and pore volume. Spectroscopic and colorimetric methods were applied to track changes in the OM composition, resulting from preferential sorption of organic fractions. Hydrotalcite sorbed large amounts of natural dissolved OM (up to 135.2 ± 20.5 mg organic C g ) under chemical solution conditions comparable with those in calcareous alkaline soils. The interlayer carbonate-to-nitrate ratio affected the OM sorption under highly alkaline conditions (pH ≥ 9) but not under neutral to weakly alkaline conditions (pH 7–8). We assume the higher the portion of carbonate as charge balancing anion the lower is the mineral's surface charge. Enhanced deprotonation of surface hydroxyl groups at high solution pH (pH ≥ 9) would therefore reduce the surface charge, thus causing decreasing electrostatic attraction of organic anions. Hydrotalcite preferentially retained aromatic compounds. Sorption of OM to hydrotalcite takes place solely at external surface sites, predominately by ligand exchange for surface hydroxyl groups.
    Keywords: Layered Double Hydroxides ; Preferential Sorption ; Organic Matter ; Ligand Exchange ; Aromatic Compounds ; Specific Surface Area (SSA) ; Agriculture
    ISSN: 0016-7061
    E-ISSN: 1872-6259
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  • 8
    Language: English
    In: Geoderma, 2007, Vol.140(1), pp.140-146
    Description: Studies of organic–mineral interactions in soils often include physical fractionation according to particle size and density to separate organic debris and mineral-associated matter. For density fractions, it has been noticed that Fe hydrous compounds, although having a high absolute density, can be found in fractions supposed to contain only compounds of lower density. Probable reasons are incomplete wetting due to nanopores, thus reduced apparent density, the slow settlement during centrifugation because of the small particle size, and association with organic matter, reducing the absolute density and favouring dispersion. Allocation of associations of organic matter and hydrous mineral phases to light fractions may affect the estimation of mineral-associated organic matter in soils. We tested the distribution of goethite, 2-line ferrihydrite, and amorphous Al(OH) ), associated with different amounts of organic matter, across density fractions obtained by floating in Na polytungstate solutions of 1.6, 2.0, and 2.4 g cm density, followed by centrifugation at 5000  . Without organic matter, the two Fe minerals with absolute densities 〉 3.9 g cm , were recovered almost completely in the fraction 〉 2.4 g cm , irrespective the mode of dispersion (manual agitation or sonification with an energy input of 300 kJ ml ). Amorphous Al(OH) , having a density of 2.2 g cm , was recovered entirely in the fraction 2.0–2.4 g cm when agitated manually while little material (〈 5%) was found in the fraction 1.6–2.0 g cm after ultrasonic dispersion. Thus density fractionation properly separates pure mineral particles according to their absolute density, neither affected by small particle size nor by incomplete wetting. Organic–mineral associations, resulting either from sorptive interactions or co-precipitation, had smaller absolute densities than the pure mineral phases, according to their content of organic matter. When agitated manually, 〉 95% of them were completely recovered in the appropriate density fraction, except for associations with extremely large organic contents (co-precipitates), of which small portions were found in the next lighter fraction. The application of ultrasonic energy, in contrast, resulted in up to 100% of the organic–mineral materials to be found in lighter fractions. The portion of material recovered in lighter fractions related positively to the content of mineral-associated organic matter. The results suggest that the application of ultrasonic energy to disrupt soil aggregates can cause dispersion of negatively charged organic–mineral associations in Na polytungstate solution, resulting in erroneous allocation to density fractions.
    Keywords: Organic–Mineral Associations ; Sorption ; Co-Precipitation ; Porosity ; Dispersion ; Agriculture
    ISSN: 0016-7061
    E-ISSN: 1872-6259
    Source: ScienceDirect Journals (Elsevier)
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  • 9
    Language: English
    In: Geoderma, 2003, Vol.113(3), pp.293-310
    Description: During the last decade, research in sedimentary systems led to the paradigm of sorptive stabilization of organic matter (OM). Studies on soils also show that sorptive interactions between dissolved organic matter (DOM) and mineral phases contribute to the preservation of soil OM. In the first part of the paper, we summarize evidence for sorptive stabilization of OM in forest soils including (a) pronounced retention of DOM in most subsoils, (b) strong chemisorptive binding exhibiting strong hysteresis, and (c) similarity in the composition of DOM and OM in illuvial soil horizons and clay-sized separates. However, the capacity of soils for sorption of DOM is not infinite. In the second part of the paper, we present a case study where we relate the yearly retention of dissolved organic carbon (DOC) in the mineral soil to the available sorption capacity of seven forest soils. We estimate that the saturation of the sorption complex would occur within 4-30 years. Assuming these soils are in steady-state equilibrium with respect to carbon cycling, this suggests a mean residence time of the sorbed organic carbon (OC) of about the same time, therefore providing little evidence for a long-term stabilization of sorbed OM. One explanation for this discrepancy may be because in forest soils most surfaces are not characterized by juvenile minerals but are covered with OM and colonized by microorganisms. This is the case mainly in topsoil horizons but occurs also along preferential flow paths and on aggregate surfaces. Biofilms develop particularly at sites receiving high input of nutrients and organic substrates, i.e., DOM, such as preferential flow paths. The OM input enhances the heterotrophic activity in the biofilm, converting the DOM into either organic compounds by microbial resynthesis or inorganic mineralization products. Recent studies suggest that Fe hydrous oxides embedded within the biofilms may serve as a sorbent and shuttle for dissolved organic compounds from the surrounding aqueous media. We assume that sorption of DOM to the biofilm does not lead to a stabilization of OM but is a prerequisite for its rapid turnover. Only when DOM is transported by mass flow or diffusion to fresh, juvenile mineral surfaces, may sorption effectively stabilize OM. This stabilization would involve complexation of functional groups, changed conformation, and incalation in small pores. Abstract Copyright (2003) Elsevier, B.V.
    Keywords: Organic Matter ; Organic Carbon ; Stabilization ; Biofilms ; Dissolved Organic Matter ; Sorption ; Agriculture
    ISSN: 0016-7061
    E-ISSN: 1872-6259
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  • 10
    Language: English
    In: Geoderma, 2003, Vol.113(3), pp.177-178
    Keywords: Agriculture
    ISSN: 0016-7061
    E-ISSN: 1872-6259
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