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  • 1
    Language: English
    In: Geochimica et Cosmochimica Acta, 01 May 2016, Vol.180, pp.284-302
    Description: We evaluated the impact of nano-structural characteristics of allophanic compounds and Fe oxide speciation on the efficiency of organo-mineral interactions in an allophanic derived from volcanic ash (Eifel mountains, Germany). The samples selected for our work represented a gradient from: (i) a pure synthetic allophane and (ii) model organo-mineral mixtures to (iii) particle size fractions of the natural Andosol. We thus aimed to link the processes operating at the individual molecular scale to the phenomena active at the aggregate scale. For a non-destructive characterization of the samples, we applied Xe NMR spectroscopy of adsorbed Xe atoms (to identify the mineral nano-structure and surface acid centres), ESEM (verifying the nano-spherical structure of allophane), C CPMAS NMR (for the nature of the soil organic matter (SOM)), Fe Mössbauer spectroscopy (Fe oxide speciation), and N adsorption (contribution of micro- and mesoporosity). By using the atomic probe Xe, we obtained evidence for a mechanism of adsorption onto allophane requiring both the narrow pores (voids formed by the primary nano-spherules) and the acid centres located at the defect surfaces of the primary spherules. The validity of this coupled mechanism for the sorption of organic matter was confirmed by the concomitant blocking of acid centres ( Xe NMR data) and the decrease of the N -available pore volumes ( and ) in the model samples DOM/- and NOM/allophane (DOM = dissolved OM, NOM = natural OM). In the Andosol, the high resistance of SOM against oxidation (OC = 15–50%) was combined with preferential accumulation of certain organic compounds, e.g. potentially labile substrates such as carbohydrates, and the low molecular weight species such as amino acids. This feature was attributed to the peculiar microporous tortuous structure of allophane aggregates that likely impose certain criteria for the chemical nature and size of mineral-bound SOM. On the other hand, the revealed dominance of nanoparticulate Fe oxyhydroxides (57% ferrihydrite) and Fe-substituted allophane (supposedly formed due to co-precipitation of the Al, Si and Fe in the course of volcanic soil formation) may substantially contribute to the formation of highly resistant organo-mineral associations through the enhanced extent of reactive surface groups in nanoparticles, increased surface charge density and electron accepting properties of substituting Fe species that supposedly enhance the proportion of oxidised organic components.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 2
    Language: English
    In: Soil Biology and Biochemistry, February 2014, Vol.69, pp.168-178
    Description: The stabilization of soil organic matter (SOM) is triggered by three main mechanisms: (i) low bioavailability due to aggregation, (ii) recalcitrance due to the chemical structure, and (iii) association of the SOM with mineral surfaces. In the present study we used particle size SOM fractions (sand, silt and clay), derived from the Ah soil horizon from a Norway spruce forest in Southern Germany, to study the effects of different stabilization mechanisms on the bioavailability of soil organic carbon (SOC) in a one year incubation experiment. The respired CO was hourly recorded, additionally CO was analysed 20 times and CO three times during the incubation experiment. To better differentiate between particulate OM (POM) and mineral associated OM (MIN), the incubated fractions and bulk soil were separated according to density (1.8 g cm ) after the incubation experiment. C-CPMAS NMR spectroscopy was used to study the chemical composition of the incubated samples. We demonstrate a clear increase in SOM bioavailability due to aggregate disruption, as the calculated theoretical CO evolution of the SOM fractions recombined by calculation was 43.8% higher in relation to the intact bulk soil. The incubated sand fraction, dominated by POM rich in O/N-alkyl C, showed a prolonged bioavailability of SOC moieties with mean residence times (MRT) of 78 years. Interestingly, the silt fraction, dominated by highly aliphatic, more recalcitrant POM, showed low mineralization rates and slow MRT's (192 years) close to values for the clay fraction (171 years), which contained a large amount of mineral-associated SOM. The recorded CO signatures showed a high depletion in C during the initial stage of the incubation, but an enrichment of the respired CO of up to 3.4‰ relative to the incubated SOM was observed over longer time periods (after 3 and 4 days for bulk soil and sand, respectively, and after 14 days for silt and clay). Therefore, we found no evidence for a C enrichment of SOM as driven by metabolic isotopic fractionation during microbial SOM mineralization, but an indication of a change in the isotopic composition of the C-source over time.
    Keywords: 13co2 ; 14co2 ; Laboratory Incubation ; Heterotrophic Respiration ; 13c-Cpmas NMR Spectroscopy ; Particle Size Fractionation ; Density Fractionation ; Mean Residence Time ; Microbial Biomass ; Agriculture ; Chemistry
    ISSN: 0038-0717
    E-ISSN: 1879-3428
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  • 3
    In: PLoS ONE, 2014, Vol.9(12)
    Description: Ongoing climate change will lead to more extreme weather events, including severe drought periods and intense drying rewetting cycles. This will directly influence microbial nitrogen (N) turnover rates in soil by changing the water content and the oxygen partial pressure. Therefore, a space for time climate change experiment was conducted by transferring intact beech seedling-soil mesocosms from a northwest (NW) exposed site, representing today's climatic conditions, to a southwest (SW) exposed site, providing a model climate for future conditions with naturally occurring increased soil temperature (+0.8°C in average). In addition, severe drought and intense rainfall was simulated by a rainout shelter at SW and manual rewetting after 39 days drought, respectively. Soil samples were taken in June, at the end of the drought period (August), 24 and 72 hours after rewetting (August) and after a regeneration period of four weeks (September). To follow dynamics of bacterial and archaeal communities involved in N turnover, abundance and activity of nitrifiers, denitrifiers, N 2 -fixing microbes and N-mineralizers was analyzed based on marker genes and the related transcripts by qPCR from DNA and RNA directly extracted from soil. Abundance of the transcripts was reduced under climate change with most pronounced effects for denitrification. Our results revealed that already a transfer from NW to SW without further treatment resulted in decreased cnor and nosZ transcripts, encoding for nitric oxide reductase and nitrous oxide reductase, respectively, while nirK transcripts, encoding for nitrite reductase, remained unaffected. Severe drought additionally led to reduced nirK and cnor transcripts at SW. After rewetting, nirK transcripts increased rapidly at both sites, while cnor and nosZ transcripts increased only at NW. Our data indicate that the climate change influences activity pattern of microbial communities involved in denitrification processes to a different extend, which may impact emission rates of the greenhouse gas N 2 O.
    Keywords: Research Article ; Biology And Life Sciences ; Ecology And Environmental Sciences
    E-ISSN: 1932-6203
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  • 4
    In: Global Change Biology, February 2014, Vol.20(2), pp.653-665
    Description: Sequestration of atmospheric carbon (C) in soils through improved management of forest and agricultural land is considered to have high potential for global mitigation. However, the potential of soils to sequester soil organic carbon () in a stable form, which is limited by the stabilization of against microbial mineralization, is largely unknown. In this study, we estimated the C sequestration potential of soils in southeast Germany by calculating the potential saturation of silt and clay particles according to Hassink [ (1997) 77] on the basis of 516 soil profiles. The determination of the current content of silt and clay fractions for major soil units and land uses allowed an estimation of the C saturation deficit corresponding to the long‐term C sequestration potential. The results showed that cropland soils have a low level of C saturation of around 50% and could store considerable amounts of additional . A relatively high C sequestration potential was also determined for grassland soils. In contrast, forest soils had a low C sequestration potential as they were almost C saturated. A high proportion of sites with a high degree of apparent oversaturation revealed that in acidic, coarse‐textured soils the relation to silt and clay is not suitable to estimate the stable C saturation. A strong correlation of the C saturation deficit with temperature and precipitation allowed a spatial estimation of the C sequestration potential for Bavaria. In total, about 395 Mt CO‐equivalents could theoretically be stored in A horizons of cultivated soils – four times the annual emission of greenhouse gases in Bavaria. Although achieving the entire estimated C storage capacity is unrealistic, improved management of cultivated land could contribute significantly to mitigation. Moreover, increasing stocks have additional benefits with respect to enhanced soil fertility and agricultural productivity.
    Keywords: Agricultural Management ; Climate Change ; Mitigation ; Soil Organic Carbon Stocks ; Soil Fractionation ; Stabilization Of Soil Organic Matter
    ISSN: 1354-1013
    E-ISSN: 1365-2486
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  • 5
    Language: English
    In: Soil Biology and Biochemistry, March 2013, Vol.58, pp.323-331
    Description: Our main objective was to trace and to quantify the stabilization of nitrogen released from litter decomposition in different functional soil organic matter fractions. To identify the fate of nitrogen in a free-range experiment, N-labeled beech litter was deposited on the bare soil surface of three 2 m × 2 m plots on a Rendzic Leptosol under beech ( L.) with mull humus form near Tuttlingen (Swabian Jura, Germany). The N composition of bulk soil and soil fractions was monitored for three years by sampling the litter layer and the Ah horizon (0–5, 5–10 cm) after 140, 507, and 876 d. A combined density and particle size fractionation procedure allowed the isolation of different functional soil organic matter fractions: free light fraction, occluded organic matter, and organo-mineral associations. The first flush in the N enrichment was observed in the bulk soil within 140 d, due to plant debris transferred to the free light fraction by probably bioturbation and soluble compounds being leached from the litter directly to the clay fractions. The observed rates within the first 140 d indicated a quick transfer of N-enriched compounds from litter into the free light fraction, with a rate of 0.07 μg kg  d , and to the clay fractions, with a rate of 0.31 μg kg  d . In contrast, transfer to the occluded light fractions was delayed, with rates of 0.01 μg kg  d (〉 20 μm) and 0.001 μg kg  d (〈 20 μm), respectively. After 876 d, we recovered 9% of the added label in the 0–10 cm soil horizon, of which more than 4% was found in the organo-mineral fraction (0–5 cm), nearly 3% in the light fractions (0–5 cm), and another 2% unspecified in the bulk soil of 5–10 cm depth. We therefore conclude that the clay fractions act as the main sink for the recovered N. The rapid incorporation and the high preservation of N in the clay fractions revealed the dominant role of organo-mineral associations in the stabilization of nitrogen in the investigated soil. ► Rapid transfer of N from litter into the clay fractions completed within 140 days. ► Dominant role of fine organo-mineral associations for stabilization of nitrogen. ► Occluded organic matter is protected by slow aggregate turnover. ► After 876 d, we recovered 9% of the added N label in the mineral soil (0–10 cm).
    Keywords: 15n ; Physical Fractionation ; Light Fraction ; Organo-Mineral Fraction ; Clay ; Fagus Sylvatica L ; Nitrogen Storage ; Field Experiment ; Transfer Rates ; Decomposition ; Agriculture ; Chemistry
    ISSN: 0038-0717
    E-ISSN: 1879-3428
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  • 6
    Language: English
    In: Soil Biology and Biochemistry, January 2014, Vol.68, pp.241-251
    Description: Prolonged summer droughts are projected to occur as a consequence of climate change in Central Europe. The resulting reduced soil water availability may lead to alterations in rates of soil processes such as nitrogen partitioning among soil organic matter fractions and stabilization within soil. To study the effect of climate change-induced drought on (1) the distribution of nitrogen among soil organic matter fractions and (2) nitrogen stabilization, we performed a space-for-time climate change experiment. We transferred intact plant–soil–microbe mesocosms of a Rendzic Leptosol with a young beech tree from a slope with northwestern exposure in southern Germany characterized by a cool-moist microclimate across a narrow valley to a slope with southwestern exposure with a warm-dry microclimate, which reflects projected future climatic conditions. A control transfer was also done on the northwest-facing slope within the same area of origin. We combined a homogenous N labeling approach using ammonium nitrate with a physical fractionation procedure and chemical soil extraction protocols. Our aim was to follow the partitioning of N in different soil organic matter fractions, i.e. light fractions, organo-mineral fractions, and extractable soil fractions including microbial biomass, ammonium, nitrate, and dissolved organic nitrogen. Within less than one growing season, we observed a modified partitioning of recently applied inorganic N between different soil fractions in relation to drier summer conditions, with attenuated nitrogen turnover under drought and consequently significantly higher N concentrations in the relatively labile light fractions. We ascribed this effect to a decelerated mineralization immobilization turnover. We conclude that prolonged summer droughts may alter the stabilization dynamics because the induced inactivity of microorganisms may reduce the transfer of nitrogen to stabilization pathways. A retarded stabilization in organo-mineral associations enhances the risk of nitrogen losses during extreme rainfall events, which are projected to increase in the 21st century predicted by future climate change scenarios for Central Europe.
    Keywords: Nitrogen Stabilization ; Density Fractionation ; 15n Labeling ; Climate Change ; Fagus Sylvatica L ; Field Experiment ; Light Fraction ; Microbial Biomass ; Mineralization Immobilization Turnover ; Transplant Study ; Agriculture ; Chemistry
    ISSN: 0038-0717
    E-ISSN: 1879-3428
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  • 7
    Language: English
    In: Soil Biology and Biochemistry, November 2014, Vol.78, pp.263-273
    Description: To better understand how carbon and nitrogen mineralization are linked in soils, we conducted a long-term incubation experiment and compared carbon and nitrogen dynamics in the bulk soil and in soil fractions. Topsoil of a Rendzic Leptosol from a beech forest site near Tuttlingen, Germany, was separated into three particle size classes: sand (2000–20 μm), silt (20–2 μm), and clay (〈2 μm). Bulk soil and particle size fractions were incubated in replicate, allowing periodic destructive sampling of triplicates at day 0, 14, 42, 84, 140, 210, and 280. We monitored CO –C respiration, NH –N emissions, nitrogen mineralization, pool sizes of total and salt extractable (0.01 M CaCl ) organic carbon and nitrogen, and microbial biomass carbon and nitrogen. The chemical composition of selected samples was further characterized by C-NMR spectroscopy. Fractionation did not influence carbon mineralization (∑ fractions ≈ bulk soil), which decreased in the order sand 〉 clay 〉 silt. The fractions respired between 10.4% (sand fraction), 8.8% (clay fraction) and 4.4% (silt fraction) of total soil organic carbon. However, nitrogen mineralization was affected by the fractionation procedure (∑ fractions 〈 bulk soil) and followed the order clay 〉 silt 〉 sand. Fractionation increased the surface area and hence provided accessory mineral surfaces, which allowed new binding of especially nitrogen-rich compounds, in addition to ammonium fixation via cation exchange. As indicated by lower metabolic quotients, microbial carbon mineralization was more efficient in the bulk soil compared to the calculated sum of fractions. In the clay fraction, carbon mineralization rates, salt extractable organic carbon contents, and microbial biomass carbon and nitrogen contents declined strongly towards the end of the incubation. This indicates that in the clay fraction, organic carbon was not available for microbial degradation and that microorganisms were strongly carbon-limited causing a subsequent inhibition of nitrogen immobilization. Density fractionation revealed that organic matter in the sand fraction consisted mainly of particulate organic matter present as light material containing partly decomposed plant remnants. The organic matter in the clay fraction was mostly adsorbed on mineral surfaces. Organic matter in the sand and in the clay fraction was dominated by O/N-alkyl C indicating low recalcitrance, but the C/N ratio of organic matter narrowed with decreasing particle size. Our results suggest that carbon and nitrogen mineralization are decoupled in the mineral-associated fractions of the soil. The specific interactions of both carbon and nitrogen containing components with the mineral matrix strongly modulate the mineralization dynamics. Therefore, isolated considerations of C/N or alkyl C to O/N-alkyl C ratios of organic matter are insufficient as indicators for decomposition in plant residues. The combined consideration of C/N and alkyl C to O/N-alkyl C ratios provides a first impression about the degree of decomposition in plant residues. However, bioavailability in fractions where organic matter is mainly stabilized by spatial inaccessibility and by organo-mineral interactions cannot be explained by these ratios, but can be examined by an incubation approach.
    Keywords: Laboratory Incubation ; Som Stabilization ; Rendzic Leptosol ; European Beech ; Heterotrophic Respiration ; NMR ; Agriculture ; Chemistry
    ISSN: 0038-0717
    E-ISSN: 1879-3428
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  • 8
    Language: English
    In: Soil Science Society of America Journal, 2011, Vol.75(6), p.2158
    Description: To characterize biogeochemical interface properties in soil with respect to oxide surfaces, the contribution of weakly crystalline and crystalline oxides to soil specific surface area (SSA) of particle size fractions was determined. Three arable topsoils with intermediate to high clay content were subjected to ultrasonic dispersion and particle size fractionation. The obtained silt and clay fractions were treated with hydrogen peroxide, extracted with oxalate and dithionite and the SSA of all fractions was determined using BET-[N.sub.2]. Results show that stable microaggregates were present in the coarse and medium silt fractions of all soils that could not be dispersed physically even at the highest ultrasonic dispersion energy and were probably stabilized by organic matter and iron oxides. Iron oxides were a major contributor to the SSA of all particle size fractions and the losses of carbon after oxalate and dithionite extraction showed that a major part of the organic matter in all particle size fractions was stabilized by iron oxides, even in these clay-rich soils. Weakly crystalline oxide surface area did not increase with decreasing partide size and calculated negative surface areas for some of the fine fractions indicated that weakly crystalline oxides were present as coatings on other minerals. The results demonstrate the importance of (iron) oxides for microaggregation and stabilization of organic matter in soil. However, the actual interface provided by these oxides depends on particle size and crystallinity due to the possible occlusion of mineral surfaces by organic matter and weakly crystalline oxides. Abbreviations: OC, organic carbon; OM, organic matter; SSA, specific surface area; XRD, X-ray diffraction. doi: 10.2136/sssaj2010.0455
    Keywords: Iron Oxides -- Chemical Properties ; Iron Oxides -- Environmental Aspects ; Loams -- Chemical Properties ; Loams -- Composition ; Soil Chemistry -- Research ; Geochemistry -- Research;
    ISSN: Soil Science Society of America Journal
    E-ISSN: 0361-5995
    E-ISSN: 14350661
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  • 9
    Language: English
    In: PLoS ONE, 01 January 2016, Vol.11(7), p.e0158823
    Description: European beech forests growing on marginal calcareous soils have been proposed to be vulnerable to decreased soil water availability. This could result in a large-scale loss of ecological services and economical value in a changing climate....
    Keywords: Sciences (General)
    E-ISSN: 1932-6203
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  • 10
    Language: English
    In: Agriculture, Ecosystems and Environment, 01 March 2014, Vol.185, pp.208-220
    Description: The management of soils as well as the impact of land use or climate changes are often evaluated in view of the storage of total soil organic carbon (SOC). However, as soil organic matter (SOM) is composed of different compounds with different degrees of stability and turnover times, there is the need for a soil- and land use-specific quantification of functional SOC pools. In this study, the amount of active, intermediate and passive SOC pools was determined for major soil types and land uses of Bavaria in southeast Germany. At 99 locations, soil horizons down to the parent material were fractionated according to the method of . The results showed that in cropland and grassland soils around 90% of total SOC stocks can be assigned to the intermediate and passive SOC pool. High SOC stocks in grassland soils are partly related to a higher degree of soil aggregation compared to cropland soils. The contribution of intermediate SOC in cropland soils was similar to that in grassland soils due to an increased proportion of SOM associated with silt and clay particles. The cultivation-induced loss of SOC due to aggregate disruption is at least partly compensated by increased formation of organo-mineral associations as a result of tillage that continuously promotes the contact of crop residues with reactive mineral surfaces. Contrary, forest soils were characterized by distinctly lower proportions of intermediate and passive SOC and a high amount of active SOC in form of litter and particulate organic matter which accounted for almost 40% of total SOC stocks. As both the amount of intermediate and passive SOC were lower in forest soils, we conclude that cropland and grassland soils may be more advantageous for long-term SOC storage in Bavaria. The high amount of labile SOC in forest topsoils poses the risk of considerable SOC losses caused by wildfire, mechanical disturbances or increasing temperatures.
    Keywords: Soil Organic Matter ; Soil Fractions ; Carbon Sequestration ; Climate Change ; Agriculture ; Environmental Sciences
    ISSN: 0167-8809
    E-ISSN: 1873-2305
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