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  • 1
    Language: English
    In: Geoderma, 01 July 2019, Vol.345, pp.63-71
    Description: Soil structure is not static but undergoes continuous changes due to a wide range of biotic and abiotic drivers such as bioturbation and the mechanical disturbance by tillage. This continuous alteration of soil structure beyond the pure swelling and shrinking of some stable structure is what we refer to as soil structure dynamics. It has important consequences for carbon turnover in soil as it controls how quickly soil organic matter gets occluded from or exposed to mineralization. So far there are hardly any direct observations of the rate at which soil pores are formed and destroyed. Here we employ are recently introduced labeling approach for soil structure that measures how quickly the locations of small garnet particles get randomized in soil as a measure for soil structure dynamics. We investigate the effect of desiccation crack dynamics on pore space attributes in general and soils structure turnover in particular using X-ray microtomography for repeated wetting-drying cycles. This is explored for three different soils with a range of soil organic matter content, clay content and different clay mineralogy that were sieved to a certain aggregate size fraction (0.63–2 mm) and repacked at two different bulk density levels. The total magnitude of desiccation crack formation mainly depended on the clay content and clay mineralogy. Higher soil organic matter content led to a denser crack pattern with smaller aperture. Wetting-drying cycles did not only effect visible macroporosity (〉8 μm), but also unresolved mesoporosity. The changes in macroporosity were higher at lower bulk density. Most importantly, repeated wetting-drying cycles did not lead to a randomization of distances between garnet particles and pores. This demonstrates that former failure zones are reactivated during subsequent drying cycles. Hence, wetting-drying resulted in reversible particle displacement and therefore would not have triggered the exposure of occluded carbon that was not already exposed during the previous drying event.
    Keywords: Soil Structure ; Desiccation Cracks ; X-Ray Tomography ; Macropores ; Clay Mineralogy ; Carbon Turnover ; Agriculture
    ISSN: 0016-7061
    E-ISSN: 1872-6259
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  • 2
    Language: English
    In: Biogeosciences Discussions, 03/08/2019, pp.1-31
    ISSN: Biogeosciences Discussions
    E-ISSN: 1810-6285
    Source: CrossRef
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  • 3
    Language: English
    In: Geoderma, 15 September 2019, Vol.350, pp.61-72
    Description: During soil formation, the interaction of different biota (plants, soil fauna, microbes) with weathered mineral material shapes unique structures depending on the parental material and the site specific climatic conditions. While many of these interactions are known, the relative importance of the different biota is difficult to unravel and therefore difficult to quantify. Biological soil structure formation is often superimposed by soil management and swell-shrink dynamics, making it even more difficult to derive mechanistic understanding. We here explore soil structure formation within a “space-for-time” chronosequence in the Rhenish lignite mining area. Loess material from a depth of 4–10 m has been used for reclamation in a standardized procedure for 24 years. Changes in soil pore system are characterized by properties such as connectivity (Euler number) and pore size distribution using undisturbed soil columns with a diameter of 10 cm. They were taken from two different depths (0–20 cm and 40–60 cm) at different sites ranging in age from 0 to 24 years. X-ray CT is used for scanning the original columns as well as undisturbed subsamples of 3 and 0.7 cm diameter. This hierarchical sampling scheme was developed to overcome the trade-off between sample size and resolution. For the first time also information on the development of biopores could be measured by separating them from other structural pores based on their unique shape. The data were complemented by destructive sampling and determination of root length with WinRHIZO to give an estimate of how many biopores are filled with roots. Furthermore HYPROP measurements of water retention curves were conducted and showed a general agreement with the image-derived pore size distribution merged across three scales. An increase in biopore density throughout year zero to year 12, in particular in 40–60 cm soil depth, was observed. The biopore length densities of approximately 17 cm/cm obtained in year 12 was similar to the one measured in year 24, suggesting that equilibrium was reached. Only about 10% of these biopores were filled with roots. In the topsoil (0–20 cm) the equilibrium value in biopore density is already reached after six years due to a higher root length density. Ploughing lead to higher mean pore size and to lower connectivity compared to the well-connected, very stable pore network in 40–60 cm depth. This study shows how fast plant roots create a stable and connected biopore system and how this is disrupted by soil tillage, which produces completely contrasting pore characteristics.
    Keywords: Biopores ; Structure Formation ; X-Ray CT ; Tilled Soil ; Agriculture
    ISSN: 0016-7061
    E-ISSN: 1872-6259
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  • 4
    Language: English
    In: Geoderma, 15 July 2019, Vol.346, pp.52-62
    Description: Some soil physical properties can easily be measured using classical laboratory methods. However, explicit valuable information on the real morphology of the pore structure as well as soil physical properties cannot be obtained at the same time with classical methods. This requires non-destructive measurements such as X-ray computed tomography (CT). However, explicit valuable information on the real morphology of the pore structure as well as soil physical properties cannot be obtained at the same time with classical methods. This paper combines parameters obtained from CT analysis (mean macropore diameter, macroporosity, pore connectivity, anisotropy) and classical laboratory methods (dry bulk and aggregate density, saturated hydraulic conductivity, mechanical precompression stress) to analyse soil compaction, exemplified on samples from two tillage treatments (cultivator and plough) and at two moisture states (6 and 1000 kPa matric potential) on a Chernozem collected at a soil depth of 16–22 cm (texture 0–30 cm: silty clay loam). The study shows that the matric potential can have a decisive impact on the mechanical stability of soil. In the loose but less stable plough treatment a more negative matric potential was clearly beneficial to the mechanical stability. In already dense soil structures, as in the cultivator treatment, a reduction of water content was less effective in increasing soil stability. The CT parameters were all closely and uniquely related to each other. The shown CT parameters can be used for a standardized characterization of the soil. Ploughing has a positive effect on soil structure which persists only as long as macroporosity and mean macropore diameter remain high. Plough maintains higher pore connectivity when compacted under dry conditions.
    Keywords: X-Ray CT ; Mechanical Soil Analysis ; Conservation Tillage ; Conventional Tillage ; Soil Compaction ; Precompression Stress ; Agriculture
    ISSN: 0016-7061
    E-ISSN: 1872-6259
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  • 5
    Language: English
    In: Geoderma, 01 January 2019, Vol.333, pp.90-98
    Description: Secondary treated wastewater, a commonly used water resource in agriculture in (semi-)arid areas, often contains salts, sodium, and organic matter which may affect soil structure and hydraulic properties. The main objective of this study was to jointly analyse the effects of long-term irrigation with treated wastewater on physicochemical soil characteristics, soil structure, and soil water dynamics in undisturbed soils. X-ray microtomography was used to determine changes in macro-porosity (〉 19 μm), pore size distribution, and pore connectivity of a sandy clay loam and a loamy sand. Differences in the pore network among soils irrigated with treated wastewater, fresh water that replaced treated wastewater, and non-irrigated control plots could be explained by changes in textural composition, soil physicochemical parameters, and hydraulic properties. In this study we showed that irrigation led to the development of a connected macro-pore network, independent of the studied water quality. The leaching of silt and clay particles in the sandy soil due to treated wastewater irrigation resulted in an increase of pores 〈 130 μm. While this change in texture reduced water retention, the unsaturated hydraulic conductivity was diminished by physicochemical alteration, i.e. induced water repellency and clay mineral swelling. Overall, the fine textured sandy clay loam was much more resistant to soil alteration by treated wastewater irrigation than the loamy sand.
    Keywords: Soil Structure ; Treated Wastewater Irrigation ; Clay Dispersion ; Unsaturated Hydraulic Conductivity ; Soil Water Retention ; X-Ray Microtomography ; Agriculture
    ISSN: 0016-7061
    E-ISSN: 1872-6259
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  • 6
    Article
    Article
    Language: English
    In: Vadose Zone Journal, 01 July 2019, Vol.18(1)
    Description: Soil hydrology is a key control for the functioning of the terrestrial environment. Many environmental issues that we need to tackle today are directly linked to soil water dynamics. This includes agricultural production and food security, nutrient cycling and carbon storage, prevention of soil degradation and erosion, and last but not least, clean water resources and flood protection. However, these problems need to be addressed at the scales of fields, regions, and landscapes, while soil water dynamics and soil hydraulic properties are well understood and typically measured at much smaller scales—the comfort zone of soil physics. An obvious problem is how to link these vastly different scales and how to profit from small-scale understanding to improve our capability to predict what is going on at the large scale. In this update, this problem is discussed based on insights gained during the last decades. As a synthesis, a two-step scaling approach is proposed for modeling soil water dynamics from local to landscape scales where the scale of the soil profile is the stepping stone.
    Keywords: Agriculture
    ISSN: 1539-1663
    E-ISSN: 1539-1663
    Source: Directory of Open Access Journals (DOAJ)
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  • 7
    In: Water Resources Research, August 2019, Vol.55(8), pp.6653-6672
    Description: —a key process for water exchange between soil and atmosphere—is controlled by internal water fluxes and surface vapor fluxes. Recent studies demonstrated that the dynamics of the water flow in corners determine the time behavior of the evaporation rate. The internal water flux of the porous media is often described by capillary flow assuming . Particularly, the crucial influence of , that is, the nonlinear dependency of the capillary flow has been neglected so far. The focus of the paper is to demonstrate that SiO‐surfaces can exhibit contact angles of about 40°. This reduces the internal capillary flow by 1 order of magnitude compared to complete wetting. First, we derived the contact angle by inverse modeling. We conducted a series of evaporation experiments in a 2‐D square lattice microstructure connected by lognormal distributed throats. We used an explicit analytical power series solution of the single square capillary model. A contact angle of 38° ± 1° was derived. Second, we directly measured the contact angle of the Si‐SiO wafer using the Drop Shape Analyzer Krüss 100 and obtained an averaged contact angle of 42° ± 2°. The results support the single square capillary model as an appropriate model for the description of the evaporation process in an ideal square capillary. Evaporation rate dependence on contact angle and temperature: Influence of capillary, viscous, and gravitational forces Visualization micromodel experiments of corner flow: Micromodels produced by a new interval‐based ICP‐DRIE technology Analytical solution for 1‐D corner flow and analysis of the fluid‐fluid patterns and geometric characteristics of the evaporation process
    Keywords: Water Flow ; Silicon Dioxide ; Water Exchange ; Surfaces ; Internal Water ; Power Series ; Water Exchange ; Soil Water ; Evaporation ; Contact Angle ; Dependence ; Evaporation Rate ; Water Flow ; Water Exchange ; Microstructure ; Evaporation ; Porous Media ; Porous Media ; Silica ; Evaporation ; Water Flow ; Capillary Flow ; Atmospheric Models ; Wetting ; Silicon Dioxide ; Fluxes ; Soils ; Evaporation ; Evaporation Rate ; Water Exchange ; Contact Angle;
    ISSN: 0043-1397
    E-ISSN: 1944-7973
    Source: John Wiley & Sons, Inc.
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  • 8
    Language: English
    In: Geoderma, 01 January 2019, Vol.333, pp.149-162
    Description: The capacity of soils to store organic carbon represents a key function of soils that is not only decisive for climate regulation but also affects other soil functions. Recent efforts to assess the impact of land management on soil functionality proposed that an indicator- or proxy-based approach is a promising alternative to quantify soil functions compared to time- and cost-intensive measurements, particularly when larger regions are targeted. The objective of this review is to identify measurable biotic or abiotic properties that control soil organic carbon (SOC) storage at different spatial scales and could serve as indicators for an efficient quantification of SOC. These indicators should enable both an estimation of actual SOC storage as well as a prediction of the SOC storage potential, which is an important aspect in land use and management planning. There are many environmental conditions that affect SOC storage at different spatial scales. We provide a thorough overview of factors from micro-scales (particles to pedons) to the global scale and discuss their suitability as indicators for SOC storage: clay mineralogy, specific surface area, metal oxides, Ca and Mg cations, microorganisms, soil fauna, aggregation, texture, soil type, natural vegetation, land use and management, topography, parent material and climate. As a result, we propose a set of indicators that allow for time- and cost-efficient estimates of actual and potential SOC storage from the local to the regional and subcontinental scale. As a key element, the fine mineral fraction was identified to determine SOC stabilization in most soils. The quantification of SOC can be further refined by including climatic proxies, particularly elevation, as well as information on land use, soil management and vegetation characteristics. To enhance its indicative power towards land management effects, further “functional soil characteristics”, particularly soil structural properties and changes in the soil microbial biomass pool should be included in this indicator system. The proposed system offers the potential to efficiently estimate the SOC storage capacity by means of simplified measures, such as soil fractionation procedures or infrared spectroscopic approaches.
    Keywords: Clay Mineralogy ; Specific Surface Area ; Metal Oxides ; Microorganisms ; Soil Fauna ; Soil Aggregation ; Soil Texture ; Soil Type ; Natural Vegetation ; Land Use and Management ; Topography ; Parent Material ; Climate ; Agriculture
    ISSN: 0016-7061
    E-ISSN: 1872-6259
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  • 9
    Language: English
    In: Biogeosciences, Sept 27, 2019, Vol.16(18), p.3665
    Description: pSoil denitrification is the most important terrestrial process returning reactive nitrogen to the atmosphere, but remains poorly understood. In upland soils, denitrification occurs in hotspots of enhanced microbial activity, even under well-aerated conditions, and causes harmful emissions of nitric (NO) and nitrous oxide (N.sub.2 O). The timing and magnitude of such emissions are difficult to predict due to the delicate balance of oxygen (O.sub.2) consumption and diffusion in soil. To study how spatial distribution of hotspots affects O.sub.2 exchange and denitrification, we embedded microbial hotspots composed of porous glass beads saturated with growing cultures of either Agrobacterium tumefaciens (a denitrifier lacking N.sub.2 O reductase) or Paracoccus denitrificans (a "complete" denitrifier) in different architectures (random vs. layered) in sterile sand that was adjusted to different water saturations (30thinsp;%, 60thinsp;%, 90thinsp;%). Gas kinetics (O.sub.2, CO.sub.2, NO, N.sub.2 O and N.sub.2) were measured at high temporal resolution in batch mode. Air connectivity, air distance and air tortuosity were determined by X-ray tomography after the experiment. The hotspot architecture exerted strong control on microbial growth and timing of denitrification at low and intermediate saturations, because the separation distance between the microbial hotspots governed local oxygen supply. Electron flow diverted to denitrification in anoxic hotspot centers was low (2thinsp;%-7thinsp;%) but increased markedly (17thinsp;%-27thinsp;%) at high water saturation. X-ray analysis revealed that the air phase around most of the hotspots remained connected to the headspace even at 90thinsp;% saturation, suggesting that the threshold response of denitrification to soil moisture could be ascribed to increasing tortuosity of air-filled pores and the distance from the saturated hotspots to these air-filled pores. Our findings suggest that denitrification and its gaseous product stoichiometry depend not only on the amount of microbial hotspots in aerated soil, but also on their spatial distribution. We demonstrate that combining measurements of microbial activity with quantitative analysis of diffusion lengths using X-ray tomography provides unprecedented insights into physical constraints regulating soil microbial respiration in general and denitrification in particular. This paves the way to using observable soil structural attributes to predict denitrification and to parameterize models. Further experiments with natural soil structure, carbon substrates and microbial communities are required to devise and parametrize denitrification models explicit for microbial hotspots.
    Keywords: Nitrogen Oxides – Analysis ; Soil Microbiology – Analysis ; Denitrification – Analysis ; Soil Carbon – Analysis
    ISSN: 1726-4170
    E-ISSN: 17264189
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  • 10
    Language: English
    In: Vadose Zone Journal, 01 June 2019, Vol.18(1)
    Description: Biological activity in soil causes fluxes of O into and CO out of the soil with significant global relevance. Hence, the dynamics of CO concentrations in soil can be used as an indicator for biological activity. However, there is an enormous spatial and temporal variability in soil respiration, which has led to the notion of hotspots and hot moments. This variability is attributed to the spatiotemporal heterogeneity of both plant–soil–microbiome interactions and the local conditions governing gas transport. For the characterization of a given soil, the local heterogeneities should be replaced by some meaningful average. To this end, we introduce a line sensor based on tubular gas-selective membranes that is applicable at the field scale for a wide range in water content. It provides the average CO concentration of the ambient soil along its length. The new technique corrects for fluctuating external conditions (i.e., temperature and air pressure) and the impact of water vapor without any further calibration. The new line sensor was tested in a laboratory mesocosm experiment where CO concentrations were monitored at two depths during the growth of barley ( L.). The results could be consistently related to plant development, plant density, and changing conditions for gas diffusion toward the soil surface. The comparison with an independent CO sensor confirmed that the new sensor is actually capable of determining meaningful average CO concentrations in a natural soil for long time periods.
    Keywords: Agriculture
    ISSN: 1539-1663
    E-ISSN: 1539-1663
    Source: Directory of Open Access Journals (DOAJ)
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