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  • 1
    In: European Journal of Soil Science, May 2019, Vol.70(3), pp.454-465
    Description: Soil structural traits provide links between soil structure and ecosystem functioning. The size and stability of soil aggregates are assumed to provide information on aggregate formation and turnover. A standard method to analyse these traits is to determine the mass distribution on sieves. The major drawback of this method is the small size resolution because of a small number of size classes. A promising, yet still unexplored, method for size distribution analysis in soil science, is dynamic image analysis, which foremost allows a much larger diameter resolution and the assessment of both size and shape distributions. The aim of our study was to validate the applicability of dynamic digital image analysis in combination with sonication to characterize the size and shape distribution and the stability of aggregates. We used two different heterogeneous reference materials and three different soil samples with different aggregate stabilities to test this method. The soil samples were chosen based on increasing clay, humus and calcium carbonate contents. The method yielded reproducible results for diameter and shape distributions for both reference materials and soil samples. The most important improvement compared to well‐established methods was the extremely large size resolution. This allows specification of the pattern of diameter‐dependent breakup curves by relating them to specific soil properties. The information on sphericity adds supplementary information on the aggregates released. We found much lower sphericity of 1‐mm aggregates mobilized from topsoil samples formed from the activity of living organisms than aggregates mobilized from subsoil samples formed mainly by physicochemical processes. Highlights Our aim was to validate dynamic digital image analysis to characterize soil aggregates.Dynamic image analysis allows high resolution and shape analysis compared to established methods.The method yielded reproducible results for diameter and shape distributions.We established high‐resolution disruption patterns of aggregates enabling new approaches in future research.
    Keywords: Aggregate Breakdown Dynamics ; Particle‐Size Distribution ; Ultrasonic Dispersion
    ISSN: 1351-0754
    E-ISSN: 1365-2389
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  • 2
    Language: English
    In: Journal of Hydrology X, 01/2019, Vol.2, C, p.100014
    ISSN: 25899155
    Source: Elsevier (via CrossRef)
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  • 3
    Language: English
    In: Forests, 01 August 2019, Vol.10(9), p.726
    Description: The compaction of forest soils can deteriorate soil aeration, leading to decreased CH4 uptake and increased N2O efflux. Black alder (Alnus glutinosa) may accelerate soil structure regeneration as it can grow roots under anaerobic soil conditions. However, symbiotic nitrogen fixation by alder can have undesirable side-effects on greenhouse gas (GHG) fluxes. In this study, we evaluated the possible trade-off between alder-mediated structure recovery and GHG emissions. We compared two directly adjacent 15-year old beech (Fagus sylvatica) and alder stands (loamy texture, pH 5−6), including old planted skid trails. The last soil trafficking on the skid trails took place in 1999. GHG fluxes were measured over one year. Undisturbed plots with beech had a moderately higher total porosity and were lower in soil moisture and soil organic carbon than undisturbed alder plots. No differences in mineral nitrogen were found. N2O emissions in the undisturbed beech stand were 0.4 kg ha−1 y−1 and 3.1 kg ha−1 y−1 in the undisturbed alder stand. CH4 uptake was 4.0 kg ha−1 y−1 and 1.5 kg ha−1 y−1 under beech and alder, respectively. On the beech planted skid trail, topsoil compaction was still evident by reduced macro porosity and soil aeration; on the alder planted skid trail, soil structure of the uppermost soil layer was completely recovered. Skid trail N2O fluxes under beech were five times higher and CH4 oxidation was 0.6 times lower compared to the adjacent undisturbed beech stand. Under alder, no skid-trail-effects on GHG fluxes were evident. Multiple regression modelling revealed that N2O and CH4 emissions were mainly governed by soil aeration and soil temperature. Compared to beech, alder considerably increased net fluxes of GHG on undisturbed plots. However, for skid trails we suggest that black alder improves soil structure without deterioration of the stand's greenhouse gas balance, when planted only on the compacted areas.
    Keywords: Soil Compaction ; Skid Trails ; Black Alder ; Alnus Glutinosa ; Greenhouse Gas Fluxes ; Soil Structure Recovery ; Forestry
    E-ISSN: 1999-4907
    Source: Directory of Open Access Journals (DOAJ)
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  • 4
    Language: English
    In: Agricultural and Forest Meteorology, 15 February 2019, Vol.265, pp.424-434
    Description: Molecular diffusion is commonly assumed as main physical process of gas transport in soils. However, non-diffusive gas transport processes like the so-called pressure-pumping effect can affect soil gas transport significantly. The pressure-pumping effect has only been detected indirectly and the underlying mechanisms remain unclear. Using a novel method the soil gas transport at a conifer forest site was monitored over a seven-week period. Airflow and air pressure were simultaneously measured above and below the forest canopy and air pressure was also measured in the soil. During episodes of high above-canopy wind speed, the effective soil gas diffusivity temporarily increased due to pressure-pumping. The enhancement of the gas transport rate in the topsoil reached up to 30%. We found that the best meteorological proxy explaining this effect was related to air pressure fluctuations measured at soil surface and not the mean wind speed directly above ground. While sub-canopy wind speeds continuously decreased from the bottom of the tree crown to the soil surface, amplitudes of the air pressure fluctuations were nearly constant in the whole sub-canopy profile and in the soil. We hypothesize that the air pressure fluctuations responsible for pressure-pumping are related to characteristics of above-canopy airflow rather than to airflow directly above the soil surface.
    Keywords: Pressure-Pumping ; Soil Gas Transport ; Air Pressure Fluctuations ; Tracer Gas ; Agriculture ; Meteorology & Climatology
    ISSN: 0168-1923
    E-ISSN: 1873-2240
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