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  • 1
    Language: English
    In: Soil & Tillage Research, 2011, Vol.114(2), pp.78-85
    Description: ▶ With the same load conditions, soil stress level in soil was higher when the soil strength was high. ▶ Vertical stresses were more concentrated along the load axis when the soil strength was low. ▶ Higher vertical stresses were measured at 0.9 m depth for a dry than a wet soil. ▶ A two-layer model of stress calculation is needed for arable soils, due to the low strength of the topsoil. The transmission of stress in soils is extremely sensitive to changes in water content. According to the elasticity theory, for a given load applied to a given soil, an increase in soil water content yields a higher concentration of stresses under the centre of the load and a deeper propagation of stresses. We quantified the effect of soil water content of topsoil/subsoil layers (wet/wet, wet/dry, and dry/dry) on stress transmission. 3D measurements of vertical stresses under a towed wheel (800/50R34) were performed in situ in a Stagnic Luvisol. The tyre was loaded with 60 kN, and we used the recommended tyre inflation pressure for traffic in the field (100 kPa). Seven stress transducers were inserted horizontally from a pit with minimal disturbance of soil at each of three depths (0.3, 0.6 and 0.9 m) and covering the width of the wheeled area. The vertical stresses at the tyre–soil contact area were measured in separate tests. Increase of water content in the topsoil by 114% increased the contact area by 149%, decreased the vertical stresses at the tyre–soil interface by 50%, and decreased the maximum vertical stress at 0.3 and 0.6 m depth by 46 and 63%, respectively. Stress attenuation with depth decreased with an increase in soil water content, yielding approximately equal maximum stresses at 0.9 m depth for the wet/wet and wet/dry treatments, while the dry/dry soil experienced significantly higher stresses than the other treatments (43, 40 and 60 kPa for treatment wet/wet, wet/dry and dry/dry, respectively). The Söhne model underestimated the vertical stresses at all depths, and the model fit was poorest at low water contents. Our data thus support the general characteristics of the elasticity theory, although the simple one-layer Söhne model gave poor quantitative stress predictions.
    Keywords: Stress Transmission ; Soil Compaction Modelling ; Soil Strength ; Soil Water Content ; Agriculture
    ISSN: 0167-1987
    E-ISSN: 1879-3444
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  • 2
    Language: English
    In: Soil biology & biochemistry, 2013, Vol.57, pp.706-712
    Description: The potential for N₂O emissions outside the main growing season may be influenced by long-term effects of cropping system. This was investigated by collecting intact soil cores (100 cm³, 0–4 cm depth) under winter wheat in three organic cropping systems and a conventional reference within a long-term crop rotation experiment. Average annual inputs of C in crop residues and manure ranged from 1.7 to 3.3 Mg ha⁻¹. A simulated freeze–thaw cycle resulted in a flush of CO₂ during the first 48 h, which could be mainly from microbial sources. Other samples were adjusted to approximately −10, −30 or −100 hPa and amended with excess ¹⁵NO₃ ⁻ prior to freezing and thawing. Denitrification was the main source of N₂O during a 72-h incubation at 22 °C, as judged from N₂O and total ¹⁵N evolution. Although the input of C in the conventionally managed cropping system was significantly less than in the organic cropping systems, it showed higher N₂O evolution at all three matric potentials. Estimates of relative gas diffusivity (DP/D₀) in soil from the four cropping systems indicated that C input affected soil aeration. Soil from the two cropping systems with highest C input showed N₂O evolution at DP/D₀ in excess of 0.02, which is normally considered a threshold for development of anaerobic sites in the soil, presumably because the oxygen demand was also high. The study shows that cropping system affects both soil gas diffusivity and C availability, and that both characteristics significantly influence the N₂O emission potential. ; p. 706-712.
    Keywords: Crop Rotation ; Long Term Effects ; Crop Residues ; Emissions ; Oxygen ; Nitrous Oxide ; Growing Season ; Freezing ; Organic Production ; Thawing ; Carbon Dioxide ; Soil Air ; Denitrification ; Aeration ; Winter Wheat ; Diffusivity
    ISSN: 0038-0717
    Source: AGRIS (Food and Agriculture Organization of the United Nations)
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  • 3
    Language: English
    In: Soil Biology and Biochemistry, February 2013, Vol.57, pp.706-712
    Description: The potential for N O emissions outside the main growing season may be influenced by long-term effects of cropping system. This was investigated by collecting intact soil cores (100 cm , 0–4 cm depth) under winter wheat in three organic cropping systems and a conventional reference within a long-term crop rotation experiment. Average annual inputs of C in crop residues and manure ranged from 1.7 to 3.3 Mg ha . A simulated freeze–thaw cycle resulted in a flush of CO during the first 48 h, which could be mainly from microbial sources. Other samples were adjusted to approximately −10, −30 or −100 hPa and amended with excess NO prior to freezing and thawing. Denitrification was the main source of N O during a 72-h incubation at 22 °C, as judged from N O and total N evolution. Although the input of C in the conventionally managed cropping system was significantly less than in the organic cropping systems, it showed higher N O evolution at all three matric potentials. Estimates of relative gas diffusivity ( / ) in soil from the four cropping systems indicated that C input affected soil aeration. Soil from the two cropping systems with highest C input showed N O evolution at / in excess of 0.02, which is normally considered a threshold for development of anaerobic sites in the soil, presumably because the oxygen demand was also high. The study shows that cropping system affects both soil gas diffusivity and C availability, and that both characteristics significantly influence the N O emission potential. ► Four experimental cropping systems differed two-fold in annual C inputs. ► A freeze–thaw induced flush in respiration from intact soil ceased within 48 h. ► Nitrous oxide emissions were highest from the system with lowest annual C input. ► Denitrification was the main source of N O, as indicated by gaseous N losses. ► Gas diffusivity was a main driver of N O, but interacted with cropping system.
    Keywords: Freeze–Thaw Cycle ; Soil Organic Matter ; 15n ; Gas Diffusivity ; Denitrification ; Agriculture ; Chemistry
    ISSN: 0038-0717
    E-ISSN: 1879-3428
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  • 4
    Language: English
    In: Agriculture, Ecosystems and Environment, 15 December 2010, Vol.139(4), pp.584-594
    Description: ▶ Increased carbon input enhanced microbial activity and N transformation processes. ▶ Manuring and catch crop incorporation prior to sowing increased grain yields in organic systems were similar to those achieved in the system where inorganic fertilizer was applied. ▶ Organic systems had higher cumulative heterotrophic CO respiration than inorganic fertilizer-based systems. Organic and conventional farming practices differ in the use of several management strategies, including use of catch crops, green manure, and fertilization, which may influence soil properties, greenhouse gas emissions and productivity of agroecosystems. An 11-yr-old field experiment on a sandy loam soil in Denmark was used to compare several crop rotations with respect to a range of physical, chemical and biological characteristics related to carbon (C) and nitrogen (N) flows. Four organic rotations and an inorganic fertilizer-based system were selected to evaluate effects of fertilizer type, catch crops, of grass-clover used as green manure, and of animal manure application. Soil was sampled from winter wheat and spring barley plots on 19 September 2007, 14 April 2008 and 22 September 2008, i.e. before, during, and after the growth season. The soils were analyzed for multiple attributes: total soil organic carbon (SOC), total N, microbial biomass N (MBN), potentially mineralizable N (PMN), and levels of potential ammonium oxidation (PAO) and denitrifying enzyme activity (DEA). measurements of soil heterotrophic carbon dioxide (CO ) respiration and nitrous oxide emissions were conducted in plots with winter wheat. In April 2008, prior to field operations, intact soil cores were collected at two depths (0–5 and 5–10 cm) in plots under winter wheat. Water retention characteristics of each core were determined and used to calculate relative gas diffusivity ( / ). Finally, crop growth was monitored and grain yields measured at harvest maturity. The different management strategies between 1997 and 2007 led to soil carbon inputs that were on average 18–68% and 32–91% higher in the organic than inorganic fertilizer-based rotations for the sampled winter wheat and spring barley crops, respectively. Nevertheless, SOC levels in 2008 were similar across systems. The cumulative soil respiration for the period February to August 2008 ranged between 2 and 3 t CO –C ha and was correlated ( = 0.95) with average C inputs. In the organic cropping systems, pig slurry application and inclusion of catch crops generally increased soil respiration, PMN and PAO. At field capacity, relative gas diffusivity at 0–5 cm depth was 〉50% higher in the organic than the inorganic fertilizer-based system ( 〈 0.05). Crop yields in 2008 were generally lower in the low-input organic rotations than in the high-input inorganic fertilizer-based system; only spring barley in rotations with pig slurry application and incorporation of a catch crop prior to sowing obtained grain yields similar to levels achieved in the system where inorganic fertilizer was applied. These results suggest that within organic cropping systems, both microbial activity and crop yields could be enhanced through inclusion of catch crops. However, the timing of catch crop incorporation is critical.
    Keywords: Catch Crop ; Denitrifier Enzyme Activity ; Gas Diffusivity ; Inorganic Fertilizer ; Microbial Biomass ; Potential Ammonium Oxidation ; Potentially Mineralizable Nitrogen ; Agriculture ; Environmental Sciences
    ISSN: 0167-8809
    E-ISSN: 1873-2305
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  • 5
    Language: English
    In: Soil Biology and Biochemistry, December 2012, Vol.55, pp.17-19
    Description: There are no methods at hand with a long and proven record for assessing the relative contribution of fungi and bacteria to decomposer activity in soil. Whereas a multitude of methods to determine fungal and bacterial biomass are available, activity assays traditionally relied on the substrate-induced respiration (SIR) inhibition approach. Here we compare fungal contribution to the microbial active biomass assessed by the SIR inhibition method with the contribution of fungal-feeding nematodes to the microbial-feeding nematode community. Four cultivation systems on the same soil that differ in carbon inputs with a factor two ranked exactly the same with the two methods. A conventionally farmed rotation with low organic input had the lowest fungal fraction, while three organically farmed soils ranked higher. ► Two methods: antibiotic inhibition and nematode trophic groups were employed. ► The methods agree in assessing ratio of fungal activity to bacterial activity in soil. ► Fungal fraction of decomposer activity higher in organic than conventional farming.
    Keywords: Substrate-Induced Respiration ; Inhibitors ; Bactericide ; Fungicide ; Nematode ; Trophic Group ; Fungi ; Bacteria ; Agriculture ; Chemistry
    ISSN: 0038-0717
    E-ISSN: 1879-3428
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  • 6
    Language: English
    In: Geoderma, 15 June 2018, Vol.320, pp.115-125
    Description: Compaction of the subsoil is an almost irreversible damage to the soil resource. Modern machinery exerts high mechanical stresses to the subsoil, and a range of studies report significant effects on soil functions. There is an urgent need for quantitative knowledge of soil strength in order to evaluate sustainability of current field traffic. The aim of this study was to identify the most important drivers of soil precompression stress, σ , and to develop pedotransfer functions for prediction of σ . We revisited previously published data on σ for a silty clay loam soil at a range of soil matric potentials. σ was estimated from the original stress-strain curves by a novel, numerical method for estimating the stress at maximum curvature, assumingly partitioning the curve into elastic and plastic sections. Multiple regression was used to identify the drivers best describing the variation in σ data. For the plough layer, σ increased with bulk density (BD), which explained 77% of the variation. For the subsoil layer just beneath the ploughing depth, the model best describing σ data included the drivers BD and pF, with pF defined as the log to the negative matric potential. The model was strongly significant with R  = 0.90. The same trend was found for three subsoil layers from 0.35–0.95 m depth, but the model accounted for only 16% of the variation in σ . A model involving samples from all soil layers and including BD, pF and soil clay content accounted for 38% of the variation. This model predicted σ to be constant at pF ~2 across soil clay contents for a given soil BD. For pF 〈 2, σ was predicted to be higher for sandy soils than for soils rich in clay. In contrast, σ increased with clay content for dryer conditions (pF 〉 2). Model predictions correlated well with measured data in two independent data sets from the literature. However, the predictions were approximately double those of one of the data sets. This may relate to the longer stress application used in laboratory compression tests for these data compared to the other calibration data set and to the procedure used in this study. We encourage further studies of the effect of stress application procedures in compression tests. The prediction equations established in this investigation have to be verified based on measurements of σ for a range of soil types, soil horizons and soil moisture conditions.
    Keywords: Pedotransfer Function ; Bulk Density ; Matric Potential ; Soil Texture ; Agriculture
    ISSN: 0016-7061
    E-ISSN: 1872-6259
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  • 7
    Language: English
    In: Soil Science Society of America Journal, 2017, Vol.0(0), p.0
    Description: Air permeability affects a range of soil functions and is useful in the quantification of soil pore characteristics. Measurements of air flow used to quantify air permeability are mostly performed at a fixed pressure difference, assuming a linear relation between flow and pressure. However, evidence exists that nonlinear pressure losses may occur even at low pressure gradients. We constructed an apparatus that allows automatic measurement of air flow at a range of pressures. The new methodology was applied to eight soil samples deriving from a loamy, Stagnic luvisol. Three artificial cores were also tested: a solid cylinder of plastic with drilled, vertical holes; a cylinder of autoclaved aerated concrete (AAC), and an AAC cylinder with drilled holes. The historical Forchheimer approach, including a polynomial regression of flowAaAeAeAupressu data, was applied to derive the true Darcian flow based on the coefficient to the linear part of the relation. Flow-pressure data appeared to be curvilinear for all test specimens, except for one of the soil samples. The results showed up to 65% errors in estimates of air permeability if the nonlinear pressure losses were ignored when applying a pressure difference as low as 100 Pa. Our results strongly suggest use of the Forchheimer approach based on measurements of flow and pressure difference at a range of air pressures. We suggest an index for soil pore tortuosity, which appears to reflect the pore characteristics of the artificial samples tested. More studies are needed to evaluate the applicability of the index for soil samples.
    Keywords: Soil Permeability – Analysis ; Soil Moisture – Analysis ; Computer Simulation – Usage;
    ISSN: Soil Science Society of America Journal
    E-ISSN: 0361-5995
    E-ISSN: 14350661
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  • 8
    Language: English
    In: Soil Research, 2018, Vol.56(2), p.129
    Description: Subsoil compaction is a serious long-term threat to soil functions. Only a few studies have quantified the mechanical stresses reaching deep subsoil layers for modern high wheel load machinery. In the present study we measured the vertical stresses in the tyre–soil contact area and at 0.3, 0.6 and 0.9 m depths of a sandy loam soil at field capacity water content. The soil was ploughed annually to a depth of 0.25 m and was tested in the spring following autumn ploughing but before secondary tillage. The machinery tested was a tractor–trailer system for slurry application with a total weight of 52 Mg. Wheel loads ranged from approximately 20 to 70 kN. The tyres were all radial ply with volumes ranging from 0.63 to 1.23 m. The tyre inflation pressures were generally above those recommended by the manufacturer and ranged from 170 to 280 kPa. The stress distributions in the contact area were highly skewed. Across tyres, the maximum stress in the contact area correlated linearly with, but was much higher than, the mean ground pressure. For each of the three soil depths, the maximum stresses under the tyres were significantly correlated with the wheel load, but not with other loading characteristics. The data predict a 6.6-kPa increase in vertical stress at 0.9 m depth for each 1-Mg addition to the wheel load. The soil stress observations support a simple rule of thumb combining wheel load and inflation pressure in calculation of subsoil vertical stress. We measured vertical stresses up to 300, 100 and 45 kPa at soil depths of 0.3, 0.6 and 0.9 m respectively. Comparing these with the data in the literature regarding soil strength and measured compaction effects on the soil studied, we conclude that the traffic event investigated is likely to induce serious effects on soil properties and functions to a depth of at least 0.7 m.
    Keywords: precompression stress; vertical stress.;
    ISSN: 1838-675X
    E-ISSN: 18386768
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  • 9
    Language: English
    In: Soil & Tillage Research, 2011, Vol.114(2), pp.57-70
    Description: ► Reduction of topsoil strength by recent tillage optimized the stress distribution at the tyre–soil interface, but did not significantly affect the propagation of vertical stresses to deeper layers. ► Söhne's model was not able to predict vertical stresses satisfactorily because of the vertical heterogeneity of soil mechanical properties, even when the soil had not been tilled for 1 1/2 years. ► The model is working well when deformations were small as in the subsoil. Transmission of stresses in the soil profile is one crucial ingredient of the ‘chain of cause and effect’ leading to either persistent deformation or elastic deformation. This study is the first of a series of three, where effects of contact stress and soil conditions on the stress distribution in the soil profile were evaluated. Measurements of vertical stresses were performed in an undisturbed Stagnic luvisol in three dimensions during one pass of one wheel (in depth, across and along the driving direction). In the present study, we tested the effect of loosening the topsoil by ∼20 cm ploughing on the stress distribution in the soil profile. The reference soil had not been ploughed or otherwise tilled for 18 months. The distribution of vertical stress near the tyre–soil interface was fitted by a model combining a power function and an exponential function for the stresses, respectively, along and across the driving direction (FRIDA model). The tillage-induced reduction in topsoil strength lead to more even stress distribution at the tyre–soil interface but did not significantly affect the measured vertical stresses at 0.3, 0.6 and 0.9 m depth. The vertical stresses at 0.3 m depth were equivalent to the peak stresses measured in the contact area between tyre and soil (approx. 200 kPa) and appeared more scattered when the top 0.2 m had been recently ploughed. Taking the FRIDA-estimated vertical stresses in the tyre–soil interface as input to the analytical Söhne model, vertical stresses in the subsoil were underestimated for both treatments. The precompression stress of the topsoil for this arable soil was much lower than the subsoil even for the treatment not tilled for 18 months. Hence, the vertical heterogeneity of soil for both treatments did not obey the model assumptions of isotropic soil properties. However, the model performed well from 0.3 to 0.9 m depth. Hence, an alternative to the model of Söhne is needed for the calculation of stress transmission in the frequently tilled topsoil of arable soils from the tyre–soil interface.
    Keywords: Stress Propagation ; Agricultural Soil ; Söhne Model ; Naturally Structured Soil ; Agriculture
    ISSN: 0167-1987
    E-ISSN: 1879-3444
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  • 10
    Language: English
    In: Soil & Tillage Research, 2011, Vol.114(2), pp.71-77
    Description: ▶ A precise description of the contact stresses is important for calculation of the stress propagation in the soil profile: vertical stresses in the upper subsoil (0.3 m depth) were influenced by the contact stresses. ▶ Vertical stresses in the deeper subsoil (0.9 m) correlated closely to the wheel load: vertical stresses in the soil profile are additive, which is an implication of the principle of elasticity. ▶ A practical implication from our results is that the (ever) increasing weight of agricultural machinery will inevitably increase the stresses reaching deep subsoil layers. ▶ The Söhne model underestimated stresses in the upper subsoil and overestimated stresses in the deeper subsoil. We urgently need increased quantitative knowledge about stress transmission in real soils that suffer heavy loads of agricultural machinery. 3D measurements of vertical stresses under tracked wheels were performed in an annually ploughed Stagnic Luvisol continuously cropped with small grain cereals. The tests took place in the spring at field capacity when the topsoil had not been tilled for 1 1/2 years. Two Nokian ELS Radial-ply tyres (800/50R34 and 560/45R22.5) were loaded with two specific loads (30 kN and 60 kN), leading to four treatments labelled 800-30, 800-60, 560-30 and 560-60. We used rated tyre inflation pressures for traffic in the field (≤10 km h driving speed). Seven load cells were inserted horizontally from a pit with minimal disturbance of soil at each of three depths (0.3, 0.6 and 0.9 m), covering the width of the wheeled area. The position of the wheel relative to the transducers was recorded using a laser sensor. Finally, the vertical stresses near the tyre–soil interface were measured in separate tests by 17 stress transducers across the width of the tyres. The level of maximum stress at 0.3 m depth was related to the surface-related stress expressions like the mean ground pressure and the tyre inflation pressure. The maximum stresses measured at 0.9 m depth were correlated with the wheel load (57 and 60 kPa at 60 kN load; 27 and 25 kPa at 30 kN load) and did not reflect the surface-related stress expressions. Our results show that the use of wide, low pressure tyres (within the technical opportunities available today) has no real effect on the stresses reaching deep subsoil layers. Our results further support the principle behind the elasticity theory. However, if fitting the Söhne model to stress measurements at all three depths, the stresses were underestimated at 0.3 and 0.6 m depth, and overestimated at 0.9 m depth. A fit of the model based on data only at 0.3 m depth indicates that stresses were transmitted nearly without attenuation through the 0–0.3 m soil layer, which cannot be described by the model of Söhne. We thus interpret the poor fit for the total profile as being due to differences in strength for the frequently tilled topsoil and the subsoil. Our results thus qualitatively confirm the principle of elasticity, but highlight the need to model arable soil as a two-layer system.
    Keywords: Stress Transmission ; Tyre Size ; Tyre Inflation Pressure ; Contact Area ; Wheel Load ; Söhne Model ; Agriculture
    ISSN: 0167-1987
    E-ISSN: 1879-3444
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