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Agricultural topsoils are characterized by a heterogeneous mixture of structural units with a considerable difference in soil mechanical properties. Mechanistic modeling of compaction for this layer is challenging because of the need to parameterize model input parameters. This study quantified soil deformation during loading in semi-confined conditions and searched for drivers decisive for observed strains with no a priori assumptions of material properties. Undisturbed soil samples were collected in the plough layer of eleven sandy and loamy soils in Denmark. The samples were adjusted to either of the following matric potentials: − 30, − 50, − 75, − 100, − 160 and − 300 hPa (pF from ∼1.5 to ∼2.5). Cores at each water condition were loaded with an annulus covering one third of the soil surface at either of the following normal loads: 30, 60, 90, 120, 150, 180 kPa. Multiple regression was performed to estimate the best model describing the variation in soil compressibility. Measured strain, ε, decreased with increase in soil organic matter, bulk density and pF, but the effect of pF was affected by soil clay content. The model explained ∼84 % of the variation in data and predicted well measured strain for two independent data sets. The regression model is suggested for prediction of soil deformation of arable topsoils in field traffic. A procedure is described for the prediction, including calculation of the relevant stress based on loading characteristics.

Intensive tillage, non-diversified crop rotations and reduction in the return of organic residues to soil imply decreasing levels of soil organic carbon (SOC). Straw removal for (bio)energy production may increase in the future, given the attempts to phase out fossil energy sources. The decreasing SOC contents may deteriorate soil structure with adverse effects on soil ecosystem services. In this study, the long-term (28–36 years) effect of repeated experimentation with removal, burning or incorporation of straw was quantified at a coarse sandy and two sandy loam soils. At the end of experimentation, remolded soil was sampled and analyzed for SOC and a range of other chemical characteristics. Undisturbed soil cores were collected in the plough layer (10 cm depth) and the upper subsoil (30 cm depth). The cores were drained to a range of matric potentials (ψ), and air permeability measured at ψ = -30 and ψ = -100 hPa. At all three locations, incorporation of straw increased SOC by ∼ 13% and ∼ 15% relative to removal and burning, respectively. Leaving straw in the field (chopped or burned) increased exchangeable potassium found in the soil layer 0–80 cm depth with approximately 110–400 kg per hectare, highest for the most clay-holding soil. Addition of straw decreased soil dry bulk density and increased the volume of soil pores holding water available for plants (0.2–30 µm), while no significant effect was found for pores>30 µm. For the two sandy loam soils, air permeability tended to be highest for soil with incorporation of straw. For one of the sandy loam soils, the effect was significant when normalized to unit volume of air-filled pore volume. No effects of straw management were found on the volume of soil pore fractions in the subsoil. However, the measurements of air permeability indicated more continuous pores for straw-amended soil in the generally dense plough pan layer. The long-term effects of straw may affect important soil functions and ecosystem services.

Combining digital imaging, physical models and laboratory measurements is a step further towards a better understanding of the complex relationships between the soil pore system and soil functions. Eight natural 100-cm3 soil cores were sampled in a cultivated Stagnic Luvisol from the topsoil and subsoil, which we assumed had contrasting pore systems. Artificial 100-cm3 cores were produced from plastic or from autoclaved aerated concrete (AAC). Eight vertical holes of each diameter (1.5 and 3 mm) were drilled for the plastic cylinder and for one of the two AAC cylinders. All natural and artificial cores were scanned in an X-ray CT scanner and printed in 3D. Effective air-filled porosity, true Darcian air permeability, apparent air permeability at a pressure gradient of 5 hPa and oxygen diffusion were measured on all cores. The active pore system characteristics differed between topsoil (sponge-like, network of macropores of similar size) and subsoil (dominated by large vertical macropores). Active soil pore characteristics measured on a simplified pore network, that is, from artificial and printed soil cores, supported the fundamental differences in air transport by convection and diffusion observed between top- and subsoil. The results confirm the suitability of using the conceptual model that partitions the pore system into arterial, marginal and remote pores to describe effects of soil structure on gas transport. This study showed the high potential of using 3D-printed soil cores to reconstruct the soil macropore network for a better understanding of soil pore functions.

Subsoil compaction caused by heavy traffic affects the soil pore system, resulting in long-term damage to soil functions. The study contrasted two treatments from compaction experiments conducted at three different sites in Denmark: non-trafficked control soil and soil subjected to four annual traffic events (2010–2013) with a wheel load of 58 to 78 kN. A cover crop of fodder radish (Raphanus sativus L.) was grown in half of the initial experimental plots after completion of the compaction treatments (2013 and onwards). In the spring of 2017, undisturbed soil cores were sampled at 0.3 and 0.5 m depth. The air-filled porosity (ε a), air permeability (k a) and gas diffusivity (D s/D o) were quantified for samples equilibrated to –100 hPa matric potential. Soil pore structural indicators were estimated from the combination of ε a, k a, and D s/D o. The ratio of non-Darcian to Darcian k a (R) was also used as a pore morphology indicator. For all sites and depths, compaction reduced ε a, D s/D o, k a-Darcy, PO1 (k a-Darcy/ε a) and the effective radius ([(8k a-Darcy)/D s/D o] 0.5) compared to control soil (p s/D o and ε a tended to be smaller for compacted soil, significantly for one of the sites. Compacted soils were also characterised by a significantly smaller R-ratio at high levels of k a-Darcy (> 32 μm 2), but also by having a tendency for the R-ratio to decrease rapidly with increasing pore air velocity compared to the control. The results reflect a compaction-induced reduction in the number of marginal pores connected to large arterial pores, promoting a simple pore system formed by continuous vertical pores. The compaction effect was not affected by the cover crop. Neither natural recovery nor fodder radish-induced mitigation of soil compaction was evident for the studied soils.

Soil thermal conductivity, λ, is decisive for a range of soil ecosystem services related to soil temperature. Soil functions affected by temperature comprise microbial as well as abiotic processes of importance to crop growth, leaching of nutrients, carbon sequestration, and emission of greenhouse gases. Models for crop production and climate change include λ for simulation of heat transfer in the natural environment. Nevertheless, most prediction equations for λ are based on measurements deriving from physically disturbed soil. We identified nine Danish soils with a gradient in clay from ~ 0.03 to 0.35 kg kg−1 and measured λ in situ at six soil depths from 5 to 85 cm with the heat pulse method. Soil cores were collected and used for measurements of λ at a range of moisture conditions in the laboratory. The degree of soil water saturation, S, ranged from 0.01 to 0.98, and measured λ was in the range 0.18–2.98 W m−1 K−1. A model for λ identified by multiple regression across all laboratory measurements explained 87% of the variation in data and pointed out a convex polynomial relation between S and λ. It included significant positive effects of bulk density (BD) and soil organic matter (SOM), while λ decreased with soil clay content for a given S. The stochastic model predicted λ for three independent data sets from the literature with little bias and root mean square errors in the range 0.15–0.23 W m−1 K−1. A polynomial regression of λ versus S was performed for combinations of site and soil depth. By this λ was estimated at completely dry conditions, λdry, and at the fully saturated state, λsat. The estimates of λdry as well as λsat had a wider range than reported in the literature. This study calls for re-evaluation of existing pedotransfer functions for λdry. Measurements of λ in soil of undisturbed structural conditions are encouraged.

Soil compaction caused by the use of heavy agriculture machinery in non-optimal soil conditions hampers crucial soil functions. Tyre inflation pressure and wheel load are well-known key drivers of compaction. The effects of traction and repeated wheeling are, however, not yet fully understood but may be of critical importance. We aimed to quantify the effects of traction and repeated wheeling on some soil structural properties independently in a compaction experiment conducted on a sandy loam at a soil water content near field capacity, using a tractor-trailer combination at two loads. The trailer was used in ‘standard’ and ‘offset’ steering modes, the latter referring to the mode in which measurements could be taken for the pass of a single towed and thereby passive trailer wheel (with one, two, three and six wheel passes). Penetration resistance was measured to 0.71 m depth and 100-cm 3 soil cores were collected around 0.16 m depth for structural property measurements. In the offset steering mode, the measurements were made in the tractor tracks to investigate the effect of traction and in the trailer's wheel track to investigate the effect of repeated wheeling (at 6.0 Mg static wheel load). The measurements were also collected for the standard steering configuration with the high towed load and from reference plots. Measurements on the soil cores comprised bulk density, porosity, and air permeability. We found a clear effect of traction on deformation of some soil structural properties, with no significant effects of the low drawbar pull but with substantial effects of the high drawbar pull. Reductions in air permeability and specific permeability were significant (89 % and 83 %, respectively) and indicated a densification and homogenisation of the soil. The effect of repeated wheeling was gradual and reasonably well explained by a linear fit to the number of wheel passes. After six passes with the passive wheel caused some soil structural properties to differ significantly from the reference soil, and resulted in approximately similar levels of bulk density, porosity, air permeability and specific permeability as the tractor with high drawbar pull. Still, the deformation was stronger for the single pass of the tractor with the high drawbar pull than for six repeated wheeling of the passive trailer wheel. These results highlight the substantial effect of traction on soil compaction. Yet, the mechanisms causing this deformation remain speculative until the propagation of horizontal stress through the soil profile is better understood.

Distortion of soil from shear stresses during field traffic may be even more damaging to soil functions than compression related to isotropic stresses. A torsional shear test was applied to undisturbed topsoil samples drained to either of six matric potentials (range -30 to -300 hPa) and tested at either of six normal loads, NL (range 30–180 kPa)(Series A). Soil clay and organic matter (SOM) content ranged 0.038−0.157 and 0.021−0.033 kg kg−1, respectively. Bulk density (BD) varied from 1.19 to 1.71 g cm-3. The preload suction stress, PSS, was calculated as the product of soil pore water saturation and the water suction in hPa. PSS ranged from 17 to 234 hPa. Data from two independent data sets with contrasting soil texture (Series B: loess soil) or management (Series C: long term fertilization) but tested with the same methodology were included for validation of prediction equations established from Series A soils. Multiple regression revealed that soil cohesion taken as the intercept term in a linear regression of shear strength, τ, and applied NL correlated positively to PSS, BD and SOM, while the internal friction estimated as the slope in regression was poorly explained by soil properties. A model combining NL in tests with PSS, clay, BD and SOM accounted for more than 90 % of the variation in τ and is suggested as a pedotransfer function (ptf) for prediction of τ for given soil and loading conditions. The model predicted well τ measured in Series B and C soils. Also subsoil τ measured for similarly textured samples was reasonably predicted by the suggested ptf. In accordance with theory, PSS seems to be a key driver of soil mechanical strength. More measurements of soil shear strength and further studies of the effect of PSS are encouraged.

Soils are the foundation for agricultural production, ecosystem functioning and for human well-being, but paradoxically only limited attention has been given towards sustainable soil management in most national and European policies. Preventing subsoil compaction is essential for ensuring soil functions and ecosystem services, because subsoil compaction is virtually persistent, reduces yields, as well as increases greenhouse gas emissions and the leaching of pollutants. However, it is also challenging as the subsoil compaction risk is dynamic and difficult for stakeholders to observe and address. Hence, this article explores the drivers of soil degradation and discuss the opportunities and barriers for a sustainable governance of the soil resource, based on a case study of subsoil compaction in Danish farming. The article draws on a mixed method case study incorporating qualitative and quantitative elements. Findings suggest that current agricultural practice entails a large risk for subsoil compaction, particularly manure distribution and harvest operations. While farmers, in general, are concerned about the effect of the agricultural practice on their fields, a number of barriers prevent them from addressing subsoil compaction. These include knowledge deficit, technological barriers, responsibility outsourcing, pragmatic tradeoffs, as well as the systemic and wicked problematic nature of subsoil compaction. Hence, we argue for a systemic response including: 1) Competence development, 2) visualization of the compaction risk, 3) changing incentives of field practice, 4) technological innovation and 5) a policy framework. This could systemically address the subsoil compaction risk, integrating the multiple factors that influence how farmers decide on their practices.

Traffic in agricultural fields with very high wheel loads imposes a risk of severe structural damage deep into the subsoil. However, there is a paucity of studies quantifying these effects. This study focuses on heavy traffic-induced changes in soil structure for a sandy loam soil in a temperate region. The treatments included no compaction (Control), compaction with ∼3 Mg (M3) and ∼8 Mg (M8) wheel loads with multiple (4–5) wheel passes, and compaction with a single-pass wheel load of ∼12 Mg (S12). The compaction treatments were replicated four consecutive years. Subsoil structural quality was evaluated visually by the SubVESS method, and soil pore characteristics were quantified for minimally disturbed soil cores sampled at 30, 50, 70 and 90 cm depth two years after the end of the experiment. Our results indicate that M8 significantly affected soil structural properties to >50 cm depth in terms of reduced subsoil structural quality, air-filled pore space, air permeability, gas diffusivity, pore volume and increased bulk density. Results also showed that the degree of compactness was ≥95% for M8 at 30 and 50 cm depth. Even though a pre-existing dense soil matrix was described in the studied soil, results confirmed that high wheel loads may cause significant subsoil compaction at >50 cm depth. Surprisingly, the S12 treatment did not show marked signs of decreasing structural quality at depth. Thus, our results indicate that primarily traffic applying multiple passes with high wheel loads compromises soil structure at depth. The S12 results further suggest the need to investigate the influence of factors other than wheel load and inflation pressure on the risk of subsoil compaction.

Scale is a key issue in soil studies. The idea that a sample must be of adequate size to embody a specific physical property comes from the representative elementary volume (REV) concept that defines minimum sample size for consistent results. Other approaches to describe soil spatial heterogeneity rely on the fractal dimension concept (FDC), which assumes structure changes with scale continuously, and the discrete hierarchy concept (DHC), which assumes change occurs discretely. This study considered using gas transport measurements combined with X-ray computed micro-tomography (μCT) for multi-scale analyses. Specifically, 24 large (“L cores”, 628.3 cm3) core volume samples were collected from two farms and two soil depths (3–11 cm and 20–28 cm) in northeast Italy. Gas transport parameters, such as air-filled porosity, air permeability, and gas diffusivity, were measured on the original cores and on successively sub-sampled medium (“M cores”, 100.4 cm3) and small (“S cores”, 4.7 cm3) cores. X-ray μCT–derived porosity indices were calculated for the two smaller scales. Soil core sub-sampling resulted in reduced soil gas transport property measurements, especially in the deepest depth when related to large and continuous bio-pore decreases in root channels and wormholes. In small core volumes, the pore network was dominated by small isolated pores, which might obstruct gas diffusion at that scale. All three concepts named above could be reconciled with our data. The limited numbers of samples and observation scales hindered identifying which model described soil spatial heterogeneity best. Finally, our results suggested the importance of considering scale effects on soil physical properties and their measurement consistency.

Highlights • The rate of change in soil structural stability and SOM fractions were quantified. • SOC was mainly affected by C input and tillage. • It was faster to lose than to gain SOC. • At macroscale, it was faster to gain than to lose soil structural stability. • SOM fractions were not able to explain the dynamics at macroscale., The combination of concurrent soil degradation and restoration scenarios in a long-term experiment with contrasting treatments under steady-state conditions, similar soil texture and climate make the Highfield land-use change experiment at Rothamsted Research unique. We used soil from this experiment to quantify rates of change in organic matter (OM) fractions and soil structural stability (SSS) six years after the management changed. Soil degradation included the conversion of grassland to arable and bare fallow management, while soil restoration comprised introduction of grassland in arable and bare fallow soil. Soils were tested for clay dispersibility measured on two macro-aggregate sizes (DispClay 1–2 mm and DispClay 8–16 mm) and clay-SOM disintegration (DI, the ratio between clay particles retrieved without and with SOM removal). The SSS tests were related to soil organic carbon (SOC), permanganate oxidizable C (POXC) and hot water-extractable C (HWC). The decrease in SOC after termination of grassland was greater than the increase in SOC when introducing grassland. In contrast, it was faster to restore degraded soil than to degrade grassland soil with respect to SSS at macro-aggregate scale. The effect of management changes was more pronounced for 8–16 mm than 1–2 mm aggregates indicating a larger sensitivity towards tillage-induced breakdown of binding agents in larger aggregates. At microscale, SSS depended on SOC content regardless of management. Soil management affected macroscale structural stability beyond what is revealed from measuring changes in OM fractions, underlining the need to include both bonding and binding mechanisms in the interpretation of changes in SSS induced by management.

Traffic-induced long-term damage to agricultural subsoils is a serious threat in modern, mechanized agriculture. Soil failure due to shear stresses is often not considered in soil compaction models but probably contributes significantly to soil structure deterioration. We measured shear strength for a total of 720 undisturbed soil cores collected at nine different locations in Denmark. Soil clay content ranged from 0.025 to 0.375 kg kg−1. We sampled at soil depths 0.3, 0.5 and 0.8 m and drained the soil cores to either -50, -100 or −300 hPa matric potential prior to shear tests. We used a shear annulus device to apply shear stress to the soil cores. The normal load, NL, in tests was either 30, 60, 90, 120, 150 or 180 kPa. Soil shear strength, τ, was estimated as the peak (maximum) shear stress at soil failure. Soil cohesion and angle of internal friction was estimated from linear regression of τ and NL. Multiple regression indicated that soil cohesion was well predicted by soil organic matter, clay content, the initial (preload) suction stress, σw, and soil bulk density. σw was calculated from water saturation and the matric potential. It was superior to quantity expressions of soil water (volumetric water content and water ratio) in explaining the trends in soil cohesion. Quantity of soil water should not be used in prediction models across soil types. The angle of internal friction correlated poorly to soil properties. Soil shear strength at a given NL could be well described by a model combining the above soil properties with the NL. This pedotransfer function predicted reasonably well the measured shear strength from two independent data sets. More studies are needed to evaluate a range of methodological aspects and for inclusion of more clay-holding soils. We encourage the inclusion of soil shear failure prediction in soil compaction models and suggest a specific procedure for this.

Highlights • The rate of change in pore size distribution was quantified. • It was faster to degrade than to restore a complex soil structure. • Introducing grassland in degraded soil may induce densification in the short-term., Changes in land use affect the pore size distribution (PSD) of the soil, and hence important soil functions such as gas exchange, water availability and plant growth. The objective of this study was to investigate potentially damaging and restorative soil management practices on soil pore structure. We quantified the rate of change in PSD six years after changes in land use taking advantage of the Highfield land-use change experiment at Rothamsted Research. This experiment includes short-term soil degradation and restoration scenarios established simultaneously within long-term contrasting treatments that had reached steady-state equilibrium. The land-use change scenarios comprised conversion to grassland of previously arable or bare fallow soil, and conversion of grassland to arable and bare fallow soils. In the laboratory, we exposed intact soil cores (100 cm3) to matric potentials ranging from −10 hPa to -1.5 MPa. Based on equivalent soil mass, the plant available water capacity decreased after conversion from grassland, whereas no change was observed after conversion to grassland. Structural void ratio decreased after termination of grassland and introduction of grassland in bare fallow soil, while no change was seen when changing arable to grassland. Consequently, it was faster to degrade than to restore a complex soil structure. The study illustrates that introducing grassland in degraded soil may result in short-term increase in soil density.

Subsoil compaction is persistent and affects the wide diversity of ecological services provided by agriculturalsoils. Efficient risk assessment tools are required to identify sustainable agricultural practices. Vehicles shouldnot transmit stresses that exceed soil strength. Wheel load is the primary source of high stress in the subsoil.However, very low contact stress without reduction of wheel load would also help reduce stress in the subsoil.The aims of our study were to: (i) test experimentally the use of tracks instead of tires as a technical solution toincrease contact area and reduce the magnitude of contact stresses, (ii) compare effects of traffic on soil physicalproperties using tires or tracks, and (iii) evaluate a state-of-the-art method for risk assessment of soil compactionbeneath tracks or tires at the European level. We measured contact stress below a fully-loaded sugar beet harvesterequipped with either a large tire or with a rubber track in a realistic harvest situation. Seventeen stresstransducers were installed across the driving direction at 0.1m depth and covered with loose soil. Dry bulkdensity and air permeability were measured at 0.35m depth after traffic. The contact area was larger and themaximum and vertical stress smaller beneath the rubber track than beneath the tire. Nevertheless, stress distributionbeneath the rubber track was far from uniform, presenting high peak stresses beneath the wheels androllers. Dry bulk density was similar after traffic for the two undercarriage systems, but air permeability waslower after traffic using the rubber track. Measured stress distributions beneath the tire and the track were usedas input to calculate the soil profile vertical stress for comparison with soil strength at 0.35m depth. Wheel loadcarrying capacity was calculated for European soils for assessment of subsoil compaction risk when using thetire, the rubber track, and the rubber track assuming an even stress distribution. As expected from the contactarea and stress measurements, the rubber track could carry higher loads than the tire. However, the air permeabilityresults are interpreted as soil distortion due to high shear forces under the rubber track. This calls for afurther development of the risk assessment method.

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, σpc, and to develop pedotransfer functions for prediction of σpc. We revisited previously published data on σpc for a silty clay loam soil at a range of soil matric potentials. σpc 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 σpc data. For the plough layer, σpc increased with bulk density (BD), which explained 77% of the variation. For the subsoil layer just beneath the ploughing depth, the model best describing σpc data included the drivers BD and pF, with pF defined as the log to the negative matric potential. The model was strongly significant with R2=0.90. The same trend was found for three subsoil layers from 0.35–0.95m depth, but the model accounted for only 16% of the variation in σpc. 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 σpc to be constant at pF ~2 across soil clay contents for a given soil BD. For pF < 2, σpc was predicted to be higher for sandy soils than for soils rich in clay. In contrast, σpc 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 σpc for a range of soil types, soil horizons and soil moisture conditions.

Assessments of changes in soil organic carbon (SOC) stocks depend heavily on reliable values of SOC content obtained by automated high‐temperature C analysers. However, historical as well as current research often relies on indirect SOC estimates such as loss‐on‐ignition (LOI). In this study, we revisit the conversion of LOI to SOC using soil from two long‐term agricultural field experiments and one arable field with different contents of SOC, clay and particles

Reducing wheel loads has long been the key advice to reduce the risk of subsoil deformation, but this disregards other machinery-soil interactions such as the effects of traction and repeated wheeling. We conducted a field experiment to disentangle the effects of traction (described as drawbar pull) and repeated wheeling on the contact area characteristics and stresses in the upper subsoil. Experimental traffic comprised a tractor (static load 11 Mg) with activated 4 WD towing a trailer (static load 17 Mg or 24 Mg) and took place on a sandy loam with stubble of oats at soil water content near field capacity. Measurements included drawbar pull, contact area and vertical contact stress, and horizontal (in the driving direction) and vertical stresses at ∼0.36 m depth in an undisturbed soil profile. Drawbar pull was significantly higher (9.13 kN compared to 6.46 kN) for the high trailer load, but no differences were observed between the two steering modes. The contact area of the tractor’s rear tyres increased for the high drawbar pull but with no significant differences in length or width. The maximum vertical stress in the tractor rear tyre’s contact area then tended to be lower, despite the increase in the dynamic wheel load (from 3.5 Mg to 4.1 Mg). Whereas high drawbar pull improved the stress distribution in the driving direction, the effect across the tyre was complicated. We found evidence of different tyre-soil interaction for tyres with and without traction. No significant effect of repeated wheeling with a single towed tyre (5.5 Mg) on the contact area characteristics were found. For the towed tyres, horizontal soil stress increased linearly with vertical soil stress, and we suggest that this increase is intrinsic to the soil (at these experimental conditions). Traction does then influence the ratio of horizontal to vertical stress. The results confirm the importance of considering dynamic aspects of field traffic such as traction and dynamic wheel load.

Subsoil compaction is one of the majorcauses of land degradationworldwide and therefore a major threat tofuture crop productivity. The objective of this contribution was to evaluate the effects of compactiontreatments on soil structure based on the numerical visual evaluation of subsoil structure (SubVESS)method and on quantitative measurements of soil pore characteristics. The effect of soil compaction wasevaluated using treatments from a compaction experiment initiated in 2010 at Research Centre Flakkebjerg,Denmark, on a sandy loam soil usingfive levels of compaction. In this study we used i) non-compactedreference, ii) Treatment M3, where soil was subjected to multiple passes (five wheel passes per compactionevent annually) of a tractor-trailer combination with max. wheel load of3 Mg, and iii) M8, with multiplepasses (four wheel passes per compaction event annually) of a tractor-trailer combination with max.wheel load of8 Mg. The tire inflation pressure was generally above the recommended pressure in order tomimic the inflation pressures commonly used in practice. The treatments were applied track-by-track in thespring of 2010–2013 when the soil water content was close tofield capacity. Spring barley (Hordeum vulgareL.) was established every year after a shallow secondary tillage to0.05 m depth to loosen the uppermostlayer. Sampling andfield evaluation were done on May 7, 2014, i.e. after four years of compactiontreatments (2010–2013) and one year of recovery. The soil profiles were evaluated at the same time as soilcores were sampled at 0.3, 0.5 and 0.7 m depth. In the laboratory, we measured water content, totalporosity, air-filled porosity (ea), air permeability (ka) and calculated pore organization indices (PO1 = ka/eaand PO2 = ka/ea2) on the soil cores. We estimated the blocked air-filled porosity and pore continuity indexfrom the relationship between air permeability and air-filled porosity for30 to300 hPa matricpotentials. Assessment using the SubVESS method showed a marked effect of the M8 treatment on soilstructural quality down to0.65 m depth, but the effects of the M3 were not significantly differentfrom the control at any depth. This was confirmed by the laboratory-measured data, which showed thatthe M8 treatment drastically reduced total porosity, air-filled porosity, air permeability, pore sizedistribution, pore tortuosity and continuity, especially at 0.3 and 0.5 m depths.Detailed measurements of the anisotropy of soil pore characteristics at 0.3–0.4 m depth showed that forPO2 (pore size distribution) and blocked air-filled porosity the control soil was significantly anisotropic.Although compaction with the8 Mg wheel load affected the vertically and horizontally-oriented poresdifferently, it did not significantly affect the anisotropy of the different pore characteristics. Our resultsshowed that in general, there was a good agreement between thefield and laboratory methods and thus,the two can be combined to evaluate the effects of compaction in the subsoil. Subsoil compaction is one of the major causes of land degradationworldwide and therefore a major threat to future crop productivity. The objective of this contribution was to evaluate the effects of compaction treatments on soil structure based on the numerical visual evaluation of subsoil structure (SubVESS) method and on quantitative measurements of soil pore characteristics. The effect of soil compaction was evaluated using treatments from a compaction experiment initiated in 2010 at Research Centre Flakkebjerg, Denmark, on a sandy loam soil using five levels of compaction. In this study we used i) non-compacted reference, ii) Treatment M3, where soil was subjected to multiple passes (five wheel passes per compaction event annually) of a tractor-trailer combination with max. wheel load of 3 Mg, and iii) M8, with multiple passes (four wheel passes per compaction event annually) of a tractor-trailer combination with max. wheel load of 8 Mg. The tire inflation pressure was generally above the recommended pressure in order to mimic the inflation pressures commonly used inpractice. The treatments were applied track-by-track in the spring of 2010–2013 when the soil water content was close to field capacity. Spring barley (Hordeum vulgare L.) was established every year after a shallow secondary tillage to 0.05 m depth to loosen the uppermost layer. Sampling and field evaluation were done on May 7, 2014, i.e. after four years of compaction treatments (2010–2013) and one year of recovery. The soil profiles were evaluated at the same time as soil cores were sampled at 0.3, 0.5 and 0.7 m depth. In the laboratory, we measured water content, total porosity, air-filled porosity (ea), air permeability (ka) and calculated pore organization indices (PO1 = ka/ea and PO2 = ka/ea 2) on the soil cores. We estimated the blocked air-filled porosity and pore continuity index from the relationship between air permeability and air-filled porosity for 30 to 300 hPa matric potentials. Assessment using the SubVESS method showed a marked effect of the M8 treatment on soil structural quality down to 0.65 m depth, but the effects of the M3 were not significantly different from the control at any depth. This was confirmed by the laboratory-measured data, which showed that the M8 treatment drastically reduced total porosity, air-filled porosity, air permeability, pore size distribution, pore tortuosity and continuity, especially at 0.3 and 0.5 m depths. Detailed measurements of the anisotropy of soil pore characteristics at 0.3–0.4 m depth showed that for PO2 (pore size distribution) and blocked air-filled porosity the control soil was significantly anisotropic. Although compaction with the 8 Mg wheel load affected the vertically and horizontally-oriented pores differently, it did not significantly affect the anisotropy of the different pore characteristics. Our results showed that in general, there was a good agreement between the field and laboratory methods and thus, the two can be combined to evaluate the effects of compaction in the subsoil.

Soil organic matter (SOM) is important for maintaining soil structural stability (SSS). This study quantified the influence of soil organic carbon (SOC) and different organic matter components on various SSS measures. We used a silt loam soil with a wide range of SOC (8.0–42.7 g kg−1 minerals) sampled in spring 2015 from the Highfield Ley-Arable Long-Term Experiment at Rothamsted Research. Four treatments were sampled: Bare fallow, continuous arable rotation, ley-arable rotation, and grass. Soils were tested for clay dispersibility (DispClay), clay-SOM disintegration (DI, the ratio between clay content without and with SOM removal) and dispersion of particles −1 minerals (clay/SOC~10) coinciding with a change from the tilled treatments to the grass treatment. We found a greater influence of SOC, POXC and HWC on SSS at contents below the change point than above. A stronger linear relation between POXC and DispClay compared to SOC and HWC suggests that POXC was a better predictor of the variation in DispClay. POXC and HWC were less related to DI than SOC. The grass treatment had a very stable structure, shown in all SSS tests, probably due to the absence of tillage and large annual inputs of stabilizing agents. This suggests that a change in management from arable rotation to permanent grass is one effective tool for improving SSS.

Important soil pore characteristics may be revealed from air permeabilitydata. Recent research has quantified significant bias in estimates of the trueDarcian permeability when frequently reported pneumatic pressure differencesto drive the convective flow are used. An alternative to measurementat infinitesimal pressure differences is the Forchheimer approach, includinga polynomial regression of corresponding values of the superficial air velocity(v) and the pressure gradient (G) applied. However, in situations withDarcian flow at low pressure gradients, this procedure may theoretically givean overestimation of the Darcian permeability. We constructed sample plasticcores (?3.5 cm high) with different numbers (n = 1–19) of drilled tubes(holes of 1, 2, 4.5, and 5.8 mm diameter). Gas diffusivity was measured witha transient-state method. Air permeability was measured at four pneumaticpressure differences (0.5, 1, 2, and 5 hPa), and the Darcian permeabilitywas estimated with the Forchheimer approach. The ratio of apparent permeabilityat 5 hPa pneumatic pressure difference to the Forchheimer-estimatedDarcian permeability was significantly lower than unity (0.06–0.52) for alltest samples. For all 1- and 2-mm hole samples and for all four levels of G,the Reynolds number indicated nonturbulent flow, whereas turbulence waspredicted for samples with 4.5- and 5.8-mm diameters. A model combiningrelative gas diffusivity, air permeability, and the space available for gas transportindicated that the Forchheimer estimates of Darcian permeability werecorrect in situations with nonturbulent flow, whereas erroneous estimateswere generated for the larger tubes with turbulence.

Mechanical stresses from agricultural machinery affect subsoil layers, influencing pore systems and hence soil processes. The low resilience of the inflicted plastic deformation necessitates a better understanding of the impacts on soil functions and the risk of compromised soil ecosystem services. Soil samples were collected at 0.3-, 0.5-, 0.7-, and 0.9-m depths in a sandy loam subjected to repeated high wheel loads during 4 yr of slurry application at a water content close to field capacity. The 100-cm(3) soil samples were drained successively to matric potentials of -30 and -100 hPa, in which air permeability was measured via the Forchheimer approach, including estimation of apparent permeability (k(app)) at four pneumatic pressure gradients. For all soil depths, the apparent permeability at 5 hPa pneumatic pressure for both control and compacted soil was significantly lower than the true Darcian permeability (k(Darcy)) derived from the relationship between the superficial air velocity and the pressure gradient. For high permeabilities, the ratio R (k(app)/k(Darcy)) was generally lower than 0.3. This ratio was lower in compacted soil than in the control soil, significantly so for the 0.3-m depth. For this depth, the decrease in R with increases in the average pore air velocity was more pronounced and a regression model explained more of the variation in data for compacted than for control soil. We consider that severe soil compaction may reduce the complexity of the subsoil pore system, closing a considerable part of the marginal pores branching from vertical (arterial) biopores.

Accurate estimation of stress transmission in soil and quantification of compaction-induced soil pore structure is important for efficient soil use and management. Continuum mechanics have so far mostly been applied for agricultural soils, even if topsoil structure is aggregated due to regular tillage. In this study, partially confined uniaxial compression tests were carried out on intact topsoil columns placed on subsoil columns. Two methods were employed for estimation of stress transmission in soil: (i) soil deformation patterns were quantified using X-ray CT and converted to stress distributions, and (ii) a tactile sensor mat was employed for measuring stresses at the interface of the topsoil and subsoil columns. The resulting soil pore structure under applied stresses was quantified using X-ray CT and by air-permeability measurements. In topsoil discrete stress transmission patterns were observed at 275 kPa applied stress, whereas continuum-like stress transmission was observed at 620 kPa. At the interface of topsoil and subsoil, discrete stress transmission patterns were observed at all applied stresses ranging from 68 to 620 kPa, but it was less discrete as we moved from lower to higher applied stresses. Our results imply that at lower stresses the stress transmission in arable soil was discrete because the applied load was mainly transmitted through chain of aggregates. At higher applied stresses the soil/aggregates deformed and to a larger degree resembled a continuous material where continuum-like stress transmission patterns were observed. We found that continuum stress transmission patterns were well simulated with models based on the elasticity theory (e.g., Boussinesq, 1885 ) compared to discrete stress transmission patterns. The soil pore structure was affected by increasing applied stresses. Total porosity was reduced 5–16% and macroporosity (pores > 0.5 mm) 50–85% at 620 kPa for topsoils. For subsoils – serving here as the material underlying the topsoil tests columns – only a small decrease was observed in both total porosity and macroporosity. Air permeability was reduced 55–80% for topsoils and 10–20% for subsoils at 620 kPa stress.

In a concurrent paper, we compared a continuum model and a discrete element method (DEM) model in simulating stress transmission in soil under a wheel. Here, those models are evaluated with measurements of vertical normal stress (σz) under the wheels of a tractor-slurry spreader setup. It was found that the variation in the measured σz could be explained by the heterogeneous stress distribution in our structured soil, similar to what is observed in the DEM simulation. Furthermore, comparison of the continuum and DEM model showed that the lack of horizontal forces and dynamic load transfer at the boundary condition in the continuum model lead to a systematic underestimation of the measured σz. Traction and drawbar forces have a significant impact on the stress state under a wheel. The continuum model and its boundary conditions should be modified to include these forces accurately.

Most models describing soil water retention imply a uni-modal pore-size distribution (PSD). The uni-modal model presented by van Genuchten (termed vanG) is widely used although double-exponential models (termed Dex) implying a bi-modal PSD may better reflect reality. We tested the ability of vanG and Dex models to represent water retention in sandy top- and subsoils with different texture, in soil with contrasting management (Highfield), and in soil exposed to different tillage (Flakkebjerg). Soils were subjected to matric potentials from –10 hPa to –1.5 MPa. For all soils, the bi-modal Dex model showed a better fit to water retention data than the uni-modal vanG model. Neither of the models worked well for highly sorted soils. The vanG model gave a poorer fit for topsoils than for subsoils because of a more pronounced bi-modality of the PSD in topsoils caused by larger soil organic carbon (SOC) content and tillage. For Highfield soils, the root mean squared error (RMSE) of the vanG fit increased from long-term bare fallow (low C content, intensive tillage) to permanent grass (high C content, no tillage) reflecting a more distinct bi-modality of the PSD for well-structured soils. We conclude that uni-modal models should be used with great caution when describing effects of texture and management on PSD and that bi-modal models may provide a better fit to PSD.

The upper boundary condition for all models simulating stress patterns throughout the soil profile is the stress distribution at the tyre–soil interface. The so-called FRIDA model (Schjonning et al., 2008. Biosyst. Eng. 99, 119–133) treats the contact area as a superellipse and has been shown to accurately describe a range of observed vertical stress distributions. Previous research has indicated that such distributions may be predicted from tyre and loading characteristics. The objective of this study was to establish a stepwise calculation procedure enabling accurate predictions from readily available data. We used multiple regression to identify equations for predicting the FRIDA model parameters from measured loading characteristics including tyre carcass volume ( V T ), wheel load ( F W ), tyre deflection ( L ), and an expression of tyre inflation pressure ( K r ) calculated as the natural logarithm of the actual to recommended inflation pressure ratio. We found that V T and K r accounted for nearly all variation in the data with respect to the contact area. The contact area width was accurately described by a combination of tyre width and K r , while the superellipse squareness parameter, n , diminished slightly with increasing K r . Estimated values of the contact area length related to observed data with a standard deviation of about 0.06 m. A difference between traction and implement tyres called for separate prediction equations, especially for the contact area. The FRIDA parameters α and β , reflecting the tyre’s ability to distribute the stress in the driving direction and in the transversal direction, respectively, increased with increases in the relevant contact area dimension (length or width). The α -parameter was further affected by F W , while K r and L added to model performance for the β -parameter. The prediction accuracy of our models was tested on an independent data set and through a range of case studies. We found satisfactory small root mean square errors and effectively no bias in the comparisons. Further studies are needed, though, to quantify effects of topsoil consistencies deviating from those tested in this study.

Soil particle density (Dp) is an important soil property for calculating soil porosity expressions. However, many studies assume a constant value, typically 2.65 Mg m − 3 for arable, mineral soils. Few models exist for the prediction of Dp from soil organic matter (SOM) content. We hypothesized that better predictions may be obtained by including the soil clay content in least squares prediction equations. A calibration data set with 79 soil samples from 16 locations in Denmark, comprising both topsoil and subsoil horizons, was selected from the literature. Simple linear regression indicated that Dp of clay particles was approximately 2.86 Mg m − 3 , while that of sand + silt particles could be estimated to ~ 2.65 Mg m − 3 . Multiple linear regression showed that a combination of clay and SOM contents could explain nearly 92% of the variation in measured Dp. The clay and SOM prediction equation was validated against a combined data set with 227 soil samples representing A, B, and C horizons from temperate North America and Europe. The new prediction equation performed better than two SOM-based models from the literature. Validation of the new clay and SOM model using the 227 soil samples gave a root mean square error and mean error of 0.041 and + 0.013 Mg m − 3 , respectively. Predictions were accurate for all levels of SOM content in the validation data set. The model gave very precise predictions for soils with clay contents lower than 0.3 kg kg − 1 , while a moderate over-prediction was observed for soils very high in clay. Finally, we developed a texture-enhanced curvilinear model that will be useful for predicting Dp of soils with high contents of clay and in particular SOM.

The concept of precompression stress is used for estimating soil strength of relevance to fieldtraffic. It represents the maximum stress experienced by the soil. The most recently developed fitting method to estimate precompression stress (Gompertz) is based on the assumption of an S-shape stress-strain curve, which is not always fulfilled. A new simple numerical method was developed to estimate precompression stress from stress-strain curves, based solely on the sharp bend on the stress-strain curve partitioning the curve into an elasticand a plastic section. Our study had three objectives: (i) Assessing the utility of the numerical method by comparison with the Gompertz method; (ii) Comparing the estimated precompression stress to the maximum preload of test samples; (iii) Determining the influence that soil type, bulk density and soil water potential have on the estimated precompression stress. Stress-strain curves were obtained by performing uniaxial, confined compression tests on undisturbed soil cores for three soil types at three soil water potentials. The new method performed better than the Gompertz fitting method in estimatingprecompression stress. The values of precompression stress obtained from the new method were linearly related to the maximum stress experienced by the soil samples prior to the uniaxial, confined compression test at each soil condition with a slope close to 1. Precompression stress determined with the new method was not related to soil type or dry bulk density. This might be due to a too small range for both parameters, but it may also emphasize the complex effect of soil structure on soil mechanical strength.

Cultivated soils are subject to very high stresses from machinery. This may affect the soil pore system and its processes, soil functions and soil ecosystem services. Compaction experiments were performed on loamy Luvisols at three sites in Denmark: Aarslev, Flakkebjerg and Taastrup. Non-trafficked control soil was compared with soil subjected to four annual traffic events with approximately 3-, 6- or 8-Mg wheel loads from tractor–trailer combinations. A self-propelled machine with a single pass of approximately 12-Mg wheel load was tested at Aarslev. Undisturbed soil cores were sampled at 0.3m depth when the experimental plots had received either 2 years (Flakkebjerg) or 3 years (Aarslev and Taastrup) of repeated compaction treatment. The volume of air-filled pores and air permeability were quantified for soil drained to –100hPa matric potential. Freeze–thaw and dry–wet treatments were applied to soil cores in the laboratory for Aarslev and Taastrup samples. The multipass machinery significantly affected >30µm soil pores and air permeability at wheel loads of ~6 Mg or higher, whereas no or only minor effects could be detected for ~3-Mg wheel loads. Indices combining air permeabilities with air-filled porosities indicated that pore morphological features had also been affected. Estimates of hydraulic conductivity indicated critical conditions for the percolation of excess rainwater for severely compacted soil at Aarslev. Generally, the single-pass machine with a high wheel load did not affect the pores and their function. A dry–wet event was a more effective remediation of compaction than a freeze–thaw treatment. In conclusion, present-day field traffic risks creating a bottleneck soil layer for important soil functions just below the tilled topsoil.

Exact estimates of soil clay (

Soil air exchange is one of the most important soil functions that directly impacts on crop productivity and environment. Generally, conservation agriculture (CA) practices are expected to provide improved soil aeration but contrasting texture-related effects were found in the literature. The aim of this study was to evaluate the effect of CA practices on gas transport characteristics in the silty soils of the Veneto Region (North-Eastern Italy). In 2010, a field experiment comparing CA practices (no-tillage, cover crop and residues retention) to conventional intensive tillage (IT) system was established in four farms located in the Veneto low plain. In fall 2015, 144 undisturbed 100 cm3 soil cores where collected at two different layers (3–6.5 cm and 20–23.5 cm) and analysed for air-filled porosity, air permeability, gas diffusivity and soil structure indices derived. Gas transport measurements highlighted low transmission properties of the silty soils independently from agronomic management. Both air permeability and relative gas diffusivity showed poor aerated conditions being generally

Subsoil compaction is a major problem in modern agriculture caused by the intensification of agricultural production and the increase in weight of agricultural machinery. Compaction in the subsoil is highly persistent and leads to deterioration of soil functions. Wheel load-carrying capacity (WLCC) is defined as the maximum wheel load for a specific tyre and inflation pressure that does not result in soil stress in excess of soil strength. The soil strength and hence WLCC is strongly influenced by soil matric potential (h). The aim of this study was to estimate the seasonal dynamics in WLCC based on in situ measurements of h, measurements of precompression stress at various h and simulations of soil stress. In this work, we concentrated on prevention of subsoil compaction. Calculations were made for different tyres (standard and low-pressure top tyres) and for soil under different tillage and cropping systems (mouldboard ploughing, direct drilling, permanent grassland), and the computed WLCC was compared with real wheel loads to obtain the number of trafficable days (NTD) for various agricultural machines. Wheel load-carrying capacity was higher for the top than the standard tyres, demonstrating the potential of tyre equipment in reducing compaction risks. The NTD varied between years and generally decreased with increasing wheel load of the machinery. The WLCC simulations presented here provide a useful and easily interpreted tool to guide the avoidance of soil compaction.

The transmission of stress induced by agricultural machinery within an agricultural soil is typically modelled on the basis of the theory of stress transmission in elastic media, usually in the semi-empirical form that includes the “concentration factor” (v). The aim of this paper was to measure and simulate soil stress under defined loads. Stress in the soil profile at 0.3, 0.5 and 0.7 m depth was measured during wheeling at a water content close to field capacity on five soils (13–66% clay). Stress transmission was then simulated with a semi-analytical model, using vertical stress at 0.1 m depth estimated from tyre characteristics as the upper boundary condition, and v was obtained at minimum deviation between measurements and simulations. For the five soils, we obtained an average v of 3.5 (for stress transmitting from 0.1 to 0.7 m depth). This was only slightly different from v = 3 for which the elasticity theory-based classical solution of Boussinesq (1885) is satisfied. We noted that the estimated v was strongly dependent on (i) the reliability of stress measurements, and (ii) the upper stress boundary condition used for simulations. Finite element simulations indicated that the transmission of vertical stresses in a layered soil is not appreciably different from that seen in a homogeneous soil unless very high differences in soil stiffness are considered. Our results highlight the importance of accurate stress readings and realistic upper model boundary conditions, and suggest that the actual stress transmission could be well predicted according to the theory of elasticity for the conditions investigated.

Management strategies like organic matter (OM) amendment and mechanical energy inputs are known to influence the soil system. A long-term (13–14-year) field experiment was used to evaluate the effects of these management strategies on soil structural formation, structural stabilization and soil tilth of a sandy loam soil in Denmark. OM was applied as manure and by retention of plant residues (ORG) to be compared with plots dressed only with mineral fertilizer (MIN). The soils were rotovated (ROT), compacted (PAC) or left undisturbed (REF) as split-plot treatments in the main plots with OM management over two years prior to sampling. In two consecutive years, undisturbed soil samples were collected from the 6 to 13 cm soil layer in the field grown with winter wheat to assess soil organic carbon (C) fractions (total organic C, polysaccharide C and microbial biomass C), total organic C and polysaccharide C of 1–2 mm macro-aggregates, bulk density, hyphal length, aggregate stability, clay dispersibility, aggregate tensile strength, direct tensile strength and shear strength. The ease of fragmentation and the torsional shear strength of soil were measured in the field as well. OM application increased all soil organic C fractions. Response patterns of organic C fractions in aggregates were the same patterns as for whole-soil. Polysaccharide C appeared to be an important agent in the aggregation process. The effect of microbial C and fungal hyphae on the aggregation process was not clear. Extensive tillage and traffic produced unfavourable tilth conditions in terms of a greater degree of clay dispersion, lower aggregate stability, higher soil tensile strength and poorer soil fragmentation. OM affected soil reaction to compressive and tensile stresses applied at differing initial bulk densities. The results also indicated the profitability of supplementing the classical laboratory analysis with in situ measurements to better evaluate management effects on soil structure.

Summary Soil structure formation is essential to all soil ecosystem functions and services. This study aims to quantify changes in soil structure and microbial activity during and after field incubation and examine the effect of carbon, organic amendment and clay on aggregate characteristics. Five soils dominated by illites, one kaolinitic soil and one smectitic soil were sieved to 2 mm, and each soil was divided into two parts and one part amended with ground rape shoots (7.5 t ha−1) as an organic amendment. Samples were incubated in the field for 20 months with periodic sampling to measure water-dispersible clay (WDC) and fluorescein diacetate activity (FDA). After incubation, WDC and FDA were measured on air-dried 1–2-mm aggregates. Tensile strength was measured on four aggregate classes (1–2, 1–4, 4–8 and 8–16 mm) and results used to assess soil friability and workability. Intact cores were also sampled to determine compressive strength. During incubation, the amount of WDC depended on soil carbon content while the trends correlated with moisture content. Organic amendment only yielded modest decreases (mean of 14% across all sampling times and soils) in WDC, but it was sufficient to stimulate the microbial community (65–100% increase in FDA). Incubation led to significant macroaggregate formation (>2 mm) for all soils. Friability and strength of newly-formed aggregates were negatively correlated with clay content and carbon content, respectively. Soil workability was best for the kaolinite-rich soil and poorest for the smectite-rich soil; for illitic soils, workability increased with increasing organic carbon content. Organic amendment decreased the compression susceptibility of intact, incubated samples at smaller stress values (

Soil productivity and other soil functions are dependent on processes in the untilled subsoil. Undisturbed soil cores were collected at the 0.3- to 0.4-m depth from a heavy clay soil in Finland subjected to a single heavy traffic event by agricultural machinery three decades before sampling. Untrafficked control plots were used as a reference. Computed tomography (CT) scanning was performed on soil cores at a field-sampled field capacity water content. Gas diffusion and air permeability were measured when the soil cores were drained to −1000 hPa matric potential (air permeability also at −100 and −300 hPa). The air-filled pore space was measured with an air pycnometer and also calculated from mass balance and CT data. Gas diffusion and air permeability were also measured on a straight model tube and on autoclaved aerated concrete. The compaction treatment had not influenced soil total porosity but had significantly lowered the volume fraction of air-filled macropores at the matric potentials investigated. The compacted soil displayed significantly lower air permeability, while gas diffusivity was not affected. Our analyses indicate that this was due to a compaction-induced reduction in the cross-sectional area of vertical, arterial macropores and in the volume of marginal pores branching from these vertical pores. We observed non-Darcian air flow during air permeability measurements and thus suggest the use of a nonlinear regression technique based on measurements at several pressure differences to arrive at true Darcian air permeability. The tests on artificial materials supported the conclusions that the dominating pores in this clayey subsoil are nearly straight, vertical macropores.

Shallow tillage (ST; typically 30 years). Bulk soil was sampled from 0 to ∼20 cm of the MP soil and from the two layers above (‘ST-upper’) and below (‘ST-lower’) ST primary tillage depth. Soil organic carbon (SOC), water content, bulk density, air-filled pore space (ɛa) and air permeability (ka) at the field-sampled water content were determined. ST increased SOC concentration in the ST-upper soil when compared to MP, whereas no difference between ST and MP was found for the ST-lower soil. When based on equivalent soil masses, the quantity of SOC did not differ between ST and MP. Bulk density and PR of the ST-lower soil were higher than at the same depth of MP soil. PR was generally close to or exceeded the often quoted 1.5 MPa critical limit for root growth. Across the 11 field experiments, the untilled ST soil at 14–18 cm generally had lower ɛa and ka than the mechanically loosened soil at the same depth for MP. Also the specific air permeability (pore organization = ka/ɛa) was lower for ST than for MP. SOC turned out to be a dominating driver of bulk density across the soils studied. Our results indicate that relatively sandy soils low in SOC display hard-setting behaviour and are little suitable for ST. Our study also identified a pedotransfer function enabling prediction of PR for the upper B-horizon soil at a water content of field capacity for soils with the same geological origin as in this study.

Persistence of subsoil compaction was investigated in a field experiment in southern Sweden. The investigation compared two treatments (control and compaction by four passes track-by-track), 14 years after the experimental traffic. The compaction experiment was carried out in 1995 with a 6-row sugar beet harvester with a wheel load of c. 10.4 Mg. Investigations included penetration resistance, bulk density, water retention, saturated hydraulic conductivity, in situ near-saturated hydraulic conductivity, and dye tracing experiments. The measurements of penetration resistance and bulk density clearly showed the persistence of subsoil compaction. In addition, both macroporosity and saturated and near-saturated hydraulic conductivity were smaller in the compacted plots, although these differences were not statistically significant. Dye tracing allowed us to visualize flow patterns in the soil and to quantitatively distinguish compacted and non-compacted subsoil profiles. Despite significant soil textural heterogeneity across the experimental field, the dye tracing data showed that persistent compaction may enhance preferential flow.

Compaction of the subsoil due to heavy traffic in moist and wet soil is widespread in modern agriculture.The objective of this study was to quantify the effects from realisticfield traffic on soil penetrationresistance and barley crop yield for three Luvisols developed from glacial till. Undisturbed soil cores wereused for quantifying the precompression stress (spc) of non-compacted soil. Tractor-trailer combinationsfor slurry application with wheel loads of3,6 and8 Mg (treatments M3, M6, M8) were used for theexperimental traffic in the spring atfield-capacity. For one additional treatment (labelled M8-1), the soilwas loaded only in thefirst year. A tricycle-like machine with a single pass of wide tyres each carrying12 Mg (treatment S12) was included at one site. Traffic treatments were applied in a randomized blockdesign with four replicates and with treatments repeated in four consecutive years (2010–2013). Aftertwo years of repeated experimental traffic, penetration resistance (PR) was measured to a depth of 1 m.The yield of a spring barley crop (Hordeum vulgare L.) was recorded in all four years of the experiment. Theresults did not support our hypothesis of spc as a soil strength measure predicting resistance to subsoilcompaction. The tyre inflation pressure and/or the mean ground pressure were the main predictors of PRin the upper soil layers. For deeper soil layers, PR correlated better to the wheel load. The number ofwheel passes (M-treatments vs the S12 treatment) modified this general pattern, indicating a very strongimpact of repeated wheel passes. Our data indicate that a single traffic event may mechanically weakenthe soil without inducing major compaction but with influence on the effect of subsequent traffic evenafter as long an interval as a year (treatments M8 vs M8-1). Crop yields were much influenced bycompaction of the plough layer. Due to the repeated wheel passes for the M-treatments, significant yieldpenalties were observed, while the single-pass treatment with 12 Mg wheel load in S12 did not havesignificant effects on crop yield. Our hypothesis of 3 Mg wheel load as an upper threshold for not inducingsubsoil compaction was confirmed for the tractor-trailer treatments with repeated wheel passes but notsupported for the single-pass machinery. The results call for further studies of the potential for carryinghigh loads using wide, low-pressure tyres by crab steering/dog-walk machinery.

Compaction of the subsoil can only be alleviated by natural processes. The objective of this study was to evaluate the long-term effect of compaction on the pore system at 0.35 m depth of a heavy clay soil naturally subjected to drying and wetting, and to freezing and thawing, and biological activity in Finland. The compaction treatment was inflicted 29 years prior to investigation and included four passes with a tractor-trailer combination with wheel loads up to 4.8 Mg and inflation pressures of 700 kPa. Gas diffusion and air permeability measurements were combined with pycnometer-estimated air-filled pore volumes. The largest macropores were characterized on three-dimensional images acquired using an x-ray medical CT scanner. We wanted to evaluate, to what extent the compaction affected soil pore geometries and volumes. The combination of diffusive and advective gas transport characteristics was expected to enhance the ability to deduce how the soil pore system was affected. This included advective air flow measurements at a range of pneumatic pressure drops. Compaction at 0.35 m depth was persistent 29 years after the compaction event. Compaction diminished the diameter of vertical macropores that served as arterial pores, while the volume and role of marginal pores branching from the arterial pores were diminished. Compacted soil had the significantly lowest volume of blocked pores not in contact to the surrounding atmosphere. For this clay-holding soil, the long-term compaction effect is interpreted as a serious reduction of the aeration potential of the bulk soil matrix in between the vertical, arterial pores. Our data indicate a high degree of anisotropy of (clay-holding) subsoil pores, and that state-of-the art models are not able to describe soil diffusivity for such soils. We suggest air permeability measurements at a range of pressure drops in combination with a regression method for estimating Darcy air permeability. Care should be taken in using the specific permeability as an indication of pore tortuosity in cases, where the dimensions of the soil pores are significantly different.

Tillage and residue management influence soil organic carbon (SOC) and lead to changes in soil physical behav-iour and functioning. We examined the effect of the clay-to-carbon ratio on soil physical properties in a humid sandy loam soil with contrasting tillage and residue management. Soil was sampled at the 0–10, 10–20 and 25–30 cm depths of a sandy loam soil at Flakkebjerg, Denmark in 2013. We used the experimental plots of a long-term field experiment with mouldboard ploughing (MP) and direct drilling (DD) treatments. The residue management included straw removal (−S) and straw retention (+S). We measured SOC, clay dispersibility and tensile strength of air-dried 8–16 mm aggregates in either natural or remoulded condition. Soil friability, soil workability and specific rupture energy were calculated from the tensile strength measurements. SOC was higher in DD at 0–10 cm than MP at 0–10 cm and DD and MP soil at 10–20 cm, while MP was higher than DD at 10–20 cm depth (p b 0.05). However, there was no difference in the effect of the contrasting tillage manage-ments on carbon sequestration when an equivalent soil mass and the entire topsoil layer were considered. In the top 10 cm soil, DD decreased clay dispersibility (p = 0.09) and increased soil friability (p b 0.05) compared with the MP soil. Direct drilling with straw removal (DD − S) resulted in higher workability compared with mouldboard ploughing with straw removal (MP − S) (p b 0.05). We defined non-complexed clay as NCC = clay −10 × SOC according to Dexter et al. (2008). NCC was a better predictor of dispersible clay than total clay and SOC at all depths in natural aggregates, while tensile strength and derived parameters were generally better explained by the total amount of clay in remoulded aggregates. Remoulded aggregates had higher tensile strength and rupture energy but lower friability and workability than natural aggregates. A linear regression of total clay and SOC explained better (R2 = 0.79) the variation in tensile strength of remoulded aggregates than models including the clay-to-carbon ratio. Our results indicate that the degree of clay saturation with carbon has a significant influence on clay dispersibility in natural aggregates, while soil aggregate strength expressions of remoulded aggregates are better explained by soil clay content.