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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.

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.

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.

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.

The compressive behaviour of soil, the compression curve, is typically expressed in a semi-logarithmic diagram in the relationship between the logarithm of applied stress, σ , and the void ratio, e . The compression curve yields three important soil compression properties: the swelling index, C s , the compression index, C c , and the precompression stress, σ pc . The compression index is considered an indicator of soil resistance to compaction, C s is used as a measure of soil mechanical resilience, and σ pc is considered the soil load support capacity. The objective of this paper was to investigate the impact of soil particle and aggregate size distribution on the characteristics of the soil compression curve and on C s , C c , and σ pc for a range of Swedish soils. We found no or only weak impacts of particle size and aggregate size distribution on soil compression properties. Instead, soil compression properties were largely controlled by initial void ratio, e 0 , in such a way that C c and C s decreases, and σ pc increases with decreasing e 0 , although the correlation between σ pc and e 0 was not statistically significant for subsoil samples. We show that the soil compression properties ( σ PC , C c and C s ) and their dependency upon e 0 are partly a consequence of the compression curve being analyzed in a semi-logarithmic (log σ − e ) diagram. The use of log σ (instead of σ ) forces the compression curve to bend at a certain value of log σ , and this bend is associated with σ pc . We showed that a precompression stress, as commonly defined, can be obtained for an ideal linear-elastic material. Therefore, the use of a logarithmic stress scale may lead to misinterpretation of soil mechanical behaviour. Our empirical results and theoretical exercise emphasize the need for further studies of physically-based expressions for soil strength for use in compaction modelling.

The concept of structural and functional hierarchy prevails in studies on the turnover of organic matter (OM) in soil. We examined the degree of structural hierarchy in arable soils sampled at six locations along a naturally occurring texture gradient (11 to 45% clay, 12.9 to 15.8 mg C g −1 soil). One set of samples was incubated with 14 C labeled ryegrass for 8 months while another set remained unamended. Constituent aggregates were isolated from the soils by fragmentation along natural planes of weakness. Eight aggregate size-classes (b0.063, 0.063–0.125, 0.125–0.25, 0.25–0.50, 0.5–1.0, 1–2, 2–4 and 4–8 mm) were isolated and analysed for textural composition and contents of total-C (and 14 C). The evolution of CO2 (and of 14 CO2) during a 15-week laboratory incubation was determined for four aggregate size-classes in the range 0.125 to 2 mm. The aggregate size distributions reflected differences in whole soil texture with decreasing proportions of b1 mm aggregates and increases in N 2m m aggregates with increasing whole soil clay content. While the texture of aggregates N1 mm was similar to that of the whole soil, aggregates within the range 0.125 to 1 mm were enriched and those b0.125 mm depleted in sand. Macro-aggregates (N0.25 mm) showed a similar distribution of native and of 14 C labelled OM. For micro-aggregates, concentrations of native and labelled OM increased with decreasing size. When corrected for sand particles N63 μm, the only consistent trend in native and labelled OM across aggregate size-classes was a higher content in micro- than in macro-aggregates. While native OM showed no consistent trend in lability, the 14 C labeled OM in the 0.125–0.25 mm size-class was more labile than that in macro-aggregates. The similarity of whole soils and differently sized macro-aggregates suggests that macro-aggregates in our soils experience a rapid turnover and merely represent fragments of larger soil units (peds and clods) that constitute the soil in situ. © 2006 Elsevier B.V. All rights reserved.

Soil type effects on organic matter turnover are most often ascribed directly to differences in soil clay content. Since soil texture determines the physical characteristics of soil, aggregation and water holding capacity may be more relevant to address in the search for controls of organic matter turnover. Most studies of microbial processes in soils are based on structurally disturbed soil, where the abiotic conditions for the microbial activity may be quite different from those in intact soils. In this study, basic physical characteristics were determined for structurally disturbed and intact soil samples from differently textured soils. Bulk soil was retrieved from 0–20 cm depth at six locations along a textural gradient in an arable field on Weichselian morainic deposits in Denmark. The samples (NA1 to NA6) ranged in clay from 11 to 45% and in silt from 7 to 15%. Clay and silt-sized organomineral complexes were isolated from NA2 soil by ultrasonic dispersion and sedimentation in water. The clay and silt fractions were added individually and in varying proportions to NA1 soil, providing three clay-amended (CL2, CL4 and CL6) and three silt-amended (SI2, SI4 and SI6) soils. All 12 soils were crushed in air dry state to 100 μm). Air diffusivity and permeability measurements showed disturbed soils to have a less continuous and more tortuous pore system than undisturbed reference samples. Water-filled pore space at a critical level of air diffusion potential was significantly higher for undisturbed than for disturbed samples, especially in soils high in clay. Drop cone measurements showed disturbed soils to be structurally weaker than undisturbed ones. Intact and structurally disturbed soils were found to differ significantly in physical properties even after 17 months of soil structure regeneration. Water-filled pore space seems to reflect the potential of available water and aeration status to regulate aerobic microbial activity of structurally disturbed soil, but not of intact field soil.