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Soil organic carbon (SOC) accumulation is easily susceptible to tillage managements, which strongly affect soil structure and microbial community structure. Subsoiling is developed for mitigating soil compaction and thus improving soil structure. Coupled with straw residue incorporation, it is expected not only to enhance this improvement but also to promote organic carbon (C) storage in soil. However, how subsoiling tillage managements change soil aggregate microstructure, microbial community structure and SOC fractions, and how soil aggregate microstructure and microbial community structure affect SOC in the topsoil and subsoil layers are equally unknown. An in-situ incubation field experiment was conducted in a Mollisol in northeastern China and included conventional tillage (CT), subsoiling tillage (ST) and subsoiling tillage with straw residue incorporation (ST + S). The methods of synchrotron-based X-ray micro-computed tomography scanning, phospholipid fatty acid and SOC density fractionation were used to analyze aggregate pore characteristics, microbial community structure and SOC fractions, respectively. The results showed that both ST and especially ST + S not only improved soil aggregate microstructure but also enhanced microbial biomass and then favored SOC accrual in the topsoil (0–20 cm). In the subsoil (20–35 cm), ST + S exerted similar positive effects as those in the topsoil. However, ST only improved soil aggregate microstructure but decreased microbial community biomass and SOC in the subsoil. Moreover, soil aggregate structure characteristics explained (solely explained 6 % in the topsoil and 12 % in the subsoil, respectively) a smaller part of the variation in SOC within aggregates than soil microbial characteristics did (solely explained 15 % in the topsoil and 25 % in the subsoil, respectively). Specifically, bacteria explained 26 % and 66 % of the variation in SOC within aggregates in the topsoil and subsoil, respectively. The porosity of macropores (>100 μm) also explained 25 % of the variation in SOC within aggregates in the topsoil, whereas the porosity of micropores (

Lime and phosphorus (P) applications are common agricultural management practices. Our aim was to quantify the effects of long-term application practices on root growth and abundance of arbuscular mycorrhizal fungi (AMF) under field conditions. We assessed the effects of lime and P fertilizer applications on barley yield, root growth and AMF abundance in 2016. Treatments were no, low, medium and high liming rate corresponding to application of 0, 4, 8 and 12 Mg lime ha −1 every 5–9 years since 1942 combined with no or yearly application of 15.6 kg P ha −1 since 1944. At harvest, grain yield, root intensity (core-break) and AMF abundance at different soil depths were estimated. Root development was monitored during early growth with minirhizotrons in treatments receiving low, medium and high liming rates and P fertilization. A quadratic model relating grain yield to liming rate estimated yields to peak at 6.4 Mg lime ha −1 with yields of 4.2 and 3.2 Mg grain ha −1 with and without P fertilization, respectively. Low and medium liming rates resulted in greater AMF abundance, especially in the no P treatments. During early growth in P-fertilized treatments, 77% and 65% more roots developed in the soil profile when treated with medium and high liming rate, respectively, compared to low liming rate. We conclude that long-term application of lime in soils receiving yearly P fertilization improved conditions for root growth in soil layers below 30 cm, but at the high liming rate, this did not translate into higher yield.

The Mollisol region of Northeast China has a large soil organic carbon (SOC) storage which is important for maintaining soil fertility. SOC is susceptible to various environmental factors; however, the responses of SOC content to environmental factors in different soil layers of cropland remain unclear, particularly in deep soil layers. In this study, we collected 138 soil samples from the surface, subsurface, and subsoil layers among 46 sample sites with monocropping maize and intensive conventional tillage in this region. We assessed the relative importance and effect paths of 12 environmental factors (including geography, climate, and soil properties) on SOC content in different layers using redundancy analysis (RDA), structural equation model (SEM), and variation partitioning analysis (VPA). The VPA results showed that SOC content was mainly affected by climatic factors that explained 68% and 57% for the surface and subsurface layers, respectively. However, SOC content in the subsoil layer was greatly affected by soil properties that explained 27%. Furthermore, the SEMs results suggested that geographical factors indirectly affected SOC content by influencing the climatic factors. Mean annual temperature was the most important factor affecting SOC content directly or indirectly, and its negative effects significantly diminished with soil depth, as it explained 63%, 52%, and 17% of the variation in SOC content for the surface, subsurface and subsoil layers, respectively. In addition, the effects of soil water-holding capacity on SOC content also decreased with soil depth, whereas pH and clay content showed a contrasting pattern. This implies that pH and clay content play important roles in the sequestration of SOC in deep soil layers. Moreover, the organic C content within >53 μm aggregates was more sensitive to environmental factors. This study can be useful for forecasting SOC dynamics and establishing reasonable C management strategies under climate change conditions.

The natural and cover crop (CC)-induced anisotropy of subsoil pore characteristics is important in assessments of soil compaction but, until recently, has received limited attention. This study aims to quantify the anisotropy of soil pore characteristics and hydraulic conductivity in subsoils subjected to compaction and CC treatments in a split-plot field experiment on temperate sandy loam soils. The main factor was ±compaction and the split-plot factor was ±CC with fodder radish (Raphanus sativus L.). The compacted plots were heavily trafficked for 4 yr (2010–2013). After 4 yr under CC (2013–2016), core samples were collected at 0.3 m. The samples were taken vertically and horizontally to quantify the anisotropy of air-filled porosity (ε a) and air permeability (k a), saturated hydraulic conductivity (k sat), and bulk density (ρ b). The results showed isotropic behavior for ρ b and ε a and significant anisotropic behavior (p a, pore geometry index (PO1) (k a/ε a), ratio of non-Darcian to Darcian k a (R-ratio), and k sat. For parameters with significant anisotropy, higher values occurred vertically than horizontally for all compaction –CC combinations, except for R-ratio. This indicates that pre-existing vertical continuous pores dominated the pore system in the control subsoil. A nonsignificant trend of higher values of k a, PO1, and k sat in the +CC than in the compacted –CC plots suggest that CC could contribute to the formation of vertical biopores. Including an autumn CC in rotation with a summer cereal crop for a longer period may significantly affect the anisotropy of the soil pore and hydraulic properties.

The least limiting water range (LLWR) has been used as a soil structural quality indicator for identifying in-season water dynamics, yet studies focusing on its use for detecting in-season water stresses and their effect on crop response on severely compacted subsoils are scarce. The objectives of this study were, therefore, to examine the in-season water dynamics on a tile-drained soil with compacted subsoil in the light of two different approaches for calculating LLWR (standard LLWR by da Silva et al. [1994] and refined LLWR by Pulido-Moncada & Munkholm [2019]) and to evaluate the crop response to aboveground and belowground conditions. Information on LLWRs was obtained from soil sampling in the most contrasting treatments of a compaction experiment: with and without compaction. In-season water dynamics were measured from 2017 to 2019. The refined LLWR approach defined a wider range of water content nonlimiting for plant growth compared with the da Silva et al. approach. Compaction affected the LLWRs (p

Nitrous oxide (N2O) is the contributor to agricultural greenhouse gas emissions with the highest warming global potential. It is widely recognised that traffic and animal-induced compaction can lead to an increased potential for N2O emissions by decreasing soil oxygen supply. The extent to which the spatial and temporal variability of N2O emissions can be explained by soil compaction is unclear. This review aims to comprehensively discuss soil compaction effects on N2O emissions, and to understand how compaction may promote N2O emission hotspots and hot moments. An impact factor of N2O emissions due to compaction was calculated for each selected study; compaction effects were evaluated separately for croplands, grasslands and forest lands. Topsoil compaction was found to increase N2O emissions by 1.3 to 42 times across sites and land uses. Large impact factors were especially reported for cropland and grassland soils when topsoil compaction—induced by field traffic and/or grazing—is combined with nitrogen input from fertiliser or urine. Little is known about the contribution of subsoil compaction to N2O emissions. Water-filled pore space is the most common water metric used to explain N2O emission variability, but gas diffusivity is a parameter with higher prediction potential. Microbial community composition may be less critical than the soil environment for N2O emissions, and there is a need for comprehensive studies on association between environmental drivers and soil compaction. Lack of knowledge about the interacting factors causing N2O accumulation in compacted soils, at different degrees of compactness and across different spatial scales, limits the identification of high-risk areas and development of efficient mitigation strategies. Soil compaction mitigation strategies that aim to loosen the soil and recover pore system functionality, in combination with other agricultural management practices to regulate N2O emission, should be evaluated for their effectiveness across different agro-climatic conditions and scales.

Many soil processes and functions, including carbon storage, water and element cycling, and crop productivity, are directly or indirectly affected by soil aggregates. As a key property, aggregate tensile strength is usually quantified by the Weibull distribution. This distribution was found to depend on soil texture, organic matter content, and aggregate shape; however, effects of cementing agents (i.e., clay mineral and organic compounds) on aggregate tensile strength cannot be empirically studied independent of shape. The objective was to apply Discrete Element Modeling (DEM) to simulate the rupture of soil aggregates of different shapes but the same assumed distributions of cementing agents. The hypothesis was that the Weibull modulus of aggregate tensile strength could be used to describe the heterogeneity of the particle binding forces within the aggregate. Multiple DEM simulations of aggregate rupture were carried out for sets of randomly generated binding force distributions between particles of the same diameter. Aggregate properties and shapes were obtained from those observed in arable Gleyic Cambisol topsoil from plots with and without vegetation of a soil compaction trial; additional aggregate shapes were simulated to increase the predictive range. The simulation results showed that (i) the sensitivity of Weibull modulus of aggregate tensile strength to the heterogeneity of particle binding force distribution increased with aggregate sphericity, (ii) the value of the Weibull modulus decreased with increasing heterogeneity of this distribution, and (iii) internal heterogeneity also reduced the crack surface area produced by aggregate rupture. Results corresponded to friability theory implying that small values of Weibull modulus indicated more complex soil structural organization. The DEM simulations could help explaining complex interactions between aggregate shape cementing agents and stability. Our DEM approach also uncovered the importance of Weibull modulus to assess structural features of soil aggregates and to describe the heterogeneity of the particle binding forces within the aggregate.

Improving soil structural stability (SSS) and soil organic carbon (SOC) are critical for soil health and environmental pollution mitigation. No-tillage alters many soil properties (e.g., SOC), however, its effects on SSS in rice paddies are unclear. Therefore, we used field experiment and meta-analysis to determine the effects of no-tillage on wet stability of aggregates (WSA), clay dispersibility (ClayDis), mean weight diameter (MWD), and aggregate SOC distribution and mineralization in rice paddies. The field experiment included four tillage practices: no-tillage, rotary tillage and moldboard plow tillage with rice straw retention (NTS, RTS and CTS respectively), and moldboard plow tillage with rice straw removal (CT). The WSA at 0–5 cm soil depth was significantly higher under NTS compared with CTS. The ClayDis at 5–10 cm soil depth under NTS was 36% lower (P < 0.05) than CTS. The relationship between SOC and WSA fits a broken stick model, with an inflection point of clay/SOC ratio at 12.5. Higher SOC under no-tillage might result from the protection of >2 mm aggregates (macroaggregates) and lower SOC mineralization of < 2 mm aggregates. Additionally, the meta-analysis showed that no-tillage increased (P < 0.05) the macroaggregate content, WSA and MWD. However, the current research regarding tillage effects on paddy ClayDis is insufficient. In rice paddies, the increased macroaggregate content may contribute to increasing SOC, which improves SSS under no-tillage.

Vehicle traffic-induced subsoil compaction has negative influences on soil functions and ecosystems, which may cause the degradation of farmland. Rubber tracks are recommended for use on modern large agricultural vehicles due to its low ground pressure as compared to tyres. However, few studies have discussed the potential of the rubber tracks to reduce subsoil stress. In this study, we investigated the magnitude and distribution of soil stress below a rubber track and a tyre for a range of traffic velocities. The vertical and horizontal soil stresses were measured under the centrelines of the rubber track and tyre paths at a depth of 0.35 m by transducers buried in the soil. The SoilFlex model was adopted to simulate soil stress in the soil profile. Vertical stress was distributed with one peak under the tyre and multiple peaks under the rubber track. Horizontal stress exhibited one peak before and one peak after the passing of the axle. The maximum vertical and horizontal stresses were approximately 3.4- and 2.0-times higher, respectively, under the tyre than under the rubber track, indicating greater effectiveness of the rubber track in reducing the vertical stress than reducing horizontal stress. Both vertical and horizontal stresses presented a downward trend as the vehicle velocity increased.

The Discrete Element Method (DEM) is a suitable approach for modeling and simulating arable soil and tillage processes. In this work, we focus on modeling soil fragmentation which is a vital element of many soil treatments. We present a method for computing the indirect tensile strength of soil aggregates through simulation. DEM aggregates are constructed as spherical clusters of smaller spherical particles. A number of indirect tensile tests are conducted using simulated aggregates from which the critical polar force and tensile strength are identified. We propose a method for introducing random voids such that the distribution of simulated tensile strength corresponds to a given Weibull distribution. We propose to scale the number of voids linearly with the measured porosity of the aggregates. This approach showed better performance than introducing a constant number of voids. We identified that the friction between the sub-spheres can be related to the characteristic strength and the number of voids is related to the spread of strengths in the Weibull distribution. The calibration and validation are performed on experimental data. Based on this approach, the synthetic DEM aggregates allow for deriving the aggregate strength as a function of aggregate size. The results are based on fracturing individual aggregates but can be extended to simulations involving multiple interacting aggregates.

Wheel tracks can lower topsoil infiltrability and increase water ponding in agricultural fields. A seedbed harrow mounted with two goosefeet tine points, the eradicators, was used to investigate track loosening at different depths on a sandy loam soil as a way of mitigating compaction effects and reducing the risk of pesticide transport to surface waters. Loosening strongly affected air permeability and steady-state infiltration. The agro-ecological system model Daisy was used to simulate the effects of soil structural and hydraulic changes on pesticide leaching to subsurface drain lines over a 332-year period. Measured properties of the topsoil were combined with a representative subsoil and weather series and with realistic management scenarios. The loads of pesticide in the drains for 3 months after loosening were calculated for each year, and the risk was defined as the 90th percentile of the load. We focused on three different herbicides used in sugar beet cultivation in spring: glyphosate, metamitron and phenmedipham. Our simulations showed that for all pesticides loosening could lower the risk by 10% on average for a 3-m working width, and the tracks contribution to the risk by 34%, for all drain spacing and working width settings. Wheeling did not affect the risk but this result was sensitive to the parameterization of the hydraulic conductivity in the compacted soil layer, showing potentially higher risk under certain conditions. These results showed that wheel track loosening is an effective strategy for reducing the risk of surface water contamination from pesticides used in agriculture.

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.

In inversion tillage systems, the mouldboard plough is fundamental for producing a desirable seedbed. The desired ploughing quality is achieved when the plough layer is inverted homogeneously. This is, however, difficult to obtain in the main-headland intersection zone where the plough is lowered and elevated, as ploughed and unploughed triangles are formed. This results in zones where the soil is inverted twice, which may result in poor residue and weed incorporation and a poor seedbed quality. The design of the three-point linkage-attached mouldboard plough has not changed since the 1950s, but the number of furrows has increased, which has increased the size of the aforementioned triangles. A novel ploughing system was introduced to meet these headland challenges, where each plough section can be lowered and elevated independently. The aim of this study was to evaluate the effects of using a section-controlled mouldboard plough. Two similarly designed, randomized, field plot experiments were conducted on two different soil types (sandy loam and loamy sand) on a stubble field and grass field. The study showed that the section-controlled plough reduced the main-headland overlap area by ~98%. The results of a range of soil physical properties measurements and seedbed quality analyses showed that the section-controlled plough created a homogeneous loosened seedbed quality, improving the incorporation of crop residues and leaving fewer residues on the soil surface. Furthermore, the section-controlled plough showed additional benefits, for example wedge operations and visual line marking.

No-tillage system (NT) has been widely adopted around the world to restore the quality of soil structure in previously tilled soils, especially in tropical and subtropical climates. Our objective was to assess the legacy of chiseling in a NT in a subtropical climate by means of gas transport and pore morphology. The experiment was conducted in a split-plot design with two factors and four replications. The main plot was divided into a part under 18 years of no-tillage (18 NT) and a part with 5 years of no-tillage after receiving 13 years of chiseling tillage down to 25 cm depth (5 NT). The subplots were soybean and maize crops. Sixteen soil samples of 567 cm3 (large cores) were sampled from the 4–12 cm depth and scanned to detect pore diameter > 120 μm. Then, small subsamples of 100 cm3 (small cores) were extracted from the large ones to detect pore diameter > 74 μm using X-ray computed tomography (CT). A range of CT-derived soil pore characteristics were quantified for both sample sizes. In the small samples, air permeability (ka) and relative gas diffusivity (Ds/Do) were determined at −60, −100, and −300 hPa matric potentials. Classical laboratory results showed no significant treatment effects on bulk density (Bd), total porosity (TP), macroporosity (εa>30), ka and Ds/Do. There was no significant treatment effect on CT-macroporosity, although 5 NT tended to be higher than 18 NT. CT images showed a significant tillage effect on CT-maximum connectivity of macropores, where for 5 NT was higher (0.75) than in 18 NT (0.65) on the large samples. Therefore, a slight residual effect from chiseling was detected after 5 years of no-tillage. TP and εa>30 decreased and Bd increased significantly after subsampling. Ds/Do and ka were significantly correlated to CT-derived pore characteristics. This suggests that CT image analysis has high capacity for predicting gas transport.

Reduced use of fossil energy by removing cereal straw for bioenergy potentially threatens soil organic carbon (SOC) storage. Straw incorporation plays a key role for SOC storage in cereal-based cropping systems, but the use of cover crops (CCs) may compensate for straw removal. However, assessing the SOC sequestration potential of management changes requires field experiments with long continued treatments and frequent soil sampling. Based on a field experiment initiated in 1981 on a sandy loam soil at Askov Experimental Station (Denmark), we examined the effect of annual additions of spring barley straw (0, 4, 8 and 12 Mg ha−1) and undersown ryegrass CC on SOC storage in the 0- to 20-cm layer. The effect of straw incorporation and CC on SOC was additive. At steady-state conditions, the SOC stock based on equivalent soil mass (SOC stockFM) increased 3.9, 6.7 and 9.3 Mg C ha−1 after annual incorporation of 4, 8 and 12 Mg straw ha−1, respectively. The ryegrass CC increased SOC stockFM by 3.3 Mg C ha−1 and thus almost compensated for removal of 4 Mg straw ha−1. An asymptotic regression model best described the temporal changes in SOC and showed that the effect of straw incorporation and ryegrass CC on SOC sequestration peaks after 10–15 years when a new equilibrium between input and output of C is reached. Clearly, reliable assessments of SOC sequestration potentials following changes in management require field experiments with frequent soil sampling until reaching steady-state conditions.

The main principles of conservation agriculture (CA) are minimal soil disturbance, permanent soil cover, diversification of crop rotations, and retention of crop residues. These practices have been associated with improved soil quality, water use efficiency, biodiversity, and reduced climate impact through carbon (C) sequestration. No tillage has been implicated with reduced soil organic C (SOC) mineralization, but recently this has been contested. Residue retention and cover crops result in higher soil C inputs and are therefore expected to increase SOC. CA affects both the distribution of SOC in the profile as well as the bulk density and this complicates the analysis of total SOC stocks. Furthermore, these changes only manifests over a long time. Therefore, the effects of CA practices require long-term experiments and thorough sampling of the soil profile, and there are few studies in the literature. This study aimed to evaluate the effect of 17 years of residue retention and tillage on soil C storage in the soil profile under cool temperate humid climate conditions. The study used field trials established in 2002 on two Danish sandy loam soils with different clay and SOC contents and compared different tillage (plowing and no-till) and straw management practices (with and without straw retention). Soil samples from 0–50 cm were taken in winter 2002, 2009, and 2019 to quantify C content, and crop yields were measured annually. Straw retention reduced bulk density at both sites. It also increased the C concentration with 0.47 mg C g–1 dry soil on average from 2002 to 2019, and SOC stocks in 2019 with 23 tonne C ha–1 on average, at the site with higher initial soil C content. The effect of straw retention on soil C was higher when the soil was plowed, and no significant effect was observed in no-till soils. Straw retention did not significantly affect C concentration at the site with low initial soil C. Tillage altered the C distribution in the soil profile, but did not significantly increase total C concentration at either site, although there was a non-significant trend (P = 0.07) to increased C concentration across sites. Use of no-till and straw retention did not consistently affect crop yield across site-years, but the inter-annual variability was large, obscuring possible differences in yields. The limited effect of CA practices on soil C storage observed in this study may be due to the soil type and climatic conditions at these sites, but this needs further investigation. Core Ideas Straw retention reduced soil bulk density at sites with low or medium-high initial C level Straw retention increased C concentration and SOC stock at a site with higher C Tillage altered soil profile C distribution, but did not increase SOC storage Yield was not affected by 17 years of no-till and straw retention.

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.

There is a need for improved knowledge of the limits to the available water range for root growth in the subsoil. The objective of this study was to recalculate the upper and lower limits of the least limiting water range (LLWR) concept by using respectively the air-filled porosity (ϵa) at which 0.005 of the relative gas diffusivity (Ds/Do) is reached and readily available water (RAW). The refined upper limit estimates the variation in ea related to pore connectivity and the refined lower limit expresses the boundary at which plants suffer physiological water stress. This study was based on soil sampled in compaction trials on two sandy loam soils. Soil samples were taken from plots with no compaction (Control), and compaction with 78 kN (M8) and 58 kN (M6) wheel loads with multiple wheel passes. The soil cores were analyzed for ϵa, Ds/Do, bulk density (rb) and penetration resistance (PR). Heavy farm machinery impact of M8 and M6 led to subsoil compaction up to depth of 0.5 to 0.7 m for the soils under study. The subsoil structure was affected by compaction across depths with the decrease in ϵa (~33-46%) and Ds/Do (~37-61%) and increase in rb (~4-8%) and PR (~40-50%, at -100 hPa at 30-cm depth). The refined LLWR showed a wider water range compared to the original approach. We anticipate that the refined LLWR well reflects the limiting soil physical conditions for root growth for the studied soils, but validation by combined soil physical and plant growth measurements is needed.

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.

Clay is a basic soil constituent that governs many soil properties including pore characteristics, which in turn control a range of crucial soil properties and functions. This study explored the relationships between aggregate strength and soil pore structure characteristics for a range of clay contents along a natural gradient, and investigated the influence of clay content and matric potential on soil workability. Soil samples (100 cm3 soil cores and bulk soil) were sampled from the 5–15 cm depth in four locations in an arable field near Lerbjerg, Denmark, which ranged in clay content from 0.119 to 0.446 kg kg−1. The soil cores were drained to five matric potentials in the range −10 to −1000 hPa to obtain volumetric water content and air-filled porosity (εa). We measured air permeability and gas diffusivity and calculated tortuosity (τ) and pore organization at −100 hPa. Soil aggregates were obtained from the bulk soil and their tensile strength (Y) and specific rupture energy (Esp) were determined at −100, −300, −1000 hPa and at air-dry state. Y and Esp increased with decreasing matric potential and with increasing clay content, which consequently affected soil workability. εa at −100 hPa or air-filled macroporosity were negatively related to Y and Esp, and positively to the range of water contents for tillage (ΔθRANGE). This indicates the importance of air-filled macroporosity for soil crumbling during tillage. The findings of the study suggest that management practices that increase soil macroporosity can potentially decrease aggregate strength and increase ΔθRANGE and hence improve soil workability. We suggest that in fields with highly variable soils operations should be scheduled at periods when the range of water contents for tillage are suitable for the whole field to reduce the risk of soil structural damage induced by tillage.

Subsoil compaction in arable farming caused by increasingly heavy machinery results in increased subsoil strength and reduced aeration and water flow. This can restrict root growth and reduce crop yield. To mitigate these effects of subsoil compaction, we included a deep-rooted catch crop of fodder radish (Raphanus sativus var. oleifera) in a rotation of spring barley (Hordeum vulgare). The objective was to determine the effect of subsoil compaction on root growth and grain yield of barley, and to investigate the creation and use of biopores for subsequent root growth. Studies were carried out in 2013–2016 in a soil compaction experiment established in 2010 on a sandy loam. The soil had for four consecutive years (2010–2013) been compacted with multi-passes in an 8 Mg wheel load treatment (M8) or had not been compacted (Control). Spring barley was grown every year and fodder radish was grown as a catch crop from harvest to winter in 2013–2016. Penetration resistance to 90 cm depth was measured in 2015 at a soil water content near field capacity. Root growth and biopore reuse were investigated using horizontal minirhizotrons installed at 30, 50 and 70 cm depth from 2013–2016. Spring barley grain yield was recorded annually. We found that M8 caused critical subsoil compaction with penetration resistance peaking at 3.5 MPa in the upper subsoil (30−40 cm depth). Subsoil compaction reduced root growth by c. 50 % at 30 cm depth and barley yields by 6–8 % compared to the Control in three years after cessation of the compaction treatment. The number of biopores that were reused were higher for fodder radish than for spring barley, 4.1 % and 3.1 %, respectively, but did not differ between compaction treatments. Our study displayed a marked, negative residual effect of soil compaction on above- and belowground crop growth. There was no tendency towards a recovery within the years studied (1–3 years after compaction). Severe subsoil compaction is a widespread problem and therefore we call for more studies on the spatio-temporal dynamics of the creation and subsequent use of biopores for root growth as these are essential for the recovery of compacted subsoils.

Vehicle traffic induced soil compaction has negative effects on soil functions and ecosystems which may cause the degradation of farmland. This study investigated the magnitude and distribution of soil stress under the tracked and tired vehicles to explore the penitential of using rubber track instead of tire to reduce the subsoil compaction. The field experiment in this study included three replicates and was conducted on a sandy loam soil. Vertical and horizontal soil stress were measured under the centerlines of the rubber track and tire at a depth of 0.35m by using embedded transducers. The SoilFlex model was applied to simulate vertical and horizontal stress in the soil profile. Unevenly distributed vertical and horizontal stress were observed under the tire and rubber track. The vertical stress was characterized by one peak under the tire and several peaks under each of track wheels and rollers. The horizontal stress exhibited peaks before and after the tire and each of track wheels and rollers. The measured maximum stress was significantly higher under the tire than under the rubber track: that is, vertical and horizontal stress were approximately 3.4 and 2.0 times higher, respectively. This finding indicated that using rubber track maybe an effective method to reduce soil stress when compared with the tire, and was more effective in reducing the vertical stress than horizontal stress. Improving the uniformity of stress distribution under the track is the key to improve the ability of tracked vehicle to mitigate soil compaction.

A 22-month incubation experiment was conducted to study the effect of lime on clay dispersion, wet aggregate stability (WAS) and dissolved reactive phosphorus (DRP), using a heavy clay subsoil with an initial pH of 7.0 and 7.3 g kg−1 of soil organic carbon. Lime was applied to achieve soil pH values of 7.5, 8 and 8.4. Clay dispersion decreased linearly with increased pH (corresponding to an increase in lime amount) for both lime types (R2 = 0.44 for CaO; R2 = 0.53 for CaCO3, P < 0.05), with a decrease of 2–16 % (CaO) and 3–17 % (CaCO3) compared with the control. Both WAS and DRP followed piece-wise linear functions, with an increase and peak around pH 7.5–7.8, and a decline at higher pH (WAS: R2 = 0.73 for CaO, R2 = 0.68 for CaCO3, P < 0.001; DRP: R2 = 0.84 for CaCO3, R2 = 0.33 for CaO, P < 0.001). Wet aggregate stability increased on average by 13 % and 11 % at the lowest and intermediate levels, respectively, compared with the control. At the highest lime application rate, WAS was 6 % (CaO) and 8 % (CaCO3) lower than in the control. These differences were probably caused by changes in electrical charge and in concentrations of soluble calcium and dissolved organic carbon (DOC) as the pH increased. More studies are needed to understand the processes in detail and to draw conclusions that are more robust.

The interaction between rolling gear and soil is complex, but most important for the stress distribution in the soil profile. We explored the effect of three types of wide, low-inflation pressure tyres with similar dimensions on mean normal stress throughout the soil profile. We first tested the hypothesis that the stress is not affected by specific tyre-construction. Second, we tested the benefit of lowering the tyre inflation pressure to a minimum for the tyre with the lowest recommended inflation pressure. Finally, we tested the effect of tyres with similar tractive potential at different wheel loads, i.e. with a different weight-pull ratio. Stress measurements were made with Bolling probes at six positions simultaneously: both beneath the centreline (centre) and at 0.3 m lateral distance (+0.3 m) of the centreline of the wheel track, at 0.2, 0.4, and 0.6 m depth. The results revealed a very limited effect of tyre construction on mean normal stress. No differences were measured beneath the centre, and the differences at +0.3 m were found only at 0.2 m depth for the tyres at the rear axle. The effect of minimising tyre inflation pressure was limited to the upper parts of the soil profile for the measurements beneath the centre of the tyre (significant at 0.2 m depth and a trend at 0.4 m depth). Finally, our study did not reveal significant benefit of tyres with a lower wheel load while potentially having similar tractive performance, although the reduction of wheel load and associated lower inflation pressure potentially reduce stress in both top- and subsoil. The results emphasize that in order to reduce soil stress, tyre design and use should allow for a large contact area and low inflation pressure.

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.

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.

Maintaining good soil structural stability is an important element in agricultural sustainability. Incorporation of cereal straw and use of cover crops improve soil structural stability but the long-term individual and interactive effects of these management practices is poorly understood. We examined the impact of four rates of straw incorporation in spring barley (0, 4, 8 and 12 Mg ha-1, annually) combined with a ryegrass cover crop undersown in the barley in spring. Soil was sampled after four decades of treatments in the Askov (Denmark) straw incorporation experiment situated on a sandy loam with 12% clay. We assessed clay dispersibility measured on two macro-aggregate size fractions (ClayDis 1-2 mm and ClayDis 8-16 mm), wet-stability of aggregates, and clay dispersibility of P=0.068), when cover crops were included. The soils receiving 8 and 12 Mg straw ha-1 had a significantly lower ClayDis 1-2 mm and ClayDis 8-16 mm than soil with straw removal. Inclusion of a ryegrass cover crop did not affect ClayDis 1-2 mm, but decreased ClayDis 8-16 mm marginally (P=0.054) and decreased clay dispersibility of field-moist soil. The results suggest, that binding agents from cover crops such as roots increase stabilization of large macro-aggregates, while the increased stability due to straw incorporation was related to SOC irrespective of aggregate size.

This study, based on vegetable production fields, combined soil quality assessed by three approaches (qualitatively by farmers, semi-quantitatively by a researcher and quantitatively by laboratory analyses) with the aim of improving the integration of the different approaches. We interviewed 79 peri-urban vegetable growers in two communities within the Sunyani Municipality, Ghana. Eight of the farmers were selected to participate in the farmer-based assessment of soil quality. Further, visual evaluation of soil quality was conducted by the researcher, followed by laboratory analyses of soil properties to corroborate the farmers’ assessment of good and poor soils in their fields. Results showed that the farmers used locally-defined characteristics to describe the physical, biological and crop performance indicators of soil quality. There was, in general, limited use and understanding of soil chemical properties as indicators of soil quality. The farmers’ perception on soil quality of their fields largely influenced their decision on the type of crops they cultivate, and application regimes of mineral fertilizers. Results from the visual evaluation by the researcher agreed in some respects with the farmers’ assessment of soil quality of the good and poor soils in their respective farms. Laboratory analyses did not show specific trends for the content of chemical properties for neither good nor poor soils. The study highlighted that none of the approaches of soil quality assessment is necessarily superior in and of itself. We emphasized the need for integration to capitalize on the strengths of each approach, enhance mutual learning between farmers and soil scientists, build the capacity of farmers, and improve their decision on soil use for agricultural production.

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

An adequate and uniform seeding depth is crucial for the homogeneous development of a crop, as it affects timeof emergence and germination rate. The considerable depth variations observed during seeding operations - evenfor modern seed drills - are mainly caused by variability in soil resistance acting on the drill coulters, whichgenerates unwanted vibrations and, consequently, a non-uniform seed placement. Therefore, a proof-of-conceptdynamic coulter depth control system for a low-cost seed drill was developed and studied in a field experiment.The performance of the active control system was evaluated for the working speeds of 4, 8 and 12 km h−1, bytesting uniformity and accuracy of the coulter depth in relation to the target depth of −30 mm. The evaluationwas based on coulter depth measurements, obtained by coulter position sensors combined with ultrasonic soilsurface sensors. Mean coulter depth offsets of 3.5, 5.3 and 6.3mm to the target were registered for the depthcontrol system, compared to 8.0, 9.1 and 11.0mm without the control system for 4, 8 and 12 km h−1, respectively.However, speed did not affect the coulter depth significantly. The control system optimised coulter depthaccuracy by 15.2% and at 95% confidence interval it corresponded to an absolute reduction in the coulter depthconfidence span of 10.4 mm. The spatial variability, due to variation in soil mechanical properties was found tobe±8 mm, across the blocks for the standard drill and when activating the coulter depth control system thisvariability was reduced to±2 mm. The system with the active control system operated more accurately at anoperational speed of 12 km h−1 than at 4 km h−1 without the activated control system.

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.

Poor seedbed preparation may cause low yields and poor resource utilization. Therefore, novel sensor technology for seedbed quality evaluation is strongly needed to make sure good growing conditions are achieved efficiently. The objective of this study was to quantify the effect of the front leveling tines and the tillage depth of a cultivator on soil surface roughness and aggregate size distribution. Field tests were performed with a seedbed cultivator, using 5 different leveling intensities and 2 cultivation depths. Using a laser range scanner, the soil surface was mapped before, during and after cultivation. These surface maps were analyzed using Granulometry to estimate aggregate size distribution in the seedbed. Mean Weight Diameter (MWD) and Geometric Mean Diameter (GMD) were calculated based on these aggregate size estimates. Additionally, roughness was calculated based on the surface profiles produced by the laser range scanner. The leveling intensity showed a statistically significant effect on the MWD, GMD and roughness, however, the cultivation depth showed no evidence to suggest a significant effect. Finally, roughness calculated during and after cultivation had a good correlation, which shows that it is possible to use the laser range scanner for roughness measurements during the tillage operation.

Conservation agriculture (CA) has been suggested as a means of makingintensification of agriculture sustainable. The purpose of this study was tounderstand and quantify long-term individual and combined effects of keyconservation practices on soil physical properties and topsoil C content.Field experiments were conducted in 11- to 12-yr-old experiments on twoDanish sandy loams at Foulum and Flakkebjerg. Three crop rotations/residuemanagement treatments were compared and tillage was included as a splitplotfactor. The tillage systems were moldboard plowing to a depth of 20 cm(MP), direct drilling (D) and harrowing to a depth of 8 to 10 cm (H). Soilsampling and in-field measurements were performed in autumn 2013 andspring 2014. In the field, soil structure was visually evaluated and penetrationresistance (PR) measured. Soil C, wet stability (clay dispersion and wetaggregate stability), and soil strength were determined in the laboratory. TheMP soil had a uniform soil organic carbon (SOC) content in the 0- to 20-cmdepth of topsoil, whereas H and D resulted in SOC accumulation near thesoil surface. Plowing resulted in the best visually assessed topsoil structureand had the lowest PR. However, H and D in combination with residueretention gave the best structural stability. Residue retention alleviatednegative effects of reduced tillage on PR and improved wet stability in theMP treatment at the Foulum site. Clay and SOC correlated well with soilphysical parameters, confirming their important role in soil structureformation and stabilization. Our study showed benefits of combining key CAelements, although longer-term studies are most likely needed to reveal thefull potential.

Knowledge of soil water content for tillage is important in planning and scheduling tillage operations to avoid tillage-induced soil structural degradation, creating undesirable seedbed for crop establishment, and using high energy inputs because soil is not workable. We propose a new approach (NA), which we compare with the water retention approach (WRA) and the consistency approach (CA) for estimating the wet tillage limit, optimum water content for tillage and the dry tillage limit. The new approach uses soil water content at an air-filled porosity of 0.10 m3 m–3 to estimate the wet tillage limit and an aggregate tensile strength of 50 kPa to estimate the dry tillage limit. The optimum water content for tillage is estimated using the double-exponential water retention function as the soil water content at the break point between the textural and structural pores where in general, the structural pores have drained whereas the textural pores remain filled with water. The three approaches were compared using Highfield soil with a range of soil organic carbon contents (SOC), and Lerbjerg soil with a range of clay contents. The number of workable days for these soils were estimated using a decision support tool for assessing soil workability. Workable days were limited by the soil being either too wet or too dry. For the Highfield soils, the wet tillage limit estimated by NA was generally wetter than that for WRA and CA, whereas for Lerbjerg soils NA was identical to values estimated by WRA. Workable days were strongly influenced by SOC content, clay content, and the approach used for estimating tillage limits. The estimated average workable days in the spring and the autumn seasons over the period from 2014 to 2018 increased with increasing SOC content (from 2 days for BF to 40 days for G treatment), but decreased with increasing clay content (from 38 days for L12 to 0 day for L45) although there was an exception for WRA. Average workable days for the period investigated were more for the tillage limits estimated by WRA and NA compared to CA. Further studies are needed to test the use of fixed values defined in NA for estimating the wet and dry tillage limits for all soils. Future studies could also investigate whether the fixed values of 0.10 m3 m–3 and 50 kPa need to be refined. Field validation on a range of soil textures and in different climates could be the subject of further research to investigate the robustness of the approaches for estimating tillage limits.

Soil structure and structural stability are key parameters in sustainable soil management andoptimum cropping practices. Locally and temporally adapted precision tillage may improve cropperformance while at the same time reduce environmental impacts. The main objective of thisstudy was to improve the knowledge of precision tillage practices through characterizing the effectof varied tillage intensities on structural properties of a clay loam soil. A field experiment wasconducted using a randomized complete block design with two main factors, i.e. operational speed(OS, 2 levels) and rotovating speed (RS, 3 levels). The tillage was conducted using a PTO-drivenrotovator equipped to measure angular velocity. The effect of traffic compaction, made directlyafter tillage, was measured on soil taken from wheel track (WT) compared with soil outside wheeltrack (NWT). Soil samples from 0-3 cm depth, were collected three times, before (T0), after tillage(T1) and later after 45 days. Results show that bulk density as a grand mean was 1.22 g/cm3 at T0;0.95 and 1.42 g/cm3 (NWT and WT, respectively) at T1, and 1.05 and 1.44 g/cm3 (NWT and WT,respectively) at T2. Low OS (2.9 km/hr) combined with high RS (630 rpm) showed significantlyhigher air permeability than the other treatments. Log-transformed air permeability was 17%lower for the WT treatment than for the NWT treatments at T2. Treatments with the high OS (6.3km/h) and thus lowest tillage intensity, generally showed lower clay and silt dispersibility (CSD) atT1 and T2. Dispersibility was larger for the WT treatment than for the NWT treatments at T2.Generally, specific energy input was higher (10.2 kJ/m2) under low OS than under high OS (4.8kJ/m2). Wheel tracked soil resulted in significantly higher GMD (56 mm) compared to the nonwheeltracked soil at T1 (9 mm). We conclude that precise combination of operation and rotationspeeds will result in optimum seedbed in terms of permeability and stability of soil structure.

Soil tilth is a dynamic and multifaceted concept that refers to the suitability of a soil for planting and growing crops. A soil with good tilth is “usually loose, friable and well granulated”; a condition that can also be described as the soil’s having a good “self-mulching” ability. On the other hand soils with poor tilth are usually dense (compacted), with hard, blocky, or massive structural characteristics. Poor soil tilth is generally associated with compaction, induced by wheel traffic, animal trampling, and/or to natural soil consolidation (i.e., so-called hard-setting behavior). The soil-tilth concept dates back to the early days of arable farming and has been addressed in soil-science literature since the 1920s. Soil tilth is generally associated with soil’s physical properties and processes rather than the more holistic concepts of soil quality and soil health. Improved soil tilth has been associated with deep and intensive tillage, as those practices were traditionally considered the primary method for creating a suitable soil condition for plant growth. Therefore, for millennia there has been a strong focus both in practice and in research on developing tillage tools that create suitable growing conditions for different crops, soil types, and climatic conditions. Deep and intensive tillage may be appropriate for producing a good, short-term tilth, but may also lead to severe long-term degradation of the soil structure. The failure of methods relying on physical manipulation as means of sustaining good tilth has increased the recognition given to the important role that soil biota have in soil-structure formation and stabilization. Soil biology has only received substantial attention in soil science during the last few decades. One result of this is that this knowledge is now being used to optimize soil management through strategies such as more diverse rotations, cover crops, and crop-residue management, with these being applied either as single management components or more preferably as part of an integrated system (i.e., either conservation agriculture or organic farming).Traditionally, farmers have evaluated soil tilth qualitatively in the field. However, a number of quantitative or semi-quantitative procedures for assessing soil tilth has been developed over the last 80 years. These procedures vary from simply determining soil cloddiness to more detailed evaluations whereby soil’s physical properties (e.g., porosity, strength, and aggregate characteristics) are combined with its consistency and organic-matter measurements in soil-tilth indices. Semi-quantitative visual soil-evaluation methods have also been developed for field evaluation of soil tilth, and are now used in many countries worldwide.

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.

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.

Annual removal of crop residues may lead to depletion of soil organic carbon and soil degradation. Gasification biochar (GB), the carbon-rich byproduct of gasification of biomass such as straw and wood chips, may be used for maintaining the soil organic carbon content and counteract soil degradation if applied to soil. This study investigated the effect of straw removal and GB addition on soil biological, chemical and physical properties in a 22-months soil incubation study with a temperate sandy loam soil. Soil application of wood and straw GB (WGB and SGB) resulted in very low CO2 emissions, confirming the stability of the material against microbial degradation. Both GBs increased total organic carbon, cation exchange capacity and pH of the soil. The application of SGB and WGB did not affect aggregate stability, whereas SGB did not affect and WGB decreased clay dispersibility. In contrast, the addition of straw resulted in a high soil respiration rate, and about 80% of the added carbon was respired at the end of the incubation. However, the addition of straw increased aggregate stability and decreased clay dispersibility. Results from Fourier transformed infrared photoacoustic spectroscopy revealed a lower content of O-H and aliphatic C-H together with a higher content of aromatic groups in soils amended with GB compared to soils amended with straw. This suggested that the improvement in aggregate stability in straw treatments could be related to microbial derived aliphatics and simple sugars, and that increased stability against microbial degradation in biochar amended soil was related to highly condensed aromatic groups. Addition of nutrients (N, P and S) together with straw resulted in higher soil respiration compared to the straw treatment, but did not cause differences in other soil processes.Results from this study suggest that GB has a potential for increasing soil carbon sequestration, CEC and pH. However, the straw turnover process plays a vital role for aggregate stability and clay dispersibility. Thus, annual straw removal on loamy soil needs to be considered with care in order to avoid soil degradation and risk of soil compaction or erosion. Annual removal of crop residues may lead to depletion of soil organic carbon and soil degradation. Gasificationbiochar (GB), the carbon-rich byproduct of gasification of biomass such as straw and wood chips, may be usedfor maintaining the soil organic carbon content and counteract soil degradation if applied to soil. This study investigatedthe effect of straw removal and GB addition on soil biological, chemical and physical properties in a22-months soil incubation study with a temperate sandy loam soil.Soil application of wood and strawGB (WGB and SGB) resulted in very lowCO2 emissions, confirming the stabilityof thematerial against microbial degradation. Both GBs increased total organic carbon, cation exchange capacityand pH of the soil. The application of SGB and WGB did not affect aggregate stability, whereas SGB did notaffect and WGB decreased clay dispersibility. In contrast, the addition of straw resulted in a high soil respirationrate, and about 80% of the added carbonwas respired at the end of the incubation. However, the addition of strawincreased aggregate stability and decreased clay dispersibility. Results from Fourier transformed infrared photoacousticspectroscopy revealed a lower content of O–H and aliphatic C–H together with a higher content ofaromatic groups in soils amended with GB compared to soils amended with straw. This suggested that the improvementin aggregate stability in straw treatments could be related to microbial derived aliphatics and simplesugars, and that increased stability against microbial degradation in biochar amended soil was related to highlycondensed aromatic groups. Addition of nutrients (N, P and S) together with straw resulted in higher soil respirationcompared to the straw treatment, but did not cause differences in other soil processes.Results fromthis study suggest that GB has a potential for increasing soil carbon sequestration, CEC and pH. However,the straw turnover process plays a vital role for aggregate stability and clay dispersibility. Thus, annual straw removalon loamy soil needs to be considered with care in order to avoid soil degradation and risk of soil compaction or erosion.

Soil aggregate properties, such as strength, shape and porosity, influence a range of essential soil functions and there is a need for more detailed understanding of the effect of soil management on these aggregate properties. There is also a need for improved knowledge on the link between aggregate and bulk soil properties. The objective of this study was to quantify the long-term effect of rotation and tillage on aggregate shape, strength and pore characteristics, to evaluate the influence of aggregate shape and pore characteristics on aggregate strength and soil friability and to correlate aggregate properties to bulk soil properties. Soil core samples were taken in spring 2010 from the long-term rotation and tillage trial (initiated in 1980) at the University of Guelph, Canada. The rotations included were continuous corn (R1) and a diverse rotation (R6), and the tillage treatments were mouldboard ploughing (MP) and no-tillage (NT). The soil cores were exposed to a drop shatter test and air-dried before separation into different size fractions. Ten aggregates from the4-9.2 mm size fraction per core sample (i.e. 320 in all) were X-ray micro-CT scanned and the size, shape and porosity determined using image analysis. Subsequently, aggregate tensile strength was determined in an indirect tension test. For the aggregates, rotation had a more pronounced than tillage treatment on the different properties. The diverse rotation resulted in higher aggregate total porosity and more rounded aggregates than the continuous corn rotation. Surprisingly, there was no treatment effect on X-ray micro-CT resolvable porosities. Aggregate strength decreased with both total and X-ray micro-CT resolvable porosity even though the correlation was weak. A stronger correlation was found to aggregate sphericity although only around 10% of the variation in tensile strength could be explained by this property. Our study highlights that caution must be taken when trying to predict aggregate strength from general aggregate characteristics. For both bulk soil and aggregates, the R6-MP had highest and R1-NT lowest porosity. However, tillage had strongest effect on bulk soil porosity whereas aggregate total porosity was only affected by rotation. We conclude that the scale of observation is important when evaluating the influence of soil management. A strong correlation was found between aggregate strength and pore characteristics and soil fragmentation in a drop shatter test, i.e. 55% of the variation could be explained. Our study indicates therefore that bulk soil fragmentation behavior can be predicted from aggregate characteristics. Soil aggregate properties, such as strength, shape and porosity, influence a range of essential soil functions and there is a need for more detailed understanding of the effect of soil management on these aggregate properties. There is also a need for improved knowledge on the link between aggregate and bulk soil properties. The objectives of this study were to quantify the long-term effect of rotation and tillage on aggregate shape, strength and pore characteristics, to evaluate the influence of aggregate shape and pore characteristics on aggregate strength and soil friability and to correlate aggregate properties to bulk soil properties. Soil core samples were taken inspring 2010 from the long-term rotation and tillage trial (initiated in 1980) at the University of Guelph, Canada. The rotations includedwere continuous corn (R1) and a diverse rotation (R6), and the tillage treatments were mouldboard ploughing (MP) and no-tillage (NT). The soil coreswere exposed to a drop shatter test and airdried before separation into different size fractions. Ten aggregates fromthe 4–9.2mmsize fraction per core sample (i.e. 320 in all)were X-ray micro-CT scanned. The size, shape and porosity of the aggregateswere determined using image analysis with 40 μm voxel size. Subsequently, aggregate tensile strength was determined in an indirect tension test. Rotation had a more pronounced effect than tillage treatment on the different aggregate properties. The diverse rotation resulted in higher aggregate total porosity and more rounded aggregates than the continuous corn rotation. Surprisingly, therewas no treatment effect on X-ray micro-CT resolvable porosities. Aggregate strength decreased with both total and X-ray micro-CT resolvable porosity even though the correlations were weak. Significant correlation was also found to aggregate sphericity although only around 10% of the variation in tensile strength could be explained by this property. Our study highlights that caution must be taken when trying to predict aggregate strength from general aggregate characteristics. For both bulk soil and aggregates, the R6-MP had highest and R1-NT lowest porosity. Tillage had strongest effect on bulk soil porosity, whereas aggregate total porosity was only affected by rotation. Our results suggest that the scale of observation is importantwhen evaluating the influence of soil management. A strong correlationwas found between aggregate strength and pore characteristics and soil fragmentation in a drop shatter test, i.e. 55% of the variation could be explained. Our study indicates therefore that bulk soil fragmentation behaviour can be predicted fromaggregate characteristics. It needs to be highlighted that our results are based on one long-term experiment on a silt loam soil. The results need to be verified for soils with different soil types, climates and management histories.