Mathieu Lamandé

Changes of pore morphology, infiltration and earthworm community in a loamy soil under different agricultural managements

Abstract Earthworm activity produces changes at different scales of soil porosity, including the mesoporosity (between 1.000 and 30 μm eq. dia.) where both water retention and near-saturated infiltration take place. At this scale, the structural changes are poorly described in temperate agricultural systems, so we do not yet fully understand how these changes occur. The present study was conducted to determine the relationships between the morphology of the mesopores, which is mainly affected by earthworm activity, and the hydrodynamic behaviour (near-saturated infiltration) of topsoil under different agricultural managements inducing a large range of earthworm populations. Investigations were carried out at the soil surface in three fields under different management practices giving rise to three different earthworm populations: a continuous maize field where pig slurry was applied, a rye-grass/maize rotation (3/1 year, respectively) also with pig slurry, and an old pasture sown with white clover and rye-grass. Pore space was quantified using a morphological approach and 2D image analysis. Undisturbed soil samples were impregnated with polyester resin containing fluorescent pigment. The images were taken under UV light, yielding a spatial resolution of 42 μm pixel−1. Pores were classified according to their size (which is a function of their area) and their shape. Hydraulic conductivity K(h) was measured using a disc infiltrometer at four water potentials: −0.05, −0.2, −0.6, and −1.5 kPa. The abundance and ecological categories groups of earthworms were also investigated. Continuous soil tillage causes a decrease in both abundance and functional diversity (cf. maize compared with old pasture) when soil tillage every 4 years causes only a decrease in abundance (cf. rotation compared with old pasture). There were no relationships between total porosity and effective porosity at h=−0.05 kPa. Image analysis was useful in distinguishing the functional difference between the three managements. Fewer roots and anecic earthworms resulted in fewer effective tubular voids under maize. There were fewer packing voids in the old pasture due to cattle trampling. Greater hydraulic conductivity in the pasture phase of rotation may arise from a greater functional diversity than in the maize and absence of cattle trampling compared with the pasture. We point to some significant differences between the three types of agricultural management. A better understanding is required of the influence of agricultural management systems on pore morphology. This study provides a new methodology in which we consider the earthworm activity as well as community in order to assess the effects of agricultural management on soil structure and water movement.

X-ray CT and Laboratory Measurements on Glacial Till Subsoil Cores: assessment of inherent and compaction-affected soil structure characteristics

Abstract The aim of this study was to articulate the potential of medical computed tomographic (CT) scanning for analyzing soil structure (macroporosity, soil matrix density, number of macropores) and how these estimates compare with, and complement, traditional laboratory measurements (bulk density, total porosity, effective air‐filled porosity, and air permeability). Undisturbed soil cores were sampled at two depths (0.35 and 0.7 m) in a long‐term soil compaction experiment in southern Sweden 14 years after its establishment. Persistence of subsoil compaction was detectable by CT‐estimated soil matrix density, bulk density, and total porosity. Vertical distribution of CT‐estimated air‐filled macroporosity between 0.25‐ and 0.45‐m depth showed that biological activity effect on macroporosity was largest in the top of the soil columns from the compacted plots, whereas reduction of macroporosity was significant at the bottom of the same columns. This was not detectable by classical laboratory measurements. Variations in air permeability could be related to the CT‐estimated number of pores but not to the CT‐estimated air‐filled macroporosity. Despite using a coarse resolution, the combination of visualization and traditional laboratory measurements proved valuable in identifying the persistent effects of subsoil compaction and the differences in soil structure among the two investigated subsoil layers. However, we recommend to systematically perform a sensitivity analysis to the segmentation threshold before any further analysis of CT‐estimated parameters.

Correlating Gas Transport Parameters and X-Ray Computed Tomography Measurements in Porous Media

Abstract Gas transport parameters and X-ray computed tomography (CT) measurements in porous medium under controlled and identical conditions provide a useful methodology for studying the relationships among them, ultimately leading to a better understanding of subsurface gaseous transport and other soil physical processes. The objective of this study was to characterize the relationships between gas transport parameters and soil-pore geometry revealed by X-ray CT. Sands of different shapes with a mean particle diameter (d50) ranging from 0.19 to 1.51 mm were used as porous media under both air-dried and partially saturated conditions. Gas transport parameters including gas dispersivity (&agr;), diffusivity (DP/D0), and permeability (ka) were measured using a unified measurement system (UMS). The 3DMA-Rock computational package was used for analysis of three-dimensional CT data. A strong linear relationship was found between &agr; and tortuosity calculated from gas transport parameters ( ), indicating that gas dispersivity has a linear and inverse relationship with gas diffusivity. A linear relationship was also found between ka and d50/TUMS2, indicating a strong dependency of ka on mean particle size and direct correlation with gas diffusivity. Tortuosity (TMFX) and equivalent pore diameter (deq.MFX) analyzed from microfocus X-ray CT increased linearly with increasing d50 for both Granusil and Accusand and further showing no effect of particle shape. The TUMS values showed reasonably good agreement with TMFX values. The ka showed a strong relationship when plotted against deq.MFX/TMFX2, indicating its strong dependency on pore size distribution and tortuosity of pore space.

Macropores and Macropore Transport: Relating Basic Soil Properties to Macropore Density and Soil Hydraulic Properties

Abstract Preferential transport of water through soil macropores is a governing process in the facilitated transport of strongly sorbing compounds. The aim of this study was to investigate the relationships between macropore density and the hydraulic conductivity of the soil and to test the sampling representativeness of soil columns for the measurement of saturated hydraulic conductivity. Macropore density was determined in three horizons in four typical Danish soil types (third year of pasture), and saturated hydraulic conductivity and near-saturated hydraulic conductivity were measured in the laboratory on undisturbed soil columns (6,280 cm3) in the same three horizons. A strong relationship between macropore density and the mean particle diameter was found in the B and C horizons. A poor relationship was found between macropore density and the hydraulic conductivity at a matric potential of −10 hPa, whereas a stronger relationship was found at saturation. Results of the test of the number of soil columns needed for a representative distribution of macropores for the measurement of saturated hydraulic conductivity indicated that approximately six soil columns were enough. Our work suggests that integrating knowledge of the abundance of macropores in relation to soil type and land use will increase the performance of pedotransfer functions in predicting saturated hydraulic conductivity.

Associations between soil texture, soil water characteristics and earthworm populations in grassland

Abstract The aim of the present study was to investigate the relationships between soil physical characteristics and earthworms in a regional-scale field study in Denmark. The earthworm populations along within-field gradients in soil texture were quantified at five field sites, representing dominant soil types of Denmark. Eleven earthworm species were found, but populations were mainly dominated by Aporrectodea tuberculata and A. longa. Despite considerable variation in soil parameters across the five study sites the results suggest that the biomass of anecic worms (or A. longa as a species) was not causally associated with the soil parameters studied. This indicates that there must be other causal factors associated with the abundance (and composition) of anecic worms that are not among the soil texture and structure parameters studied. On the other hand, soil texture (Coarse sand) was associated with the abundance of the dominant endogeic species, A. tuberculata, but not endogeic worms in general. It was hypothesized that anecic and endogeic earthworms might respond to local soil water characteristics rather than soil texture, but this hypothesis could not be confirmed with the present data.

Upper subsoil pore characteristics and functions as affected by field traffic and freeze–thaw and dry–wet treatments

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

Soil mechanical stresses in high wheel load agricultural field traffic: a case study

Subsoil compaction is a serious long-term threat to soil functions. Only a few studies have quantified the mechanical stresses reaching deep subsoil layers for modern high wheel load machinery. In the present study we measured the vertical stresses in the tyre–soil contact area and at 0.3, 0.6 and 0.9 m depths of a sandy loam soil at field capacity water content. The soil was ploughed annually to a depth of 0.25 m and was tested in the spring following autumn ploughing but before secondary tillage. The machinery tested was a tractor–trailer system for slurry application with a total weight of 52 Mg. Wheel loads ranged from approximately 20 to 70 kN. The tyres were all radial ply with volumes ranging from 0.63 to 1.23 m3. The tyre inflation pressures were generally above those recommended by the manufacturer and ranged from 170 to 280 kPa. The stress distributions in the contact area were highly skewed. Across tyres, the maximum stress in the contact area correlated linearly with, but was much higher than, the mean ground pressure. For each of the three soil depths, the maximum stresses under the tyres were significantly correlated with the wheel load, but not with other loading characteristics. The data predict a 6.6-kPa increase in vertical stress at 0.9 m depth for each 1-Mg addition to the wheel load. The soil stress observations support a simple rule of thumb combining wheel load and inflation pressure in calculation of subsoil vertical stress. We measured vertical stresses up to 300, 100 and 45 kPa at soil depths of 0.3, 0.6 and 0.9 m respectively. Comparing these with the data in the literature regarding soil strength and measured compaction effects on the soil studied, we conclude that the traffic event investigated is likely to induce serious effects on soil properties and functions to a depth of at least 0.7 m.

Soil organic matter widens the range of water contents for tillage

Highlights • Soil organic carbon (SOC) affected the mechanical properties of soil aggregates.• Water contents for tillage is determined using water retention and consistency approaches.• There is a strong positive relation between SOC and range of water contents for tillage (ΔθRANGE).• ΔθRANGE determined based on the consistency approach is recommended over the water retention approach.

Seed drill depth control system for precision seeding

Abstract An adequate and uniform seeding depth is crucial for the homogeneous development of a crop, as it affects time of emergence and germination rate. The considerable depth variations observed during seeding operations - even for modern seed drills - are mainly caused by variability in soil resistance acting on the drill coulters, which generates unwanted vibrations and, consequently, a non-uniform seed placement. Therefore, a proof-of-concept dynamic 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, by testing uniformity and accuracy of the coulter depth in relation to the target depth of −30 mm. The evaluation was based on coulter depth measurements, obtained by coulter position sensors combined with ultrasonic soil surface sensors. Mean coulter depth offsets of 3.5, 5.3 and 6.3 mm to the target were registered for the depth control system, compared to 8.0, 9.1 and 11.0 mm 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 depth accuracy by 15.2% and at 95% confidence interval it corresponded to an absolute reduction in the coulter depth confidence span of 10.4 mm. The spatial variability, due to variation in soil mechanical properties was found to be ±8 mm, across the blocks for the standard drill and when activating the coulter depth control system this variability was reduced to ±2 mm. The system with the active control system operated more accurately at an operational speed of 12 km h−1 than at 4 km h−1 without the activated control system.

Reducing the risk of soil compaction by applying ‘Jordværn Online’® when performing slurry distribution

Abstract Through a case study, it is shown that the potential of applying JORDVÆRN Online® as a decision-making tool aimed at supporting sustainable in-field traffic provided the farmer with the potential to improve and maintain the soil as a continued growth medium for the future agri-food production. The combination of CTF and optimal selected tyre configurations provides the farmer with the potential of cropping agricultural land without creating permanent soil compaction. Additional, the case study indicates an urgent need for the development of alternative systems for in-field traffic in wet conditions, e.g. multiple-axle trailers with real low-inflation tyres.