SSS6.4

The structure of soils, i.e. the macroscopic organization of aggregates and pores, conditions the storage and transport of water and gas in the soil, and strongly determines the physico-chemical environment of soil organisms (plants, micro and macro-organisms). The description of the soils structure dynamics constitutes a major issue in the current context of global change, at the scientific, environmental and agronomic level. However, few tools are available to monitor this dynamic non-destructively and in situ. We therefore propose to develop a new method based on the analysis of acoustic emissions (AE) spontaneously emitted by soils during the evolution of their structure. A laboratory feasibility study was conducted to explore the links between variations in soil structure and the AE emitted during soil desiccation.

Two undisturbed soil columns (8 cm in diameter, 5 cm high) were sampled in an agricultural field (near Chartres in France), in the surface horizon of a Glossic Retisol. These cylinders were air dried (20°C during 9 days), and the AE produced during drying were monitored using piezoelectric sensors place at the soil surface. The concomitant soil structure changes were followed through 3D images, acquired by X-ray tomography (CIRE platform, INRAE, Nouzilly) all along the experiment. These images, with a resolution of 168 µm, were used to characterize the pore network (porosity, surface density, connectivity, etc.).

The dynamics of the EAs recorded during the drying of the samples is comparable for the two samples: the AE rates are maximum at the start of the experiment and then reach a plateau. Changes in soil structure follow the same dynamics, e.g. considering porosity or surface density of the pores. If we analyze the relationship between the signals recorded by the surface sensors (EA rate) and the porosity, we observe a linear relationship (R² of 0.79). This relationship, although encouraging, remains to be consolidated by additional results.

To go further, it is also necessary to define the necessary conditions to perform such a measurement in situ, and to improve the acoustic signal processing to characterize the EA produced during soil desiccation. Indeed, a major objective of our work is to differentiate, thanks to EAs, the various factors responsible for the evolution of soil structure (physical and biological), by determining their "acoustic signature".

How to cite: Lacoste, M., Giot, G., Seger, M., and Cousin, I.: Passive acoustic emissions: a dynamic tool to monitor soil structure variations?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12184, https://doi.org/10.5194/egusphere-egu21-12184, 2021.

Adopting integrated measurement techniques may enhance our understanding of hydropedological processes within the critical zone. To investigate lateral subsurface flow due to lithological discontinuities, a ponding infiltration test, two GPR surveys, and soil penetration resistance (PR) measurements were conducted on a 1 m² plot in a vegetated area located in the university campus of Doua (Lyon, France). A GPR grid with 0.2 m intervals was established. In the center of the grid, around the root system of a hawthorn shrub, an infiltration test was conducted using an automated single-ring infiltrometer proposed by Concialdi et al. (2020), to infiltrate a shear-thinning viscous solution (1 g L⁻¹ Xanthan gum powder). The viscous solution was expected to fill preferential pathways due to the roots, with limited infiltration into the soil matrix, and thus reveal complex geometries or macropore networks in highly heterogeneous soils. To create three-dimensional (3D) representations of the infiltrated solution, two GPR surveys were carried out just before and 20 min after the infiltration test, using a GSSI (Geophysical Survey System Inc., Salem, NH) SIR 3000 system with a 900 MHz antenna. A total of 24 radargrams were collected in time mode by moving the antenna along the survey lines and recording the markers position along the survey line intersections. After the second GPR survey, PR was measured at each of the 36 intersection points of the grid using an electronic hand-pushed cone penetrometer. The cone had a 30° angle and a base area of 1 cm², inserted into the soil at a constant speed of 2 cm s⁻¹ to a depth of 0.8 m. These measurements were aimed to highlight contrasting penetration resistance characteristics between different soil horizons. We also determined the soil bulk density from 24 undisturbed soil cores (~ 100 cm³) collected at different depths from 0 to 50 cm. Finally, an auger was used to extract a 0.69-m-depth soil core for the direct observation of lithological heterogeneities.

Differenced radargrams from pre- and post-infiltration surveys allowed to detect the 3D infiltration bulb, which was vertically elongated and irregularly shaped, but with an evident horizontal divergence between the depth of 20 and 30 cm. Below 30 cm depth, a significant increasing of soil PR and BD (respectively higher than 2.5 MPa and 1.50 g cm^-3, between 30 and 50 cm depth) was detected, indicating the presence of a underlying layer, which was also identifiable by visual observation of the soil core. This dense layer impeded water flow. Consequently, the liquid solution partially diverged laterally and accumulated upside this layer, and partially infiltrated into the dense layer along preferential flow paths in correspondence with the plant root system, as detected by the 3D GPR diagram. Summing up and considering every aspect, this study allowed to identify water perching above a shallow restrictive layer for a better understanding of the water dynamics of the investigated soil. This study shows the benefits to couple different types of soil physics approaches to relate hydrological processes to the soil hydraulic and mechanical properties.

How to cite: Capello, G., Biddoccu, M., Di Prima, S., and Lassabatere, L.: Combining new techniques to investigate water dynamics above a shallow restrictive layer., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12220, https://doi.org/10.5194/egusphere-egu21-12220, 2021.

Cover crops are widely known for their capacity to improve the soil biological properties and soil structural stability. Nevertheless, the cover crop residues quantity necessary to improve these soil properties is not yet really known. A 30-day incubation experiment was conducted to explore the effect of oilseed rape (Brassica napus) residues (ORR) as a cover crop on the soil aggregate stability of sandy loam soil. The fresh ORR was mixed with the soil at different rates starting from 1.0 to 6.0 g C kg^-1 of soil. The experiment consisted of five treatments: bulk soil (I), soil mixed with ORR at a rate of 1 g C kg^-1 of soil (II), soil mixed with ORR at a rate of 2 g C kg^-1 of soil (III), soil mixed with ORR at a rate of 4 g C kg^-1 of soil (IV), soil mixed with ORR at a rate of 6 g C kg^-1 of soil (V). During 30 days of incubation the soil moisture, soil water stable aggregates, and microbial substrate induced respiration rates were measured. The aggregate stability significantly increased after 30 days only in the treatment with 1 g C kg^-1 of soil. In turn, the ORR applied at a rate of 6 g C kg^-1 of soil significantly decreased the soil aggregate stability. The higher the ORR addition rate the lower was the soil basal respiration and substrate induced respiration. The general conclusion was that the higher quantity of ORR increased the soil moisture which subsequently created unfavorable conditions for the soil microbial activity and led to soil aggregate stability degradation. However, this conclusion must be validated in a field study where the soil moisture and temperature conditions are much more variable compared to our incubation experiment.

How to cite: Stegarescu, G., Reintam, E., and Tõnutare, T.: The influence of different cover crop residues quantities on soil structural stability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3877, https://doi.org/10.5194/egusphere-egu21-3877, 2021.

Biological soil crusts (biocrusts) composed of cyanobacteria, algae, lichens and bryophytes have a stabilizing effect on the soil surface. This effect is mostly studied in arid climates, where biocrusts are the main biological agent to steady and bind together soil aggregates. Nevertheless, biocrusts are also an integral part of the soil surface under semi-humid and humid climate conditions, mainly covering open spaces in forests and on fallow lands. As such, they often develop after vegetation disturbances, when their ability to compete with higher plants is increased. To better understand how biocrusts mediate changes in soil aggregate stability under different climatic conditions, we analyzed soil substrates taken under biocrust communities from four national parks in Chile using dry and wet sieving. These samples cover soils from a large climate gradient from arid (Pan de Azúcar), semiarid (Santa Gracia), mediterranean (La Campana) to humid (Nahuelbuta). 
Biocrust communities were dominated by cyanobacteria in Pan de Azúcar and Santa Gracia, bryophytes and lichens in La Campana and bryophytes in Nahuelbuta. They showed a stabilizing effect on the soil surface in three of the four investigated climates. Their presence increased the Mean Weight Diameter of the aggregates (MWD) by 102% in Pan de Azúcar, 208% in Santa Gracia and 82% in La Campana. In Nahuelbuta there was no significant increase to the condition without biocrust, because the abundance of permanent soil covering higher vegetation does not allow the effect of biocrusts to manifest. The stabilization differed between the aggregate fractions studied, being most pronounced for smaller aggregates >2 mm. The Geometric Mean Diameter (GMD) showed similar results as MWD, but with a clear effect of drying and wetting conditions, as an increase in the stability directly related to precipitation and the climatic gradient. Bulk density (BD) changed from high mean values of 1.50 g cm^-3 in Pan de Azúcar and 1.63 g cm^-3 in Santa Gracia (where cattle grazing was observed) to 1.16 g cm^-3 in La Campana and the lowest mean of 0.62 g cm^-3 in Nahuelbuta, where we observed a more developed soil structure and high organic matter content (21.58% in average). Accordingly, here we also found pronounced hydrophobicity of the soil. These preliminary findings indicate not only differences in the stability of the aggregates, but also in the state of conservation and management of the soils. Results will now be extended by further statistical analyses, which will additionally be presented at vEGU21.

How to cite: Riveras Muñoz, N., Seitz, S., Gall, C., Pérez, H., Kuehn, P., Seguel, O., and Scholten, T.: Biological soil crusts mediate changes in soil aggregate stability along a climate gradient in Chile, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7914, https://doi.org/10.5194/egusphere-egu21-7914, 2021.

The aggregate composition is one of the key characteristics of soils. Based on the use of aggregates of different sizes, one can draw conclusions about the anthropogenic load on soils in conditions of agricultural use. We investigated the aggregate composition of soil samples of the 0-20 cm layer of soils with application of four tillage techniques: plowing, energy-saving (surface tillage, without plowing), longline (surface tillage + chisel tillage), anti-erosion (chisel tillage). Stationary experience laid in 1995. The сrops were: bare fallow, busy fallow and wheat. Soil samples were sieved with a standard set of sieves:> 10; 10-7; 7-5; 5-3; 3-2; 2-1; 1-0.5; 0.5-0.25; <0.25 mm, water resistance was determined by the Savvinov method with sieves of:> 5; 5-3; 3-2; 2-1; 1-0.5; 0.5-0.25; <0.25 mm with daily water saturation prior to analysis. Rheological parameters were studied using the amplitude sweep method on a modular compact rheometer MCR-302 (Anton Paar, Austria) with a parallel plateau PP-25 measuring system.

All 24 plots with all four treatments demonstrated an excellent water-resistant structure of agro-gray soils. The soils have a good structural state in terms of agronomically aggregates content and the structural coefficient. Water resistance of the structure is excessively high and good as well. The average diameter of the aggregates is from 3.6 to 6.1 mm, the average value is 4.7 mm. This gives us an idea of the structural condition in general, but we cannot track the structural condition of plots with different treatments in the field.

The Principal Component analysis and cluster analysis was used to determine the differences in soils of different types of use. We used STATISTICA. These statistics method successfully classified soils structural relatively treatments.

The study of the rheological properties of agro-gray soils with different processing methods showed that in the zone of linear viscoelastic behavior (LV[С1] B) during all treatments, the range of LVB did not differ significantly and averaged 0.0048% deformation. Differences were noted at Shear Stress max, for the energy-saving treatment application, the τ (632.67 Pa) was lower than the other treatments, an average of 661.83 Pa for 12 repetitions. The crossover occurred at 1.48% strain for an average of 12 reps. The smallest value for the deformation at which crossover occurred (1.22%) was observed for the variant with the use of anti-erosion treatment. The highest (1.72%) for energy efficient processing.

Thus, the use application of aggregate and rheological analyses has shown the promotion of energy-saving technologies application to form a relatively favorable structural condition of the studied soils.

The work was supported by RFBR grant No 19-29-05021 mk

How to cite: Mishchenko, A., Khaydapova, D., Karpova, D., Abdulkhanova, D., and Shulga, P.: The structural state of agro-grey soil in various tillage conditions (based on aggregate and rheological analyses), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10206, https://doi.org/10.5194/egusphere-egu21-10206, 2021.

N₂O emission in soils is a consequence of the activity of nitrifying and denitrifying microorganisms and potentially abiotic processes. However, the large microscale variability of the soil characteristics that influence these processes and in particular the location of anoxic microsites, limits prediction efforts. Better understanding of denitrification activity on microscopic scales is required to improve predictions of N₂O emissions.

This study explored the role of soil microstructure on N₂O emission. To fulfill this objective we sampled 24 soil columns (5 cm diameter, 6 cm height) in the surface layer of a same plot in a cultivated soil (Luvisol, La Cage, Versailles, France). The soil samples were saturated with a solution of ammonium nitrate (NH₄NO₃), and equilibrated at a matrix potential of -32 cm (pF 1.5). The emitted fluxes of N₂O were measured during 7 days. At the end of the experiment, the soil columns were scanned in a X-ray micro tomograph, at the University of Poitiers. A 32 µm voxel resolution was achieved for the 3D reconstructed images.

In order to reduce noise and segment the 3D images, the same protocol was implemented for all columns. The reduction of noise consisted of passing a non-local mean filter, a non-sharp mask and a radial correction. Such combination of steps succeeded in removing both ring artifacts and the radial dependence of the voxel values. Due to the variety of material densities in the soil, a local segmentation based on the watershed method was implemented to classify the soil constituents in four classes (based on its density value): air, water and organic matter (OM), soil matrix and minerals. This method is good for detecting thin pores and avoids missclassification of voxels undergoing partial volume effect, which can lead to false organic coatings around macropores.

The soil columns exhibited a large variability of accumulated N₂O after 7 days (from 107 to 1940 µgN kg^-1 d.w. soil). The size of OM clusters varied between a couple and up to thousands of voxels. No correlation was found between the emission of N₂O and the porosity, nor between the N₂O emission and the connectivity of the air phase. Based on the premise that the less accessible is the oxygen to the OM, the bigger should be the N₂O emission of the soil column, we proposed and computed a microscopic spatial descriptor, I_gd, based on the notion of the geodesic distance between clusters of OM and air for each soil column 3D image. We expect to find a correlation between I_gd and the N₂O emission.

How to cite: Ortega-Ramirez, P., Pot, V., Laville, P., Schlüter, S., Hadjar, D., Basile-Doelsch, I., Henault, C., Caurel, C., Mazurier, A., Lacoste, M., and Garnier, P.: Role of soil microstructure on the emission of N2O in intact small soil columns, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12438, https://doi.org/10.5194/egusphere-egu21-12438, 2021.

Bioenergy crop cultivation is suggested as one of the promising options to increase soil organic carbon (SOC) stock, and thereby sequester atmospheric carbon dioxide. Yet, the increase in SOC varies greatly depending on the cropping system, with high plant diversity in particular appearing to be positive for carbon storage. This is recently linked to, among other things, the formation of a pore architecture favorable for microbial function and the storage of microbial degradation products. However, little is known about whether this observation holds true for a wide range of soil textures. Therefore, the objective of this research was to compare the abundance of pores with different sizes and SOC contents in soils with contrasting texture and plant diversity. Soil cores and surrounding soil samples were taken on seven long-term field experiments of monoculture switchgrass and restored prairie sites in Michigan, USA. In addition to texture and SOC analyses in disturbed soil samples, undisturbed cores with a diameter of 5 cm were scanned by micro-computer tomography (µCT) at a resolution of 18 µm. These will be used to analyze pore characteristics.

such as pore size distribution.

Results reveal, in highly sandy soil, high plant diversity was less effective to form narrow mid-size pores, and thus did not enhance SOC, while numerically higher SOC contents were observed in the restored prairie of less sandy soil, having higher abundance of mid-size pores compared to the monoculture. In conclusion, in the highly sandy soil, restored prairie with plant diversity was less effective to form pores in the mid-size range, and thus it couldn’t enhance the capability of C sequestration.

How to cite: Lee, J. H., Lucas, M., Guber, A., and Kravchenko, A.: Pore architecture and soil carbon accrual in soils under monoculture switchgrass vs. prairie soils, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14035, https://doi.org/10.5194/egusphere-egu21-14035, 2021.

The ploughing of soils drastically alters soil structure and at the same time reduces its stability against external stresses. A fragmentation of these artificially produced soil clods during winter time is often observed in areas with air temperatures fluctuating around the freezing point. In this study, the cumulative effects of multiple freeze-thaw cycles (FTCs) on soil structure and soil hydraulic properties were analyzed for two different soil textures, a silty clay loam with a substantial amount of swelling clay minerals and a silty loam with less swell/shrink dynamics. The soil material was brought into two different initial states: (i) undisturbed soil cores taken from the topsoil from a grassland, and (ii) cylinders repacked with soil clods taken from a ploughed field nearby. FTCs were simulated under controlled conditions in the lab, changes in soil structure ≥48 µm were regularly recorded using X-ray µCT. After 19 FTCs, the impact on hydraulic properties were measured and the resolution of structural characteristics were increased to 10 µm by subsampling.

The effect of FTC on soil structure was found to be dependent on the initial structure, soil texture and number of FTCs. Freezing and thawing induced a consolidation of the repacked soil clods taken from both field sites, resulting in a systematic reduction in pore sizes and macro-pore connectivity. The macro-pore system of the undisturbed samples was only slightly affected. Fragmentation of soil elements larger than 0.8 to 1.2 mm increased the connectivity of pores smaller than 0.5 to 0.8 mm. Frost action increased the unsaturated hydraulic conductivity of all treatments, while the water retention was only slightly affected. This leads to the conclusion that multiple FTCs enforces a well-connected meso-pore system at the expense of a fragile macro-pore system. A change in soil structure that benefits farmers but could be reduced in the face of milder winters due to global warming.

How to cite: Leuther, F. and Schlüter, S.: The impact of freeze-thaw-cycles on soil structure and soil hydraulic properties, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1089, https://doi.org/10.5194/egusphere-egu21-1089, 2021.