Physical degradation of agricultural soils manifests itself in different ways: decrease in infiltration rate and water storage, poor aeration and, compaction. All these symptoms have a common cause: the deterioration of the soil's natural structure due to the usual agricultural management practices.

Soil water retention curves are a valuable tool for diagnosing the physical state of the soil. Soil properties are the ones that regulate the shape of this curve, with special relevance to texture and structure. For example, changes in macroporosity –associated to soil aggregates and therefore to its structure– would be reflected in changes in the shape of this function in the low suction range.

This work evaluates no-tillage as an alternative to conventional tillage in a typical soil of Navarre (Spain), based on the analysis of soil water retention curves (SWRC).

Two plots were selected, identical in soil type and use, but contrasting in their management: (i) no-tillage (18 continuous years) after conventional tillage and (ii) conventional tillage. In both treatments, undisturbed soil samples were taken (0-5 cm). From these, SWRCs were obtained in the laboratory using the Hyprop device. Dexter’s S index was determined for each SWRC.

The S index did not show significant differences between the two treatments. However, the SWRCs present significant differences between treatments regarding pore size distribution. The tilled soil showed higher macroporosity (gravitational water). Therefore, the soil (surface horizon) under no-tillage could store ca. 10 % more water for the crop.

How to cite: Aldaz, A., Giménez, R., Virto, I., Campo, M. Á., and Arregui, L. M.: Evaluation of no-tillage as an alternative management for the improvement of the physical condition of agricultural soils through the analysis of water retention curves., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3576, https://doi.org/10.5194/egusphere-egu22-3576, 2022.

The interactions between mineral particles and soil organic matter (SOM) are an important factor for soil structure formation. Percolating dissolved organic matter (DOM) from upper soil horizons is considered an important input pathway of organic carbon (OC) into subsoils. While DOM sorption processes have been extensively studied, the effect of DOM input on soil structure formation has rarely been looked at systematically. We conducted a 30-day laboratory incubation experiment to investigate the process of DOM-induced structure formation in artificial model soils with three contrasting textures (clay loam, loam, sandy loam).

The soil texture defined the pore system and the flow characteristics of the soil solution, leading to a lower liquid retention and faster soil solution turnover in the sand-rich soils. In contrast, the OC retention was unaffected by the soil texture, indicating that only the clay minerals and iron oxides, but not the texture-defining quartz grains, contributed to the OC sorption.

The total microbial biomass, as well as the CO₂-release were unaffected by the texture. In contrast, the microbial community composition showed a texture-dependent development with a higher proportion of fungi and gram-positive bacteria in the sand-rich mixtures. This suggests that texture-related architectural features of the pore space shape the microbial community composition.

It could be shown that the biochemical processing of the percolating DOM solution was sufficient to induce the formation of large macroaggregates in all textures without requiring mechanical stress or the presence of physical OM nuclei. Very low OC concentrations (< 0.8 mg g^-1) could support the water-stability of the formed aggregates, although they were not sufficient to provide any meaningful stability against mechanical loads.

How to cite: Bucka, F. B., Felde, V. J. M. N. L., Peth, S., and Kögel-Knabner, I.: Dissolved organic matter may induce water-stable aggregates in various soil textures, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3627, https://doi.org/10.5194/egusphere-egu22-3627, 2022.

Soil structure refers to the spatial arrangement of primary soil particles, their cohesion and the pores in between them. It has a fundamental impact on a variety of soil functions including carbon sequestration and water holding capacity. Researchers in this field either approach the topic by investigating the geometry of pore networks in undisturbed soil; or they instead evaluate properties of aggregates obtained from disassembling soil clods. Which of the two approaches is chosen depends on the requirements and traditions in the respective soil science discipline. There have been surprisingly little efforts undertaken to relate both viewpoints on soil structure quantitatively. In this study, we present and evaluate methods to delineate soil aggregates in eight X-ray images of undisturbed soil samples. The approaches exploit crack formation upon shrinkage in drying soil. Comparing the image-derived aggregates to results from drop-shatter tests, we observed promising trends but overall, the results remained inconclusive. On the one hand, this was due to the very small number of studied samples. On the other hand, the presented aggregate delineation approaches have potential for improvement. We suggest to develop this line of research and apply it to larger numbers of samples, different scales and different physical aggregate isolation approaches, like dry and wet sieving. For example, it may be evaluated whether microaggregates are identifiable in still intact macroaggregates.

How to cite: Koestel, J., Fukumasu, J., Garland, G., Larsbo, M., and Nimblad-Svensson, D.: Is it possible to delineate aggregates in X-ray images of intact soil samples?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5763, https://doi.org/10.5194/egusphere-egu22-5763, 2022.

Natural permeable media like soils, sediments, but also rocks, provide niches of different quality for the inhabitation by a diversity of organisms and communities. The locations for colonization are the microstructured, frequently hierarchic organized heterogeneous biogeochemical interfaces (BGI) that evolve during weathering and pedogenesis. These BGIs are built from a vast variety of organic and inorganic materials and organisms and become manifest as crusts or (micro)aggregates that frame the void network and connect to the liquid and gaseous phases. Microaggregates, operationally defined as composite, microporous, and themselves already heterogeneous composite structures smaller than <250µm, are supposed to be fundamental structural components, because of their stability, persistence, ubiquitous presence, and growing fraction during weathering and pedogenesis. Although research on structure development and fluid-solid interaction in permeable media is an important, exciting and competitive field of soil science, in particular the co-evolution of structure and function due to the interplay of the multitude of biochemical and biophysical processes in view of the properties, functions and resilience of soils has yet to be unravelled. By now, it is well accepted that such and endeavour requires integration of soil physical, chemical, and biological disciplines and demands the development and application of joint advanced characterization and probing techniques including molecular biology within a multi- and inter-disciplinary research approach. Within the framework of the research unit 2179 “Microaggregate development and turnover in soil” and its preceding priority research program 1315 “Biogeochemical Interfaces in Soil”, collaborative research has been put in action that aim at the systematic characterization and functional exploration of aggregates structures and the associated BGI. The presentation will give a compact introduction on the propositions, concepts, and challenges of this exciting research field that aims to contribute to a fundamental understanding of the basics of the co-evolution of architecture and function and the consequences for soil based ecosystem services, and resilience of soils.

How to cite: Totsche, K. U.: Co-Evolution of Structure and Heterogeneity in Natural Permeable Media: The Emergence of Niche-Diversity and Functions During Weathering and Pedogenesis, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5800, https://doi.org/10.5194/egusphere-egu22-5800, 2022.

Sudden or extreme changes in the hydraulic and chemical conditions severely alter water flow and chemical interactions in soil. In response, this may cause an internal erosion of pore space as soil constituents are disaggregated, released and transported, which ultimately even shapes soil horizons. The resilience and resistance of soils against hydraulic and osmotic stress determines their susceptibility to internal erosion. However, the impact of single stress events cannot be observed in field experiments due to a multitude of parallel processes and boundary conditions that change simultaneously. In contrast, unsaturated column experiments using undisturbed soil monoliths offer close-to-natural packing conditions while at the same time providing full control over the boundary conditions.

To investigate how susceptible soils are for internal erosion and thus to the release of (in-)organic soil constituents, unsaturated column experiments were performed with undisturbed topsoil monoliths of a Luvisol and a Regosol formed on loess. Hydraulic and osmotic stress events were simulated by irrigation sequences with two drainage events (desiccation; hydraulic stress), two flow interrupts (ponding; hydraulic stress), and two tracer applications (osmotic stress).

After each stress event, an increase in particle concentration was measured in the effluent, most pronounced when the ionic strength of the influent decreases after the tracer breakthrough. Likewise, the release of soil organic matter (OM) responds predominantly to osmotic stress events and OM fluorescence points to the release of plant derived and microbial processed OM. Moreover, the application of X-ray µ-CT imaging on soil monoliths revealed the alteration of soil structure during the experiment. Especially, the position of secondary carbonates and macropores were identified as useful reference points to reveal structural changes such as pore refilling and soil compaction. In this way, we were able to show how the evolution of soil structure in response to the transport of (in-)organic soil constituents relates to specific hydraulic and osmotic events.

How to cite: Guhra, T., Van Overloop, L., Ritschel, T., and Totsche, K. U.: Exploring the resilience and resistance of soil against hydraulic and osmotic stress in unsaturated column experiments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7716, https://doi.org/10.5194/egusphere-egu22-7716, 2022.

Soil aggregation and the translocation of clay as well as organic matter are major processes of pedogenesis that manifest in the diagnostic soil horizons of mature soil. Yet, their onset might date to much earlier stages of soil development where host rock weathering is dominant and litter from pioneer vegetation is the only input of organic matter. To what extent aggregate formation is induced by early weathering and how clay transport facilitates aggregation is not yet comprehensively explored. Here, we present a time-lapse experiment on initial pedogenesis that reveals the formation of aggregates and clay translocation in response to irrigation with and without organic matter released from a litter layer. We show how organic matter increases total carbonate dissolution capacity with a characteristic surface morphology, but simultaneously slows down the dissolution rate. With the dissolution of carbonates, clay minerals of the host rock and iron from pyrite are released. Controlled by the presence of organic matter, both are either transported with the seepage water or form crusts and aggregates from clay minerals and freshly precipitated secondary iron oxides. The translocation and aggregation of organic matter and clay-sized minerals therefore shape soil structure already during initial pedogenesis and control the route in which soil development becomes apparent. 

How to cite: Ritschel, T., Aehnelt, M., and Totsche, K.: The evolution of early soil microstructure is governed by organic matter and its impact on the weathering rates of host rock, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8359, https://doi.org/10.5194/egusphere-egu22-8359, 2022.

Soil organisms such as arbuscular mycorrhizal fungi (AMF) and the roots they inhabit are key actors for shaping soil structure, which fosters a multitude of functions such as carbon storage and water availability. The expansion of AMF external hyphae, by being in direct contact with soil particles, can promote soil structure formation and thus induce a positive feedback on plant growth under unfavorable conditions such as drought.

Here, we aim at disentangling the complexity of the root-AMF-soil interface by partitioning the respective effects of AMF, of root, and of their interaction on soil structure formation and organic matter cycling, under both drought and well-watered conditions. To discriminate the effects of plant and AMF, we used the wild-type and two mutants of the plant species Lotus japonicus that cannot be properly colonized by AMF (ccamk and ram2-2). The mutant ccamk impairs root entry by the fungus and ram2-2 causes impaired arbuscule development. To exclude confounding factors, we used an artificial soil mixture (quartz, illite, goethite; loamy texture) that was free of microorganisms and native organic matter. The wild type and the mutants were grown in this substrate during a 60-day incubation in a climate chamber. Half of the mesocosms were inoculated with spores of the AMF Rhizophagus irregularis. We stopped the watering two weeks before the end of the experiment in half of the cylinders to create drought conditions. At the end, roots and shoots were sampled and the rhizosphere soil was separated from the non-rhizosphere soil. We analyzed root architecture, AMF traits (intraradical colonization, hyphae length), as well as aggregate distribution and their organic carbon and nitrogen contents in the rhizosphere soil.

Our results highlight the major role of AMF in promoting plant growth, with an increase of above-ground biomass, total root length and root surface area in the soil colonized with AMF, regardless of the water conditions. While plant root vigor (biomass, length, surface area) is reduced under drought conditions, the AMF are resistant to drought, with unchanged mycorrhization intensity and hyphae length in the soil that received less water. Under well-watered conditions, we quantified a higher share of macroaggregates. While AMF did not significantly affect soil structure formation, the presence of fungal hyphae resulted in an increase of carbon and nitrogen contribution of microaggregates in the rhizosphere soil. We are thus able to demonstrate that irrespective of soil water availability, AMF foster the vigor of the host plant. Furthermore, the expansion of AMF into soil, leading to higher carbon and nitrogen storage in rhizosphere soil microaggregates, is not dependent of soil moisture conditions.

How to cite: Holmer, A., Gineyts, R., Guigue, J., Zeng, T., Bucka, F., Colombi, T., Köhler, T., Gutjahr, C., Mueller, C. W., and Vidal, A.: Arbuscular mycorrhizal fungi foster carbon and nitrogen storage in soil microaggregates even under drought conditions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8486, https://doi.org/10.5194/egusphere-egu22-8486, 2022.

Soil aggregation is a key element of soil structure, providing a range of micro-niches for soil-borne microorganisms and creating fine-scale heterogeneity in physical and chemical properties. Soil microorganisms drive a range of critical terrestrial ecosystem functions. The importance of understanding the impact of soil aggregates on microbiome assembly and function is increasingly becoming appreciated. In this study, we used a long-term tomato monoculture field as a model system to investigate the impact of soil aggregates on bacterial community assembly and inhibition of the pathogen Ralstonia solanacearum. Samples were collected after harvest from experimental fields with either no fertilizer (CK), chemical fertilizer (CF), organic fertilizer (BF) or a bio-organic fertilizer (BF) and separated into categories of soil aggregates (e.g. <0.25 mm, 0.25-1 mm, 1-2 mm, >2 mm) by a wet-sieving method. Bacterial community composition was found to differ significantly across aggregate fractions, and bacterial communities from larger aggregate fractions exhibited a higher degree of phylogenetic clustering. Furthermore, we found that soil aggregate size classes differed in the relative importance of deterministic versus stochastic processes Fields with different fertilization differ in soil aggregates distribution and disease suppression. Fields with organic inputs (OF, BF) had a higher abundance of large macro-aggregates and fewer micro-aggregates than inorganic input treatments (CK, CF). Meanwhile, disease incidences were lowest in BF, then increasing in OF, CF and CK, orderly. Interestingly, only relative density of R. solanacearum in micro-aggregates was positively correlated with disease. Furthermore, in experiments involving inoculation of R. solanacearum into aggregate size fractions recovered from field samples, only micro-aggregates (<0.25 mm) from the low disease incidence soil (BF) showed significantly higher resistance against pathogen invasion as compared to the high disease incidence soil (CF). In summary, under agricultural practice, soil aggregates can mediate the ecological assembly processes of bacterial communities, thereby influences the suppression of bacterial wilt disease. Soil structure and aggregation should therefore be considered in strategies to improve soil-borne resistance to plant pathogens.

How to cite: Dong, M.: Soil aggregation impacts bacterial community assembly and suppression of Ralstonia disease in tomato, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9700, https://doi.org/10.5194/egusphere-egu22-9700, 2022.

Organic carbon (OC) in the hyper-arid Atacama Desert soils is known to be extremely low (0.1-0.01%). OC can accumulate on soil colloids (1-1000 nm) and nanoparticles (1-100 nm) due to its high specific surface area. Small-sized colloids may be transferred to deeper depth through the macropores in the soil. However, little is known about the colloidal-OC soil transfer under hyper-arid conditions. In this study, the Water Dispersible Colloids (WDCs, <300 nm) associated OC (WDC-OC) was analyzed using Asymmetric Field-Flow-Field Fractionation (AF⁴) coupled online to an Organic Carbon Detector (OCD). The experimental site is located at 1450 m altitude near Paposo (Antofagasta region, Chile) and receives <2 mm rain per annum. Samples were taken at 13 points along an alluvial fan transect, and up to a depth of 50-80 cm. Our study examined the vertical distribution of WDC-OC affected by micro-relief. Three colloidal size fractions were identified in all samples: nano-colloids (0.6-24 nm), fine colloids (24-210 nm) and medium colloids (210-300 nm). The vertical contribution of WDC-OC differed distinctively between (i) the active alluvial fan section, (ii) the older inactive alluvial fan section, related to sediment induration and soil crust development, and (iii) the edge between both fan sections. We found that WDC-OC was highest in the active fan with an average of 11.5 mg OC kg^-1 compared with the content found in crust-related older fan (0.24 mg OC kg^-1) or at the edge between the fan sections (0.19 mg OC kg^-1). Notably high WDC-OC in the fan near to the few isolated plant remains were also observed. The increase of biological activities and debris near to the plant contributes to more colloidal-OC (26.8 mg kg^-1). The relatively flat hard impermeable surface of the crust-related old fan section may induce colloids loss during high-intensity rainfalls, e.g. occurring during past El Niño periods. Furthermore, the relative percentage of WDC-OC as a part of the total was highest in the upper layer (0-1 cm) of the active fan (27-48%) and at the edge (69%), while in the older crust-related sections the highest values were observed in the subsurface (5-10 cm) (19%-29%). Near the plant remains, nano-colloids were dominated in the upper soil accounting for 48% of the WDC-OC, whereas medium colloids were predominant in the older crust-related sections (64%). Dust (colloidal-sized) particles may be deposited at the surface and then are easily trapped near plants. We conclude that WDC-OC depth profiles within the hyper-arid Atacama Desert reflects the differential surface characteristics and the age of the fan surface, i.e., the period of geomorphological inactivity. During the extremely rare rainfall events in the Atacama, both factors will lead to differential infiltration rates, which thus in turn affect the size distribution of colloidal-OC with profile depth.

How to cite: Sun, X., Tang, N., Fuentes, B., Moradi, G., Huang, W., Zhang, Q., Contreras, D., Arenas, F., May, S. M., Sibers, N., Amelung, W., Bol, R., and Klumpp, E.: Water dispersible colloids associated organic carbon along an alluvial fan transect in a hyper-arid region of the Atacama Desert, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11761, https://doi.org/10.5194/egusphere-egu22-11761, 2022.