SSS5.1 | Spatial heterogeneity and functional soil architecture shaping biogeochemical dynamics
EDI
Spatial heterogeneity and functional soil architecture shaping biogeochemical dynamics
Co-organized by BG3
Convener: Steffen A. Schweizer | Co-conveners: Nadja Ray, Kai Uwe Totsche, Nele Meyer, Sara König, Maik Lucas, Edith Hammer
Orals
| Fri, 28 Apr, 10:45–12:25 (CEST)
 
Room K2
Posters on site
| Attendance Fri, 28 Apr, 14:00–15:45 (CEST)
 
Hall X3
Posters virtual
| Attendance Fri, 28 Apr, 14:00–15:45 (CEST)
 
vHall SSS
Orals |
Fri, 10:45
Fri, 14:00
Fri, 14:00
Soil systems harbor a high spatial complexity and soil architecture with diverse functions that shape biogeochemical matter cycles. In this session, we host novel studies that illuminate functional soil architectures and the spatial heterogeneity in soils from biological, physical, and chemical perspectives related to organic matter dynamics and other biogeochemical processes.

The advent of sophisticated instrumental techniques and advanced modeling tools has enabled studying soil structure, properties, and emerging functions. Spatially-explicit approaches extend our comprehension of heterogeneously distributed microbial habitats and processes, interactions of organic matter with mineral phases, and element storage. Aggregate structures and the void network of soil systems provides a dynamic scaffolding, which can protect soil components and influence local water retention and elemental distribution. Pedogenetic soil processes drive the differentiation at pedon scale and can result from a combination of small-scale processes determining soil ecosystem fluxes. Across different scale and structures, we look forward to discuss insights from microbial microenvironments via aggregated soil architecture up to the pedon scale.

This session is of interest to soil scientists with complementary biogeochemical and physical backgrounds working at different scales. The session responds to the growing awareness of the importance of spatial heterogeneity and architecture for ecosystem-relevant soil functions, such as the occlusion of organic residues, microbial colonization, provision of water and nutrients, and many more. We aim to present and discuss recent achievements, current obstacles, and future research directions to strengthen our conceptual understanding of the linkage of spatial heterogeneity and soil architecture with soil functions and organic matter dynamics across scales.

Orals: Fri, 28 Apr | Room K2

Chairpersons: Steffen A. Schweizer, Nele Meyer, Maik Lucas
10:45–10:55
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EGU23-15721
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ECS
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solicited
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On-site presentation
Frederic Leuther

Soil structure is a dynamic property of soils, which refers to temporal changes in the spatial arrangement of pores, organic matter, and minerals. As for many chemical reactions, also soil structure can be at a state of dynamic equilibrium, in which bulk properties, such as macroporosity, average pore size, and others apparently remain constant even though pores are formed and destroyed continuously. On the long term, the creation and destruction of structural properties are in balance as long environmental conditions, such as climate or cover crops, do not change or no external disturbances, such as tillage, become effective.

The irreversible redistribution of soil constituents, i.e. soil structure turnover, itself determines essential soil functions. For example, the creation and disruption of a pore network affects water flow, water storage, and aeration. Microsites of higher densities limit the accessibility of plant residues and organic amendments for microbiology through pores, and in consequence, increase the capacity of soil to store organic carbon. However, so far there are only few experiments trying to capture these dynamic processes and quantify the contribution of different drivers. Using examples describing the relationship between soil structure and soil functions at different sites, I will show that there is a need for new long-term monitoring experiments to capture these dynamics at temporal resolution.

How to cite: Leuther, F.: Soil structure – a dynamic soil property which effects multiple soil functions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15721, https://doi.org/10.5194/egusphere-egu23-15721, 2023.

10:55–11:05
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EGU23-16345
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solicited
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On-site presentation
Stephan Peth, Daniel Uteau, Vincent John Martin Noah Linus Felde, and Svenja Roosch

Soil structure is complex and dynamic on various scales. Soil heterogeneity as an expression of soil structural complexity develops over time and is controlled by biological, physical and geochemical processes and their interactions. Biotic and abiotic mechanisms shape the soil (micro)environment by forming interconnected pore spaces and solid particle arrangements. Commonly soil development begins with a more or less homogeneous initial structure which evolves towards an increasingly heterogeneously shaped soil architecture serving as a habitat of living organisms and in turn controlling matter, energy and gas fluxes. The relationship between soil structure and function seems to result in a self-organized system of pores and biogeochemical interfaces that is in equilibrium with its boundary conditions. 

In this presentation, we will demonstrate the interaction between soil heterogeneity and function using imaging approaches. Examples will include (i) root – soil interactions and rhizosphere oxygen distribution, (ii) spatial distribution and mineralization of organic matter in soil aggregates with contrasting architecture, (iii) the effect of initial soil heterogeneity on soil structural evolution and (iv) in-situ deformation patterns upon mechanical stresses. These examples provide an insight into the internal dynamics of soil architectures and their related physical, biological and geochemical processes which are important to understand ecosystem-relevant soil functions.

How to cite: Peth, S., Uteau, D., Felde, V. J. M. N. L., and Roosch, S.: Soil heterogeneity and how it controls ecosystem functions and soil development, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16345, https://doi.org/10.5194/egusphere-egu23-16345, 2023.

11:05–11:15
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EGU23-12421
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ECS
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On-site presentation
Matthias Weber, Benedikt Prifling, Nadja Ray, Alexander Prechtel, Maxime Phalempin, Steffen Schlüter, Doris Vetterlein, and Volker Schmidt

Effective diffusion is an important macroscopic property for assessing mass transport in porous media. Numerical computations on segmented 3D CT images yield precise estimates for diffusive properties. On the other hand, geometrical characteristics of pore space like, e.g., porosity, specific surface area and further transport-related descriptors can be easily computed from 3D CT images and are closely linked to diffusion processes. In the present contribution, we consider six different soil samples of loam and sand, whose 3D microstructure is quantitatively investigated using univariate as well as bivariate distributions of geometrical descriptors of pore space. This information is used for investigating microstructure-property relationships by means of empirically derived regression formulas, where a particular focus is put on the differences between loam and sand samples. In this way, it is possible to obtain a deeper understanding for the relationship between the 3D microstructure of the pore space and the resulting diffusive properties due to the analytical nature of the prediction formulas. In particular, it is shown that formulas existing so far in the literature for predicting soil gas diffusion can be significantly improved by incorporating further geometrical descriptors such as geodesic tortuosity, chord length distribution or constrictivity. The robustness of these formulas is investigated by fitting the regression parameters on different data sets as well as by applying the empirically derived formulas to certain data that is not used for fitting. Among others, it turns out that a prediction formula based on porosity as well as mean and standard deviation of geodesic tortuosity performs best with regard to the coefficient of determination and the mean absolute percentage error. Moreover, it is shown that with regard to the prediction of diffusive properties the concept of geodesic tortuosity is superior to geometric tortuosity, where the latter is based on the skeleton of the pore space. 

How to cite: Weber, M., Prifling, B., Ray, N., Prechtel, A., Phalempin, M., Schlüter, S., Vetterlein, D., and Schmidt, V.: Quantifying the impact of 3D pore space morphology on diffusive mass transport in loam and sand, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12421, https://doi.org/10.5194/egusphere-egu23-12421, 2023.

11:15–11:25
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EGU23-14821
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ECS
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On-site presentation
Tom Guhra, Arnold Wonneberger, Katharina Stolze, Thomas Ritschel, and Kai Uwe Totsche

Soil organisms influence pedogenesis on a molecular level through the production of biopolymers which potentially interact with soil minerals depending on their molecular properties. Specifically, biopolymers can inhibit aggregation as separation agent or promote aggregation as bridging agent (Guhra et al. 2022). Mucus, a biopolymer excreted by earthworms consisting mainly of proteins, polysaccharides, and, to a lesser extent, lipids, has often been neglected so far, despite earthworm's fundamental contribution to soil quality and structuring via bioturbation. In our study, we investigate the role of cutaneous earthworm mucus (CEM) of L. terrestris during the formation of organo-mineral associations and aggregates. For this purpose, batch experiments were carried out with goethite and CEM at different pH values and increasing CEM concentrations resulting in the formation of mucus-goethite associations. Afterwards, the (homo/hetero) aggregation of these newly formed mucus-goethite associations with quartz particles was investigated in response to mucus-C loadings on mineral surfaces and CEM concentration in solution.

Our experiments showed a pH dependent CEM structure and an adsorption to goethite controlled by concentration and pH. Polysaccharides from CEM adsorb preferentially under acidic conditions (pH 3) and low CEM concentration (6 mg mucus-C/l). In contrast, a stronger adsorption of proteins was observed at higher CEM concentrations (30 mg mucus-C /l). In subsequent aggregation experiments, the hetero-aggregation rate of organo-mineral associations and quartz was decreased at low C-loadings and increased at high loadings in comparison to the CEM-free reference. Furthermore, the aggregation between goethite particles was inhibited by electrostatic/steric repulsion (separation agent) when high CEM concentrations were present in solution (mineral:mucus ratio of 17), while CEM functions as bridging agent at low relative CEM supply (mineral:mucus ratio of > 83).

The formation and the aggregation behavior of mucus-mineral associations contribute to nutrient/carbon storage as well as structure formation in soil. The composition, function, and (im-)mobilization of CEM and corresponding organo-mineral associations in earthworm-influenced soil structures is shaped by CEM availability and the structure/reactivity of CEM affected by environmental parameters.

 

 

References:

Guhra, T., Stolze, K. and Totsche, K.U. 2022. Pathways of biogenically excreted organic matter into soil aggregates. Soil Biology and Biochemistry, 164, 108483.

How to cite: Guhra, T., Wonneberger, A., Stolze, K., Ritschel, T., and Totsche, K. U.: The functional roles of mucus during aggregation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14821, https://doi.org/10.5194/egusphere-egu23-14821, 2023.

11:25–11:35
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EGU23-9140
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ECS
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On-site presentation
Sebastian Socianu, Hanna Böhme, Timo Leinemann, Patrick Liebmann, Karsten Kalbitz, Robert Mikutta, and Georg Guggenberger

Preferential flow paths (PFPs) are intertwined soil regions that link top and subsoil and through which water and consequently nutrients flow across the soil profile. PFPs enable newly available carbon sources to reach deeper soil layers, enabling soil microorganisms to flourish in an otherwise substrate-poor subsoil. A reliable assessment of organic carbon (OC) translocation into the subsurface requires an understanding of the small scale variability of dissolved organic carbon (DOC) concentrations and fluxes into the subsoil.

Using segmented suction plates over a 5-year period, we measured DOC and water fluxes, and subsequently OC translocation, at three depths in three soil profiles down to 1.5 m in a sandy Dystric Cambisol in Lower Saxony (Germany). DOC fluxes and water fluxes were correlated and decreased with depth. Overall fluxes were dependent on seasonal fluctuations of precipitation, with the winter and spring months bearing the highest water fluxes. We found significant flux variability between suction plates and soil depths. Rank analysis showed stable regions of high and low water and DOC fluxes, suggesting stable subsoil PFPs over these five years. Furthermore, the significance of small scale spatial heterogeneity as estimated by intraclass correlation was higher than the seasonal variability in each hydrological year, strengthening the idea that PFPs in a soil profile persist over years. In addition, SUVA analysis showed a decrease in OM aromaticity with depth in all three profiles and it was moderately correlated with water fluxes, indicating selective retention of complex organic matter along the soil profile.

These findings highlight the potential for long-term stability of PFPs in subsoils and their significance for the development and maintenance of biogeochemical subsoil C hotspots, and that small scale soil heterogeneity plays a major role in controlling water and nutrient movements across the soil profile.

How to cite: Socianu, S., Böhme, H., Leinemann, T., Liebmann, P., Kalbitz, K., Mikutta, R., and Guggenberger, G.: Small scale soil heterogeneity shows stable subsoil preferential flow paths of water and DOC over a 5 year period in a Dystric Cambisol, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9140, https://doi.org/10.5194/egusphere-egu23-9140, 2023.

11:35–11:45
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EGU23-6871
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ECS
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On-site presentation
Franziska B. Bucka, Vincent J.M.N.L. Felde, Stephan Peth, and Ingrid Kögel-Knabner

Aggregate forming and stabilizing processes have been intensively studied as they are closely linked to organic carbon (OC) sequestration. However, soils are no static systems and consequently, their structure is subject to constant breakup and turnover processes.

In order to study soil structure turnover with respect to the loss of OC, we designed an incubation experiment with soil microcosms, allowing OC-loss by leaching and microbial respiration, while preventing any mechanical disturbance.

We incubated intact soil cores of an arable Luvisol derived from Loess-deposits in south-east Germany for 300 days at constant water-tension and 25 °C to promote microbial activity. During the incubation, CO2-release and OC leaching from the microcosms were monitored. A subset of microcosms was sampled each month to assess the effect of progressing OC depletion on the size distribution, OC content and stability of the aggregates.

The incubation led to a reduction of the initial OC (11.2 mg g-1) by 2.2 mg per g soil and a more narrow C:N ratio, which corresponded to a reduced OC coverage of the mineral surfaces (1.7 m² g-1 to 0.9 m² g-1, as determined by N2-BET). Despite the OC reduction, the aggregate size distribution (as determined both by wet- and dry-sieving) did not change significantly, although there was a trend towards a reduced aggregate mean weight diameter (higher reduction after wet-sieving). The aggregates’ mechanical stability (as determined by dry-crushing), even slightly increased with a lower OC-content in the bulk soil.

Those observations highlight that OC depletion, without additional mechanical influence, does not immediately lead to the decay of soil structure. This suggests the existence of OC-storage sites that are not prone to OC-loss by leaching or microbial degradation. In contrast, the sites of initial OC-loss might not contribute to the structural stability of a soil.

How to cite: Bucka, F. B., Felde, V. J. M. N. L., Peth, S., and Kögel-Knabner, I.: Detecting the Disintegration: Insights into soil structure decay following OC depletion, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6871, https://doi.org/10.5194/egusphere-egu23-6871, 2023.

11:45–11:55
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EGU23-15453
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ECS
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On-site presentation
Alexander Fechner, Robert Mikutta, Klaus Kaiser, Tobias Bromm, Cordula Vogel, Jeroen Zethof, Michaela Aehnelt, Georg Guggenberger, and Stefan Dultz

Organic substances of diverse origins are known to promote the formation of microaggregates in soils. However, their contribution to the resistance of microaggregates against mechanical stress remains unclear. This study tests for possible effects of plant- and microbial-derived organic matter on the stability of microaggregates against ultrasonic dispersion, taking advantage of a 14-year field experiment with either continuous (cropland) or minimum (bare fallow) organic inputs. The idea was that minimum input will result in the depletion of organic matter and, consequently, in decreased microaggregate stability. Microaggregates were separated into three size fractions (<20, 20-53, 53-250 µm) by wet sieving and subjected to ultrasonic disturbance at various energies. The contents of organic C, total N and neutral and amino sugars in microaggregates were determined by thermal combustion and biomarker analyses, and X-ray photoelectron spectroscopy of intact and crushed microaggregates was used to analyse the spatial distribution and oxidative alteration of organic matter. The results show that most microaggregate samples under bare fallow showed little to no decline in organic C concentrations, while bulk soil C decreased from 1.2 to 0.9 %. Amino and neutral sugars, however, decreased significantly, indicating decreased contribution of microbial products. This finding is in conflict with the missing plant C input, which should have promoted microbial processing of organic matter, resulting in declining contents of organic C with increased contributions of microbially derived compounds. Microaggregate surfaces were significantly enriched in C, with no decrease under bare fallow, which might indicate that microaggregates are not built around organic cores but are structural units collecting organic matter from their surroundings. This agrees with the finding that more oxidised and microbially processed material is stored within microaggregates, while organic matter on the outer surfaces is less oxidised, i.e. less strongly processed and thus fresher. This may explain why microaggregates lost very little organic C during fallow, as degrading plant material could have provided organic matter, substituting the loss of mineralized microbial organic matter. All microaggregate size fractions showed little and rather similar resistance against mechanical stress, achieving near complete dispersion after the application of 25 J/ml. Microaggregate stability was, in agreement with organic C contents, similar for both treatments but showed no indication that the varying contribution of amino and neutral sugars was of relevance to microaggregate stability. We conclude that, despite the clear effect of bare fallow on the organic matter composition, it had little effect on microaggregate organic C contents and their resistance to mechanical stress. This indicates that the composition of organic matter may not be the primary factor for the mechanical stability of microaggregates.

How to cite: Fechner, A., Mikutta, R., Kaiser, K., Bromm, T., Vogel, C., Zethof, J., Aehnelt, M., Guggenberger, G., and Dultz, S.: Changes in soil organic matter quality during long-term bare fallow do not affect microaggregate stability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15453, https://doi.org/10.5194/egusphere-egu23-15453, 2023.

11:55–12:05
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EGU23-11593
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On-site presentation
Lukas Kohl, Petri Kiuru, Marjo Palviainen, Maari Raivonen, Markku Koskinen, Laura Matala, Mari Pihlatie, and Annamari Laurén

Spatial heterogeneity in the soil pore network is commonly understood to lead to spatially distinct biogeochemical transformations like the production of methane in anaerobic pockets in unsaturated soils. Yet, demonstrations of this heterogeneity and its linkage to soil structure (e.g., the spatial position in the soil pore network architecture) remains elusive.

We therefore developed an assay to elucidate centimeter-scale differences in biogeochemical reactions within and between peat soil cores. For this, we injects a isotope-labeled substrate (sodium 13C2-acetate) at different locations in intact peat samples (10 cm diameter x 10 cm height) and followed its conversion to 13CO2 and 13CH4 over 5 days time in an automated measurement system using a Picarro G2201-i trace gas analyser. We analyse the ratio of 13CH4 and 13CO2 produced from the amended substrate, the fraction of substrate converted to 13CH4/13CO2, and the time course of 13CH4/13CO2 release. 

To test this approach, we collected seven pairs of peat core samples (15-25cm depths, 10 cm diameter, >30m between apart) at a drained forested peatland in Southern Finland. As one of the goals was to evaluate the effects of water retention hysteresis, half of the samples were set to -15 hPa water potential after draining to -30 hPa water potential, while the other half was set to same water potential after water-saturating the samples. In three experiments per core, we injected 10 nmol sodium acetate in 1mL water at 2, 5, and 8cm depth. We find both fixed effects (of core, injection depth, water treatment) and random effects that might be governed by the position of the injection with the peat core. 

We find, for example, that while a subset of the peat cores emitted (natural abundance) CH4, these cores showed highly heterogeneous conversions of the injected label into CH4 and CO2 that could not be explained by the fixed effects, demonstrating the spatial heterogeneity of methanogenesis and heterotrophic respiration within the peat core.

In our future work, we will explore if pore networks models extracted from microtomographic images can explain these contrasting results. 

How to cite: Kohl, L., Kiuru, P., Palviainen, M., Raivonen, M., Koskinen, M., Matala, L., Pihlatie, M., and Laurén, A.: Elucidating cm-scale heterogeneity in soil biogeochemistry with a 13C pulse-chase assay, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11593, https://doi.org/10.5194/egusphere-egu23-11593, 2023.

12:05–12:15
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EGU23-17042
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On-site presentation
The fine-scale (spatial) organisation of the soil microbiome
(withdrawn)
Christina Kaiser, Eva Simon, Sean Darcy, Ksenia Guseva, Lauren Alteio, Christian Ranits, Kian Jenab, Hannes Schmidt, and Petra Pjevac
12:15–12:25
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EGU23-1207
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ECS
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Virtual presentation
Ming Wang

Hummock-hollow microtopography is common in the northern peatlands of the world, but its effects on soil organic carbon (SOC) components are still poorly understood. In this study, we investigated effects of microtopography on SOC stocks and soil labile organic carbon (LOC) fractions in a sedge peatland in Changbai Mountain in northeast China. We found that SOC and soil LOC fractions had much heterogeneity in microtopography. SOC concentration in hummocks was significantly higher than under hummocks and in hollows. On average, the total SOC stock to a depth of 0.3 m below the ground surface was 19.00 kg C/m2. 56% of the total SOC stock was stored in soils in and under hummocks, despite the hummock only covering 30% of the total area. Light fraction organic carbon (LFOC), easily oxidizable organic carbon (EOC), microbial biomass carbon (MBC) and dissolved organic carbon (DOC) in hummocks were significantly higher than under hummocks and in hollows. In addition, the cumulative soil CO2 emissions in hummocks were 2.0 and 4.5 times higher than those under hummocks and in hollows. The temperature sensitivity of soil CO2 fluxes (Q10) were 1.55, 1.67, and 1.52 in hummock, under hummock and in hollow, respectively. Redundancy analysis (RDA) identified that SOC explained most variations in soil LOC fractions (59.6%), followed by soil total phosphorus (7.4%) and soil water content (6.6%). Our findings indicate that the hummocks are important carbon pool in the sedge peatland, but they are vulnerable to global warming and human disturbance. Hummock-hollow microtopography creates heterogeneity in hydrological conditions and soil physicochemical properties, and thus influences SOC stocks and soil LOC fractions at a small scale. This study highlights the importance of microtopography in carbon storage and cycling and has direct implications for the assessment of the carbon sequestration function in northern peatlands.

How to cite: Wang, M.: Heterogeneity of soil organic carbon dynamic regulated by microtopography in boreal peatlands, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1207, https://doi.org/10.5194/egusphere-egu23-1207, 2023.

Posters on site: Fri, 28 Apr, 14:00–15:45 | Hall X3

Chairperson: Sara König
X3.126
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EGU23-15383
Elsa Coucheney, Emilien Casali, Nicholas Jarvis, and Johannes Koestel

One source of uncertainty in the prediction of soil carbon (C) dynamics is the regulation of microbial activity by soil moisture. This important factor regulates both the survival and the activity of the microbial community through the availability of water, air and substrates. The role of soil structure in the response of C mineralisation to soil moisture is not taken into account in models. We need to better understand how the heterogeneity of the soil pore space and changes in soil structure affect C mineralisation through the regulation of soil water retention and thus the distribution of air and water in the pore system. We hypothesized that soil structure has a predominant effect on the response curve close to saturation by affecting the amount and distribution of the air phase in soil, in which the diffusion of gases takes place: transport of air (O2) to microbes and transport of the mineralisation product (CO2) back to the atmosphere.

To obtain soils of contrasting structure, we sampled 8 intact cores (at a depth of 10-15 cm) from four blocks of an agricultural field experiment located in northern France a under conventional or no-till management. Each core (5 x 6.5 cm) was consecutively incubated over a period of one week after equilibration at water potentials of -2.5, -10, -20 and -30 cm and C mineralisation rates were estimated at day 1, 3 and 7. Air distributions in the soil pore networks were quantified by X-ray tomography between each equilibration/incubation period. Water retention curves, soil dry bulk density and porosity were estimated from water contents (weights) measured at each potential.

The estimated porosity varied from 0.40 to 0.52 and the Van Genuchten parameter alpha (estimated from water retention curves) varied from 0.05 to 0.09 cm-1 and both were slightly smaller under no till compared to conventional management. Air contents varied from zero to 0.09 m3 m-3 and were positively correlated to the C mineralisation rates, which varied from 18 μg CO2 g-1C h-1 near saturation to 65 μg CO2 g-1C h-1 at water potentials of -20 to -30 cm. X-ray analyses carried out at the four different water potentials further showed that C mineralisation rates were positively correlated to the volume fraction of the air-filled porosity connected to the upper surface of the cores.

These results confirm that soil structure is important in the C mineralisation response to soil moisture close to saturation by regulating the air content and its distribution in soil at water potentials ranging from 0 to -30 cm.

How to cite: Coucheney, E., Casali, E., Jarvis, N., and Koestel, J.: X-ray imaging demonstrates that air-filled porosity and its connectivity controls carbon mineralisation near saturation in intact soil cores., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15383, https://doi.org/10.5194/egusphere-egu23-15383, 2023.

X3.127
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EGU23-5864
Carmen Höschen, Steffen Schweizer, and Ingrid Kögel-Knabner

Organic matter (OM) and soil mineral constituents interact closely at the submicron scale forming structural units and providing biogeochemical interfaces. Soil structure itself plays a key role for carbon storage, microbial activity and soil fertility and pollutant mitigation. A better understanding to which extent biogeochemical processes and interactions in the soil are driven by the spatial arrangement of OM and mineral constituents requires advanced efforts to apply novel microspectroscopy approaches.

NanoSIMS, allowing unique elemental and isotopic analyses at nanometer spatial resolution, provide valuable insights into the architecture of soil organo-mineral constituents and crucial processes taking place at the microscale.

The instrument is equipped with two ion sources: the Cesium source (Cs+) convenient to detect ions related to organic matter distribution and the Oxygen source (O-) favourable to provide information on mineral phases or metals in samples. With a spatial resolution similar to the Cesium source and high stability, the upgraded radio frequency (RF) plasma Oxygen source  recently installed at the TUM is now best suited for novel analytical approaches to probe elemental and isotopic composition of soil organo-mineral constituents in soils at the microscale.

We will show examples of how the two primary ion sources, single or correlatively applied, enable novel experimental designs in soil biogeochemistry. Novel combinations of the OM distribution (12C, 13C and 14N, 15N) detected by the Cs+ source with the distribution of e.g. Si, Al, Fe, Ca, Mg, K, and Na of minerals as revealed by the O- source are now possible.

Post-processing tools for unsupervised clustering and supervised segmentation facilitate the comparison and quantitative analysis of the spatial architecture within intact soil structures. These ongoing developed tools can contribute to the extent of our understanding of biogeochemical processes taking place at organo-mineral and mineral-mineral interfaces in soil systems at the microscale.

How to cite: Höschen, C., Schweizer, S., and Kögel-Knabner, I.: New opportunities to unravel the microarchitecture of soil organo-mineral associations by NanoSIMS using the upgraded Oxygen source, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5864, https://doi.org/10.5194/egusphere-egu23-5864, 2023.

X3.128
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EGU23-4845
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ECS
Junge Hyun, Jeehwan Bae, and Gayoung Yoo

Although urban greenery is an important area for soil carbon (C) sequestration in national and international policies, there is a lack of studies on its unique soil C status. Especially the contribution of black carbon (BC) and inorganic carbon (IC), which originated from anthropogenic activities, need to be separated from ecosystem-driven organic carbon (OCeco) to accurately quantify the soil C sequestration in urban ecosystems. However, there is currently no standardized, widely used method to separate various forms of C in this soil. In this study, we suggested a robust and reliable method to discriminate the OCeco, BC, and IC contents and understand the anthropogenic effects on C in urban soils. To achieve this objective, we tested the accuracy of the “EGA with peak deconvolution approach” that derives a CO2 thermogram from an evolved CO2 gas analyzer (EGA) connected to a thermal analyzer and conducts sample-by-sample peak deconvolution. Since we used the model mixtures that had known OCeco, BC, and IC contents, the absolute accuracy of this approach could be tested. As a result, EGA with peak deconvolution approach showed high accuracy (R2 > 0.90), and the regression lines between the known and measured values were close to the 1:1 line.

Using the EGA with peak deconvolution approach, we further investigated the soils in urban greeneries. EGA with peak deconvolution approach was helpful in understanding the impacts of human intervention on the soil C cycle. Surrounding land use significantly altered the soil OCeco/TC and BC/TC but was not soil IC/TC; the OCeco/TC tended to increase with green area, while the BC/TC had a positive relationship with impervious area. The suggested method can be used to evaluate the C sequestration rate of SSM practices in the urban area. Without the information on OCeco and BC, the impacts of human intervention on soil C can be misinterpreted, which overestimates the C sequestration rate.

How to cite: Hyun, J., Bae, J., and Yoo, G.: Assessment of thermal analysis techniques for determining organic, black, and inorganic carbon contents in urban soils, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4845, https://doi.org/10.5194/egusphere-egu23-4845, 2023.

X3.129
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EGU23-11663
Nele Meyer, Jacqueline Kaldun, Andrey Rodionov, Wulf Amelung, and Eva Lehndorff

The long-term stability of soil organic carbon (SOC) is controlled by stabilization mechanisms, among which physical stabilization through microaggregate (<250µm) formation is considered to be critically important. Yet, the turnover of Carbon in aggregates is not well understood. Here, we aimed at unravelling the importance of microaggregates for long-term C storage in a soil subjected to a C3-C4 vegetation change 36 years before sampling. We hypothesized that Carbon in microaggregates is characterized by a longer mean residence time (MRT) than that of bulk soil and that SOC turnover appears predominantly at the outside of aggregates. Free and occluded size fractions (250-53 µm) were obtained by wet sieving and ultrasound. True aggregates were manually isolated from size fractions and analyzed for quantity, C content, and bulk δ13C. Additionally, we used laser ablation isotope ratio mass spectrometry (LA-IRMS) with a resolution of 20 µm to study small-scale patterns of δ13C within aggregates and on their surfaces. The calculated MRT of Carbon in occluded and free aggregates was with 62 and 105 years only slightly longer than that of bulk soil (58 years). Also the low quantity of true aggregates (<5% aggregates in size fraction) questions their importance for soil C storage. The spatial variability of δ13C within individual aggregates was considerable, both in C3 (-18.8±6.4) and C4 (-19.6 ±5.5) soil, but without difference between inside and surface locations. No aggregates being clearly older than 36 years, i.e. with only C3-derived SOC isotope signatures were found, suggesting that on the micro-scale microbial turnover processes control δ13C more than expected. In summary, aggregates seemed to be subjected to high rates of formation and decay. Altogether, it is therefore questionable whether aggregates considerably contribute to overall long-term SOC storage. Yet, results need to be treated with caution and we will present evidence that the concept of source mixing between C3 and C4-derived Carbon is not valid in small-scale approaches where differences in δ13C are dominated by C turnover processes rather than source.

How to cite: Meyer, N., Kaldun, J., Rodionov, A., Amelung, W., and Lehndorff, E.: Spatial variability of Carbon turnover in soil microaggregates and the challenge of combining multi-scale approaches, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11663, https://doi.org/10.5194/egusphere-egu23-11663, 2023.

X3.130
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EGU23-8300
Ingrid Kögel-Knabner, Franziska B. Bucka, Vincent J.M.N.L. Felde, and Stephan Peth

Percolating dissolved organic matter (DOM) from the topsoil is considered the main source of subsoil organic carbon (OC) in temperate soils. Although DOM adsorption to minerals has been extensively studied, comprehensive knowledge about its influence on subsoil OC storage and structure development is limited.

We conducted a short-term incubation experiment using artificial model soils without pre-existing aggregates to study the effects of percolating DOM within varying soil textural conditions on OC turnover and initial structure development.

The model soils were designed with contrasting texture (clay loam, loam, sandy loam), but identical mineral composition (quartz, illite, montmorillonite, goethite), mimicking subsoil conditions, where mineral surfaces free of OM come into contact with percolating DOM. The regular application of DOM under a constant suction head (-15 kPa) enabled the DOM to percolate freely through the soil matrix over the course of the experiment.

A higher sand content caused a lower porosity, which was accompanied by a lower moisture content. In contrast, the OC retention (21% of the OC input), and the microbial abundance and activity were unaffected by the soil texture. The percolating DOM created patches of OM covers on 10% of the mineral surfaces (N2-BET) within an otherwise OC-free mineral matrix.

The biochemical processing of the percolating DOM solution induced the formation of large, water-stable aggregates (wet-sieving) in all textures without requiring the presence of physical organic nuclei. Aggregate formation was pronounced in the clay-rich soils (58% mass contribution), which also exhibited a higher mechanical stability of the aggregates.

The results highlight that retention and microbial mineralization of dissolved OM are decoupled from pore sizes and soil solution exchange, but are instead driven by the mineral composition and OC input.

The biochemical processing of percolating DOM can induce large soil aggregates. Here, the presence of fine mineral particles enhances the formation and mechanical stability of the aggregates, irrespective of their surface charge or sorption properties.

How to cite: Kögel-Knabner, I., Bucka, F. B., Felde, V. J. M. N. L., and Peth, S.: Going with the flow: Initial soil structure development by percolating dissolved organic matter, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8300, https://doi.org/10.5194/egusphere-egu23-8300, 2023.

X3.131
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EGU23-17493
Hermann F. Jungkunst, Simone Kilian Salas, Paul A. Schroeder, Jens Boy, and Georg Guggenberger

Most biogeochemical models commonly obtain their soil input from pedotransfer functions based on soil texture and other crude but widely available soil data. However, soil texture based on single grain size distribution neglects the impact of actual soil structures in the field. Consequently, scientific efforts are being made to correct for this systematic bias in predicting soil functioning. Pronounced discrepancies between field measurements and model predictions occur for tropical soils: overestimated N2O emissions is a prominent example of this mismatch. A well-known characteristic of tropical soils, potentially responsible for the systematic error, are stable aggregates called pseudo-sands. In the field they are perceived as sand, but in the lab measured as clay and silt. The simple assumption that pseudo-sands act just like sands in the field seems to work satisfactorily for certain hydrological predictions, so models were easily adjusted to it. However, here we pursue the hypothesis that, biogeochemically, pseudo-sands do not act like sands. Due to their high internal surface and rough structure, pseudo-sands, unlike sands, provide a wide variety of ecological niches for a diverse community of microorganisms to establish. We will present first evidence for pseudo-sands to act more like a biophysical reactor than just another grain of sand. The long-term goal is to develop a transfer function related to the properties of pseudo-sands that will lead to improved models for tropical soils.

How to cite: Jungkunst, H. F., Kilian Salas, S., Schroeder, P. A., Boy, J., and Guggenberger, G.: Are pseudo-sands internal soil biophysical reactors?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17493, https://doi.org/10.5194/egusphere-egu23-17493, 2023.

X3.132
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EGU23-13591
Alexander Prechtel, Simon Zech, and Nadja Ray

Advanced imaging techniques now allow to take snapshots of soils even down to the nanoscale. Nevertheless, assessing the temporal evolution of elemental distributions, distinguishing different liquid phases and identifying dynamic microbial processes is experimentally still challenging. Consequently mechanistic models operating at the pore scale facilitate the study and understanding of phenomena shaping soil structures as, e.g., carbon turnover, and vice versa.

We present an overview of a versatile hybrid discrete continuum modeling approach combining cellular automata and partial differential equations, which integrates the complex coupling of biological, chemical, and physical processes. Dynamic liquid and gaseous phases, diffusive processes for solutes, mobile bacteria transforming into immobile biomass, and ions are prescribed by means of partial differential equations. Furthermore the solid phase is dynamic, e.g. through aggregation of soil particles, the addition and decomposition of particulate organic matter, or the mechanical influence of roots and their exudates. The virtual soil structures rely on micro-CT images or particle libraries derived from dynamic image analysis of water-stable aggregates.

Applications include structure formation of clay minerals, the interplay between soil structural dynamics and organic matter turnover, or the impact/importance of liquid phase connectivity and substrate supply. Finally the mathematical homogenization technique is used to show a way how to incorporate information from the pore scale to macroscale models, e.g. by coupling microscale carbon turnover to profile-scale CO2 transport.

How to cite: Prechtel, A., Zech, S., and Ray, N.: Seven years of research on process-based, mechanistic modeling of aggregation and its drivers, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13591, https://doi.org/10.5194/egusphere-egu23-13591, 2023.

X3.133
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EGU23-10958
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ECS
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Highlight
Thomas Ritschel and Kai Totsche

Soil's aggregated structure is fundamental for the functioning of soil, and aggregation is a crucial process within pedogenesis. While aggregates are often considered stable entities, bonds between aggregate forming materials can form, consolidate, and break over time. Consequently, individual aggregates are subject to permanent restructuring and do not show a final spatial configuration that remains stable. Instead, only the temporal average of aggregate features converges to a constant value and –in case the system comprises a large ensemble of aggregates– a situation of thermodynamic equilibrium will establish over time. The dynamics of disaggregation and restructuring might be equally important for the establishment of aggregate structure as the aggregation mechanisms themselves and should therefore be considered when modeling structure formation. We conducted a comprehensive numerical analysis to reveal the interplay of aggregation mechanisms and the breaking of aggregate bonds in a physicochemical framework that combines three-dimensional transport with DLVO-type surface interactions. The attractive and repulsive energies between aggregate forming materials were used to model the temporal dynamics and stability of bonds in a heuristic manner. Despite the ongoing formation and breaking of bonds, we show that aggregation approaches a thermodynamic equilibrium depending on the physicochemical environment. Specifically, an ensemble of aggregates of sufficient size to provide robust statistical averages converges to a state of constant mean properties, e.g., aggregate size and aggregate morphology. Aggregates and their structure should therefore be considered dynamic entities, where an ensemble might reach a steady-state equilibrium, but each individual aggregate does not.

How to cite: Ritschel, T. and Totsche, K.: The thermodynamics of aggregate development, structure, and size, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10958, https://doi.org/10.5194/egusphere-egu23-10958, 2023.

X3.134
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EGU23-7089
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ECS
Melanie A. Thurner, Xavier Rodriguez-Lloveras, and Christian Beer

Soil texture, i.e. its composition of clay, silt and sand, as well as organic material, is often very heterogeneous within small distances. State-of the-art land-surface models usually cannot capture this due to their coarse grid. However, neglecting small-scale soil heterogeneity may affect the estimated exchange of energy, water, and carbon between land and atmosphere strongly.

This discrepancy is especially problematic when modelling permafrost soils, where the heterogeneity-induced mismatch can make the difference between frozen and unfrozen soil, as well as waterlogged and unsaturated soil, as soil texture determines physical properties such as heat and water-storage capacity. By that, soil heterogeneity affects the build of soil ice and resulting frost heave, determines pond locations, and ultimately influences soil genesis, e.g. by inducing cryoturbation. The determination of soil geophysics also propagates into biogeochemical dynamics, affecting the arctic carbon cycle by providing the environment for either carbon stabilization or degradation.

 

To assess the effect of soil heterogeneity in detail, and quantify the potential mismatch, we develop a two-dimensional geophysical soil model with a spatial resolution of less than 10 cm at the region of interest. We apply our model at permafrost sites, because our ultimate aim is to understand cryoturbation as a permafrost-specific soil process and its relevance for the arctic carbon cycle, which will finally allow us to improve predictions of the Arctic carbon budget.

Here we present our first results, where we study the effect of fine-scale soil heterogeneity on soil temperature, water, and implications for the simulated sensible and latent heat fluxes between soil and atmosphere. By comparing simulations with and without soil texture heterogeneity, as well as with and without lateral fluxes of heat, we are able to quantify the effect of soil heterogeneity at small scale and discuss the effect on larger scales.

How to cite: Thurner, M. A., Rodriguez-Lloveras, X., and Beer, C.: Effects of pedon-scale soil heterogeneity on soil temperature and surface energy fluxes - Does it matter?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7089, https://doi.org/10.5194/egusphere-egu23-7089, 2023.

X3.135
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EGU23-16184
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Highlight
Co-Evolution of Structure, Heterogeneity, and Function During Pedogenesis
(withdrawn)
Kai Uwe Totsche and the MAD Soil Consortium

Posters virtual: Fri, 28 Apr, 14:00–15:45 | vHall SSS

vSSS.2
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EGU23-10424
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ECS
Huazheng Liu and Yanfeng Jia

Abstract :[Background] The black soil area in northeast China is an important grain production base in China, and soil erosion is serious. Soil aggregate stability has a profound influence on soil erosion process. The purpose of this study was to clarify the fragmentation characteristics of surface and bottom soil aggregates in sloping farmland under different damage mechanisms, and to evaluate the stability characteristics of aggregates under different damage mechanisms, so as to provide theoretical basis for the prevention and control of soil erosion in sloping farmland in rainy season. [Methods] The typical long straight sloping farmland in northeast Black soil region was selected as the study area. Samples were taken every 30 m along the longitudinal section of the slope length, with a sampling depth of 30 cm and a sampling length of 1020 m. The particle size distribution and stability parameters of soil aggregates were determined by Le Bissonnais (LB) method (including fast wetting (FW), slow wetting (SW) and runoff disturbance (WS) treatments. [Results] 1) Under the three treatments of LB method, FW treatment (rainstorm) had the largest damage to soil aggregate structure, SW treatment (light rain) had the least damage to soil aggregate structure, and WS treatment (disturbance) was in the middle. On the whole, the aggregate stability showed MWDSW>MWDWS>MWDFW. 2) Analysis of soil aggregates in the topsoil (0-10 cm) showed that SW treatment (light rain) caused the soil aggregates to break into aggregates of >0.2 mm. WS treatment (disturbance) caused the fragmentation of soil aggregates mainly concentrated in 2-0.2 mm grain size, indicating that the >2 mm grain size aggregates were mainly destroyed by raindrop splashing, which made them split into microaggregates. After FW treatment (rainstorm), the aggregates were broken in <1 mm size, which provided abundant loose aggregates for raindrop splash erosion and runoff erosion process, and became the main source of erosion materials in soil erosion process. 3) The aggregate fragmentation of surface layer (0-30 cm) was more sensitive to 0-10 cm soil layer after light rain. The aggregate fragmentation effect under rainstorm and irrigation was more obvious in 0-20 cm soil layer. The effects of raindrop splashing and runoff disturbance on aggregate fragmentation in 0-30 cm soil layer are similar. MWD0-10cm < MWD10-20cm < MWD20-30cm in different soil layers under the three failure mechanisms. 4) In the three treatments of LB method, 1 mm grain size was taken as the critical, and 1 mm grain size was used as the index to distinguish soil stability. Soil aggregate size >1 mm had a strong resistance to soil dissipation, clay expansion and mechanical oscillation. This result could characterize the factors affecting soil stability structure.

How to cite: Liu, H. and Jia, Y.: Study on the structure and stability characteristics of typical black soil aggregates in Northeast China based on Le Bissonnais method, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10424, https://doi.org/10.5194/egusphere-egu23-10424, 2023.