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.

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.

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 (OC_eco) 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 OC_eco, 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 OC_eco, 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 OC_eco/TC and BC/TC but was not soil IC/TC; the OC_eco/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 OC_eco 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.

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.

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.

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.

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.

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.

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.

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 MWD_SW>MWD_WS>MWD_FW. 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. MWD_0-10cm < MWD_10-20cm < MWD_20-30cm 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.