SSS5.1

Soil organic carbon turnover time (τ, year) is an important indicator of soil carbon stability and sequestration capacity. However, our understanding of the spatial heterogeneity of subsoil τ still was poorly qualified over a large scale, even though subsoil organic carbon below 0.2 m accounts for the majority of total soil organic carbon. We compiled a dataset that consisted of 630 observations in subsoil (0.2 - 1 m) from published literatures to investigate the spatial heterogeneity of subsoil τ (defined as the ratio of soil carbon stock and net primary production) and explore its main environmental drivers using structural equation modelling (SEM) in forest ecosystems across China. Results indicated that mean (± standard deviation) subsoil τ was 72.4 ± 68.6 years with a large variability ranging from 2.3 to 896.2 years. Subsoil τ varied significantly with forest types that mean subsoil τ was the longest in deciduous broadleaf forest (82.9 ± 68.7 years), followed by evergreen needleleaf forest (77.6 ± 60.8 years), deciduous needleleaf forest (75.3 ± 78.6 years) and needleleaf and broadleaf mixed forest (71.3 ± 80.9 years), while the shortest τ in evergreen broadleaf forest (59.9 ± 40.7 years). SEM suggested that soil environment was the most important factor in predicting subsoil τ. However, the dominant driver differed with forest types, i.e. soil environment for evergreen broadleaf forest and climate for evergreen needleleaf forest. This study highlights the different dominant controlling factors in subsoil τ and improve our understanding of biogeographic variations of subsoil τ. These findings are essential to better understand (and reduce uncertainty) in biogeochemical models of subsoil carbon dynamics at regional scales.

How to cite: Yu, P., Tang, X., Shi, Y., Cai, C., Han, Y., Li, Z., Deng, L., Song, C., and Li, J.: Spatial heterogeneity of subsoil organic carbon turnover times in forest ecosystems across China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1902, https://doi.org/10.5194/egusphere-egu21-1902, 2021.

Plant residues in soil create temporal and spatial hotspots of extremely high microbial activities leading to very intensive greenhouse gas (GHG) fluxes that challenge our mechanistic understanding and predictive power. Using a series of well-controlled soil microcosm experiments, we examine how abiotic processes (e.g., iron reduction-oxidation cycling) at residue/soil interfaces contribute to hotspot dynamics. We quantify for the first time the contributions of microbially-initiated Fenton reactions, which produce strongly oxidizing hydroxyl radicals (HO•), to organic matter solubilization and mineralization in hotspots 0–3 mm from the litter surface. The concentrations of ferrous iron (Fe2+), hydrogen peroxide (H2O2) and HO• were 2.1–3.0, 3.0–9.0 and 2.6–2.8 times higher, respectively, at the straw-soil interface than in the bulk soil. Thus, iron minerals, especially in concert with microorganisms, produce a burst of hydroxyl radicals that explain extremely high GHG fluxes from soil hotspots. Our findings highlight how Fe minerals and microorganisms synergistically influence global carbon cycling and stability. Our findings highlight the relevance of free radical-related mechanisms in soil to the cycling, stabilization, and storage of carbon and also extend our mechanistic understanding of processes occurring within hotspots.

How to cite: Yu, G.: Free radical-related mechanisms in soil and their relevance  to the cycling, stabilization, and storage of carbon, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3576, https://doi.org/10.5194/egusphere-egu21-3576, 2021.

Salt-marsh evolution importantly depends on complex feedbacks between hydrodynamic, morphological, and biological processes. These crucial ecogeomorphic structures support a diverse range of ecosystem services, including coastal protection and biodiversity increase. In addition, they are among the most carbon‐rich ecosystems on Earth, as their high primary production coupled with rapid surface accretion results into the ability to sequester atmospheric carbon at high rates. However, salt-marsh future is at risk today, due to the effects of climate changes and local anthropogenic disturbances, in particular sea-level rise and reduced fluvial sediment delivery to the coasts. The organic matter captured and stored by salt marshes results from the balance between inputs and outputs and may contribute to marsh surface accretion, which determines their ability to keep pace with sea-level rise. Therefore, a better understanding of the processes regulating organic matter dynamics on salt marshes is a critical step to elucidate their carbon sink potential and to address salt-marsh management and conservation issues. Toward this goal, we analysed organic matter decomposition processes within salt-marsh ecosystems by burying 712 commercially available tea bags within different marshes in the Venice Lagoon (Italy), following the Tea Bag Index protocol. The process provides the values of two key parameters: the decomposition rate (k) and litter stabilisation factor (S). Based on standardized litter bag experiments, the Tea Bag Index focuses on the effects of abiotic conditions, neglecting litter-quality influences. The mean values of the decomposition metrics from our analyses are in general consistent with previous results and indicate a quite fast decomposition of the organic matter with a remaining mass of about 34% of the initial labile mass after 90 days. We next explore the possible dependence of k and S on environmental drivers. Temperature showed the most significant relationship with decomposition processes, suggesting an organic-matter decay acceleration with warming temperature, in line with previous literature. Moreover, the statistical analysis indicated some significant trends of the decomposition rate also with surface elevation and distance from the marsh edge. This suggests that, at the marsh scale, higher and probably less frequently flooded sites are exposed to faster decomposition, likely due to greater oxygen availability enhancing microbial respiration. In conclusion, the organic matter decay we observed is rapid enough to consume all the labile material before it can be buried and stabilized, hence increased global temperatures may not have a significant effect in increasing organic matter decomposition in coastal marshes. Therefore, we argue that, at least in the short term, the remaining mass of the organic matter contributing to carbon sequestration and marsh accretion, strongly depends on the initial litter quality, recalcitrant or labile, which may differ considerably between different species and plant parts and may be affected by climate change effects.

How to cite: Puppin, A., Roner, M., Finotello, A., Ghinassi, M., Tommasini, L., Marani, M., and D'Alpaos, A.: Characterization of organic matter decomposition in the Venice Lagoon using the Tea Bag Index, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3355, https://doi.org/10.5194/egusphere-egu21-3355, 2021.

Black carbon is one of the short-lived climatically significant factors. This term refers to climate-forming substances that are located for a short amount of time in the atmosphere - from several days to several years. To identify the role of cryoconite in the conditions of a possible climatic crisis, the stabilization of organic matter isolated from cryoconite holes was assessed. Humic acids are part of the organic matter accumulating in soils and cryoconites and are heterogeneous systems of high-molecular condensed compounds formed as a result of the decomposition of organic remains of plants and animals in terrestrial and aquatic ecosystems. Climatic parameters, precursors of humification, and the local position in the landscape determine the diversity of the composition and properties of HAs. Stabilization of organic material is defined as the transformation of organic matter into a state inaccessible to soil microorganisms, and the very property of stabilization is a characteristic stage in the dynamics of carbon. Using 13C NMR spectroscopy, we determined the proportion of aromatic and aliphatic compounds in the composition of HAs in order to assess the stabilization of organic matter in cryoconites from Mount Elbrus (Caucasus Mountains, Russia), the Arctic (Severnaya Zemlya archipelago, Russia) and Antarctica (King George Island, West Antarctica).

Samples for qualitative analysis of carbon accumulated in cryoconites were carried out during fieldwork in 2020. The studied samples were analyzed at the Department of Applied Ecology, St. Petersburg State University. Humic acids (HAs) were extracted from each sample according to a published IHSS protocol. Solid-state CP/MAS ¹³C-NMR spectra of HAs were measured with a Bruker Avance 500 NMR spectrometer.

Thus, it follows from the obtained results that aliphatic fragments of humic acids predominate in all studied cryoconites. A similar composition of humic acids testifies to a single mechanism of accumulation and development of organic matter in glacier regions. Low biological activity and climatic features prevent condensation of high-molecular compounds in the organic matter of cryoconite holes. This is an essential prerequisite for high rates of carbon dioxide emissions into the atmosphere under the conditions of deglaciation of the studied regions. With the thawing of glaciers and the ingress of cryoconites into warmer conditions, an additional contribution of carbon dioxide to the atmosphere can occur and, therefore, increase the possible climate crisis on our planet.

This study was supported by Russian Foundation for Basic Research No. 19-05-50107.

How to cite: Polyakov, V. and Abakumov, E.: Stabilization of organic carbon isolated from cryoconite holes in polar and mountain systems by 13С NMR spectroscopy, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4156, https://doi.org/10.5194/egusphere-egu21-4156, 2021.

Soil organic carbon (OC) levels generally increase with increasing clay and silt content under a similar climatic zone because of increased association of OC to clay minerals and stronger occlusion inside aggregates. Surprisingly though, in Western Europe many silt loam soils actually bear low topsoil OC levels compared to lighter textured soils. Soil texture obviously also strongly controls moisture availability with consequent indirect impact on heterotrophic activity. We hypothesized that with increasingly frequent summer drought: 1) soil microbial activity in sandy soils is more likely impeded due to their limited water holding capacity retention during droughts, while soil OC mineralization in silty soils remain be less drought-limited; 2) capillary rise from sufficiently shallow groundwater would, on the other hand, alleviate the water stress in lighter textures. To test these hypotheses, we established a one-year field trial with manipulation of soil texture, monitoring of soil moisture and maize-C decomposition via ^13/12C-CO₂ emissions. The upper 0.5 m soil layer was replaced by sand, sandy loam and silt loam soil with low soil OC. Another sandy soil treatment with a gravel layer was also included beneath the sand layer to exclude capillary rise. Soil texture did not affect maize-C mineralization (C_maize-min) until April 2019 and thereafter C_maize-min rates were higher in the silt loam than in the sandy soils (P=0.01). θ_v correlated positively with the C_maize-min rate for the sand-textured soils only but not for the finer textures. These results clearly highlight that soil texture controlled C_maize-min indirectly through regulating moisture under the field conditions starting from about May, when soils faced a period of drought. By the end of the experiment, more added C_maize was mineralized in the silt loam soil (81%) (P<0.05) than in the sandy soil (56%). Capillary rise did not result in a significant increase in cumulative C_maize-min in the sandy soil, seemingly because the capillary fringe did not reach the sandy topsoil layer. These results imply that, under future climate scenarios the frequency of drought is expected to increase, the largely unimpeded microbial activity in silty soils might lead to a further stronger difference in soil OC with coarser textured soils under similar management.

How to cite: Li, H., Van den Bulcke, J., Mendoza, O., Deroo, H., Haesaert, G., Dewitte, K., De Neve, S., and Sleutel, S.: Soil texture can predominantly control organic matter mineralization in temperate climates by regulating soil moisture rather than through direct stabilization, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4424, https://doi.org/10.5194/egusphere-egu21-4424, 2021.

Clarifying the controlling factors for soil organic carbon (SOC) stabilization is a primary issue in mitigating climate change. However, the mechanisms controlling soil carbon cycle are not well-understood, especially in tropical regions. Furthermore, the mechanisms are expected to differ between topsoil and subsoil. The objectives were to clarify the controlling factors for SOC pools partitioned by their stabilities, then to compare the differences in pools and controlling factors between topsoil and subsoil.

Both top (0–15 cm) and subsoil (20–40 cm) samples were collected at volcanic regions of Tanzania and Indonesia along an elevation gradient under mostly undisturbed vegetation (23 sites). A kinetic model, including labile, intermediate, and stable pools, was fitted to accumulative SOC mineralization curve obtained from 343-day incubation to determine the sizes of the labile and intermediate SOC pools (C_L and C_I) and their mean residence times, where the size of the stable SOC pool (C_S) was measured as non-hydrolyzable carbon by fractionation. Correlation and path analyses were conducted to determine the controlling factors for each SOC pool, using the results of the model fitting and SOC fractionation and the data on climate, geochemistry, and biology (e.g., mean average temperature and precipitation, nanocrystalline mineral content (Al_o+1/2Fe_o), and microbial biomass, respectively).

The intermediate pool (56.2 ± 10.4% of SOC) predominantly contributed to the storage and stability of total SOC (10 to 157 g kg⁻¹) for both topsoil and subsoil with the mean residence time of years to decades (3400 to 31500 days). For both topsoil and subsoil, Al_o+1/2Fe_o was strongly correlated with C_I and C_S, suggesting that organo-mineral complexation is a predominant factor that controls the intermediate and stable SOC pools, rather than soil pH or texture. Also, temperature negatively affected the sizes of all three pools, which indicates the low temperature retards the decomposition of all parts of SOC. The labile SOC pool was more controlled by biotic and climatic factors (i.e., microbial biomass and excess precipitation). Concerning differences between topsoil and subsoil, SOC was more in the intermediate than in the stable pool, and the effect of temperature on C_S was more substantial in the subsoil. Moreover, Al_o+1/2Fe_o controlled the mean residence time of the intermediate SOC pool, indicating the stability of subsoil SOC that had a labile nature would be more dependent on nanocrystalline minerals.

While temperature widely influences all SOC pools, geochemical factors control more stable pools and total SOC storage, whereas biotic factors and moisture mainly alter relatively labile SOC pools. The subsoil SOC would be more sensitive to climate change than topsoil SOC. The findings helped to understand SOC stabilization mechanisms for both top and subsoils in tropical volcanic regions.

How to cite: Lyu, H., Watanabe, T., Zhong, R., Kilasara, M., Hartono, A., and Funakawa, S.: Effect of Climate and Geochemistry on Organic Carbon Pools in Top and Subsoils of Tropical Volcanic Regions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9355, https://doi.org/10.5194/egusphere-egu21-9355, 2021.

Soil C sequestration is widely regarded as the most reasonable way to mitigate global warming. Traditionally, a high amount of organic carbon (OC) input is strongly recommended to increase soil organic carbon (SOC) stocks in croplands. However, according to the whole-soil saturation theory, stable SOC (mineral-associated SOC) accumulation can be limited at a certain point, relying on silt and clay contents. Most studies based on the theory were conducted in aerobic soil condition. This relationship is still uncertain in a rice paddy that makes up 10.8% of total arable land and has an anaerobic soil environment. In this study, we investigated high OC addition can enhance soil C sequestration in a rice paddy. We added different OC levels (0.5, 2.0, 2.9, and 4.6 Mg C ha^-1 yr^-1) in rice paddy by incorporating cover crop biomass for nine years. SOC stock and soil saturation degree were determined. Unprotected, sand-associated, silt-associated, and clay-associated SOC were separated via density and size fractionation. Respired C losses (CO₂-C and CH₄-C) were monitored using the static closed chamber method. SOC stock did not linearly increase with higher amount of OC input. The carbon sequestration efficiency (i.e. the increase of SOC per unit of OC input) decreases with the amount OC added. Higher OM input significantly increased unprotected labile SOC content. Unprotected SOC (<1.85 g cm^-3) exponentially increased as the SOC saturation degree was higher. On the other hand, stable SOC content did not exhibit a linear relationship with the SOC saturation degree. The higher OC addition level exponentially increased respired C loss. In particular, C loss via CH₄ was more sensitive to high OC addition. We conclude that higher OC addition in rice paddy without consideration in terms of SOC stock saturation point can accelerate global warming by increasing labile SOC accumulation and CH₄ emission.

How to cite: Song, H., Galgo, S., Canatoy, R., Chae, H., and Kim, P. J.: High organic carbon input can accelerate global warming in rice paddy soil: increase unprotected soil organic carbon and CH4 emission, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9438, https://doi.org/10.5194/egusphere-egu21-9438, 2021.

Soils contain more carbon (C) in the form of organic matter (soil organic matter = SOM) than the entire atmosphere and global vegetation combined. They are a central component of the global C cycle and its largest dynamic reservoir. Smart agricultural practices are discussed, on the one hand, as a way to mitigate climate change because they can increase the amount of SOM and thus actively remove C from the atmosphere. On the other hand, all intensively used soils lose C in the long term. The scientific key questions in this context revolve around the extent and dynamics of C storage, as well as the associated stabilization mechanisms involved and effects of agricultural use on the C budget.

The DOK experiment is a long-term agronomic field trial near Basel (Switzerland) that compares biodynamic, organic and conventional management systems since 40 years. Within the "DynaCarb" project, we investigate how the management systems affect SOM fractions during the 40-year experimental period. We compare the unfertilized control to a purely mineral, a purely organic, and a combined fertilized, mineral-organic variant (four field replicates each) during six crop rotation cycles. By using a combined density and particle size fractionation, the SOM is separated into particulate and mineral-associated fractions and their development is quantitatively investigated in archived samples from 1982, 1989, 1996, 2003, 2010, and 2017.

"DynaCarb" investigates the medium- and long-term effects of different agricultural systems on SOM. These results are of great importance for the evaluation of the C sequestration potential of agricultural soils and for the identification of suitable management and fertilization strategies.

How to cite: Mayer, M., Fliessbach, A., Mäder, P., and Steffens, M.: Carbon distribution between density and particle size classes of differently managed soils in a 40-year agronomic long-term trial, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9801, https://doi.org/10.5194/egusphere-egu21-9801, 2021.

Soil organic matter (SOM) is a complex collection of organic molecules of varying origin, structure, chemical activity, and mineral association. A wide array of laboratory methods exists to separate SOM based on qualitative, biological, chemical, and physical characteristics. However, all present conceptual and logistical limitations, including the requirement of a substantial amount soil material.

An newly applied alternative method of fractionation relies on a conceptual analogue between biochemical stability in soil and thermal stability, e.g. more persistent SOM will require higher temperatures (greater energy inputs) to decompose than less persistent SOM. This accounts for both chemical complexity and mineral association as main factors in determining SOM persistence.

In this method, carbon is released by heating SOM to 900°C at a constant rate. The peaks of carbon release are grouped into activation energy pools, CO₂ is collected, and analyzed for ¹³C and ¹⁴C. We seek to describe in finer detail the distribution of soil radiocarbon by adding another fractionation step following a different paradigm of SOM stability, and explore mineralogical effects on SOM quality and stability using thermal analysis, radiocarbon, and gas chromatography.

Here, we analyzed bulk soil and soil fractions derived from density separation and chemical oxidation, as well as mineral horizons dominated by diverse mineralogies. Density fractions contained a wide range of radiocarbon activities and that young SOM is stabilized across multiple fractions, likely due to organomineral complexation. Initial results showed that soil minerals with limited stabilization potential released C at lower temperatures than those with diverse stabilization mechanisms. High-temperature sub-fractions contained the oldest carbon across fractions and minerals, thus supporting the assumption that thermal stability can be used as a limited analogue for stability in soil. We present a fine-scale distribution of radiocarbon in SOM and discuss the potential of this method for comparison with other fractionation techniques.

How to cite: Stoner, S., Sierra, C., Schrumpf, M., Dötterl, S., and Trumbore, S.: Thermal fractionation of soil organic matter produces fine-scale distributions of SOM age, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15968, https://doi.org/10.5194/egusphere-egu21-15968, 2021.

Advanced techniques have been recently used to obtain information on Natural Organic Matter (NOM). However, the current knowledge of the chemical structure of humic substances (HS) is still incomplete. These substances appear to be too complex mixtures of charged organic molecules, and their characterization remains one of the most stimulating challenges in modern environmental science. Knowledge of the chemical composition of NOM is of great importance for the definition of soil and water properties because it has a significant impact on the understanding of numerous molecular and global-scale processes.
This study aims to apply two-dimensional graphical methods to resolve homologous series in mass spectra of humic extracts (Suwannee River, Nordic Aquatic and Soil) obtained using FT-ICR / MS (Thermo LTQ FT, 7 Tesla) in negative ionization mode. Electrospray ionization (ESI) coupled with ultra-high resolution mass spectrometry offered by Fourier transformed ion cyclotron resonance (FT-ICR / MS) has emerged with great promise as it can provide an overview of the NOM composition and details on a molecular scale. NOM's very high-resolution FT-ICR spectra can be extremely complicated. These spectra usually contain many peaks at each nominal mass and thousands of peaks across the entire spectrum. Each peak can represent a chemically distinct compound. This complexity poses an analytical challenge to the study of spectra for structural interpretation. Two-dimensional graphing methods, such as Kendrick and van Krevelen graphs, have been successfully applied to very high-resolution mass spectra, allowing peaks to be sorted into complicated spectra from their homologous relatives across the mass range.
In van Krevelen plots, ionic signals corresponding to structural similarities between homologous series of compounds involved in the loss or gain of functional groups are found on straight lines. We identified many interesting homologous regions and compared the three humic standards with each other. Finally, we recognized the structural relationships of the homologous series obtained through Kendrick graphs.
The results showed homologous series in the Suwannee River and Nordic Aquatic samples compared to the soil-extracted samples (soil-FA and soil-HA). In particular, homologous series signals related to methylation/demethylation, hydrogenation/dehydrogenation, hydration/dehydration, and oxidation/reduction processes were lower in the soil-FA van Krevelen diagrams. On the contrary, the differences were not so evident in all the homologous series for the soil-HA samples.

How to cite: Scrano, L., Mottola, F., Stefanelli, C. M., Lelario, F., Bianco, G., and Bufo, S. A.: Graphical resolution of  Humic structures, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11108, https://doi.org/10.5194/egusphere-egu21-11108, 2021.

Soil C sequestration through improved agricultural management practices has been suggested to be a cost-efficient tool to mitigate climate change as increased soil C storage removes CO₂ from the atmosphere. In addition, improved soil organic carbon (SOC) content has positive impacts on farming though better soil structure and resilience against climate extremes through e.g. better water holding capacity. In some parts of the world, low SOC content is highly critical problem for overall cultivability of soils because under certain threshold levels of SOC, soil loses its ability to maintain essential ecosystem services for plant production. Soil organic amendments may increase soil C stocks, improve soil structure and boost soil microbial activities with potential benefits in plant growth and soil C sequestration. Additional organic substrates may stimulate microbial diversity that has been connected to higher SOC content and healthy soils.

We performed a two-year field experiment where the aim was to investigate whether different organic soil amendments have an impact on soil microbial parameters, soil structure and C sequestration.

The experiment was performed in Parainen in southern Finland on a clay field where oat (Avena sativa) was the cultivated crop. Four different organic soil amendments were used (two wood-based fiber products that were leftover side streams of pulp and paper industry; and two different wood-based biochars). Soil amendments were applied in 2016. Soil C/N analysis was performed in the autumns 2016-2018 and soil aggregate in the summer and autumn 2018, as well as measures to estimate soil microbial activity: microbial biomass, soil respiration, enzymatic assays, microbial community analysis with Biolog ®  EcoPlates and litter bag decomposition experiment. The relative share of bacteria and fungi was determined using qPCR from soil samples taken in the autumns 2016, 2017 and 2018.

Data on how the studied organic soil amendments influence soil structure and C content, as well as soil microbial parameters will be presented and discussed.

How to cite: Heinonsalo, J., Salonen, A.-R., Shrestha, R., Kalu, S., Sietiö, O.-M., and Huusko, K.: Organic soil amendments as a tool to increase biological activity and C sequestration in clay soil, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11965, https://doi.org/10.5194/egusphere-egu21-11965, 2021.

Agricultural sandy soils with high organic matter (OM) contents are generally unexpected under the current paradigm of organic matter formation and stabilization. These so-called black sand soils occur in North-Western Europe and have been related to historical heathland vegetation. The properties and mechanisms of the high OM sequestration in these soils are not clear as they exceed common observations of OM stored in coarse-textured soils. In this study, we analyzed a subset of samples with ‘black sand’ properties from the European soil database “Land Use/Cover Area frame statistical Survey” (LUCAS). Through particle size fractionation, we isolated the fine fraction <20 µm which contained, on average, 55 % of the total soil organic carbon (OC), in only 8 % of the corresponding soil mass. The fine fraction <20 µm contained 301 mg OC g^-1 with a C:N ratio of 17.4 on average and was positively correlated with the bulk soil OC. The characterization of OM composition in the fine fractions by solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy revealed that the share of alkyl C increased with OC concentrations whereas O/N-alkyl C decreased. To analyze the distribution of OM at the microscale, we analyzed five samples from the <20 µm fraction containing a gradient of 245-378 mg OC g^‑1 with nanoscale secondary ion mass spectrometry (NanoSIMS) at a spatial resolution of 120 nm. These microscale measurements revealed fine mineral particle structures associated with heterogeneously distributed OM. Using image analysis, we found that the proportion of OM-dominated area (indicated by ¹²C₂^- and ²⁶CN^-) increased from 52 to 80 % on average with increasing OC concentration of the fine fractions. A majority of OM-dominated area was correlated with higher ⁴²AlO^- counts, which might suggest a preferential co-localization. In turn, the particle area which was dominated by minerals (indicated by ¹⁶O^‑, ²⁸Si^‑, ⁴²AlO^‑ and ⁷²FeO^‑) contained less Al and more Si. This shows that the more alkylated and OM-rich fine fractions are related with distinct patterns of organo-mineral structures at the microscale.

How to cite: Schweizer, S. A., Lugato, E., Höschen, C., and Kögel-Knabner, I.: Organic matter in black sand soils related to alkyl carbon and organo-mineral structures at the microscale, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13614, https://doi.org/10.5194/egusphere-egu21-13614, 2021.

How to cite: Giannetta, B., Oliveira De Souza, D., Aquilanti, G., and Said Pullicino, D.: Redox-driven changes in the distribution of Fe minerals between aggregate-size classes in illuvial and elluvial horizons of a hydromorphic soil, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14354, https://doi.org/10.5194/egusphere-egu21-14354, 2021.