Displays

Soil microaggregates play an important role for the long-term sequestration of soil organic matter (SOM) and are the nucleus of larger soil structures. However, it is still not well understood how the interplay of microaggregate structure and composition determine their functionality. This is especially due to analytical limitations that lead to the fact that the 3D architecture of microaggregates in conjunction with the fate of OM at the sub-micron scale is still hard to analyze. Combining microscopic and spectroscopic techniques with high sensitivity and high spatial resolution enables the correlation of microscale topographic information together with elemental and isotopic composition. In the present study we use novel imaging techniques at a previously unresolved spatial resolution to analyze and reconstruct the 3D architecture of microaggregates and the associated SOM.

In this study, isotope labelling was used to understand the fate of fresh C and N within newly developed soil microstructures. To explore the role of the inherited carbon content, soils from three different management practices, namely bare fallow, three-field and direct drilling, with different OM content, from an agricultural cropland research farm were used. To investigate the impact of fresh OM differing in C/N ratio on the 3D architecture of new formed soil microstructures, we performed an amendment with a substrate lacking N (highly labelled glucose (>99% ¹³C) ) and a substrate containing N (amino acid mixture (>98% ¹³C, >98% ¹⁵N) . After the incubation, all bulk soils and fractions were measured by Isotope-ratio mass spectrometry (IRMS) for ¹³C and ¹⁵N abundances. Microaggregates, clay and fine silt after 24 hours incubation were analyzed using Helium Ion Microscope (HIM) coupled with Secondary Ion Mass Spectrometer (SIMS) system and Nano Secondary Ion Mass Spectrometry (NanoSIMS) instrument for topography and elements/isotopes distribution, respectively. HIM-SIMS produced secondary electron images with a high resolution down to 0.5 nm and SIMS images of ²⁴Mg, ²⁷Al, ³⁹K and ⁵⁶Fe with a sub 20 nm resolution. NanoSIMS was capable to locate ¹²C₂, ¹²C¹³C, ¹²C¹⁴N and ¹²C¹⁵N for organic matter and ¹⁶O, ²⁷Al¹⁶O and ⁵⁶Fe¹⁶O as mineral phases at a submicron scale.

Significant (p < 0.01) difference found between clay and fine silt fractions in amino acid treatment shows that the clay plays a more important role in fresh OM sequestration than fine silt, as more ¹³C and ¹⁵N were detected on mineral surfaces in the clay sized fraction. Interestingly, no difference between fractions were observed for the glucose (C added only) treatments, here N can be assumed to act as a limiting factor. By correlative image registration of HIM-SIMS with NanoSIMS data, the 3D architectural buildup soil microaggregates was reconstructed. The combination of HIM-SIMS and NanoSIMS analyses allows a considerable step forward in the capability to investigate soil microaggregate and their organo-mineral associations at a previously unresolved sub-micron scale. The ongoing work points to the chemical composition of the mineral microaggregate constituents as being decisive for the spatial arrangement of the organo-mineral associated OM.

How to cite: Wu, T., Ost, A., Audinot, J.-N., Wirtz, T., Buegger, F., Höschen, C., and Mueller, C. W.: Elucidating the 3D structure of soil microaggregate and the fate of organic matter, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17147, https://doi.org/10.5194/egusphere-egu2020-17147, 2020.

Aggregate formation and stabilization depends on the interaction of minerals and soil organic matter (SOM). So far, little is known about the interplay of individual organic matter qualities and soil texture within this process. We developed an experimental set-up to study early soil development and aggregate formation within a controlled lab environment. We designed artificial soil microcosms with different texture, mimicking natural soils, and added organic carbon (OC) derived from particulate organic matter (POM, milled hay litter), dissolved organic matter (DOM, solution derived from hay), and bacterial necromass (Bacillus subtilis). We performed a short-term incubation for 30 days under constant water tension and investigated microbial activity, soil structure development and OC allocation compared to a control that did not receive additional OC input. 

OC input led to the formation of mostly large, water-stable macroaggregates (3000-630 µm) and some small microaggregates (<63 µm) in all microcosms as effect of microbial processing of the added OM. The addition and microbial decay of litter pieces led to physical occlusion of the particles into mainly large (3000-630 µm), OC-rich macroaggregates independent of the texture. The addition of DOM solution also induced the formation of large macroaggregates besides small microaggregates, although the OC input was much lower. Here, the aggregate formation was impaired by higher sand content in the mixtures. The addition of bacterial necromass led to the highest microbial activity, but relatively low aggregate formation, which might be a result of less physically active organic matter nuclei.

The results show that our experimental design allows to specifically investigate selected process complexes of soil structure formation defined by the addition of OM and soil texture.

How to cite: Bucka, F., Tung, S.-Y., and Kögel-Knabner, I.: Processing of organic matter input influences aggregate formation in artificial soils with different texture, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11335, https://doi.org/10.5194/egusphere-egu2020-11335, 2020.

Microaggregates are the fundamental building blocks of soils and thus important for their structure, properties, and functions. Hence, experimental aggregate formation studies (Dultz et al. 2019) were conducted to reveal the mechanisms leading to the establishment of soil microaggregates from mixtures of the mineral building units goethite and illite. Mathematically based modeling can further illuminate the mechanisms and factors behind structure formation as well as facilitate this understanding, even if the experimental capability is limited.

To this end, we present and extend a mechanistic modeling approach (Rupp et al. 2018, Rupp et al. 2019) which is based on a cellular automaton method that resolves explicitely particles at the micrometer scale. Thus it is capable to represent structural changes originating from (electrostatic) interaction of building units (aggregate forming materials). As prototypic building units goethite and illite with needle like and platy shapes of different size and charge are implemented. The operational, comprehensive model allows studying structure formation as a function of composition and charge of such mineral mixtures. Along this line, homoaggregation as well as heteroaggregation scenarios are investigated. The resulting microaggregates are investigated with respect to size, structure, and stability. Moreover, the role of the aspect ratio for stability, the point of zero charge for aggregation, and the amount of excess particles with respect to time is illustrated. Finally, the results are evaluated and compared to experimental data given in Dultz et al. (2019), and extend the scenarios studied there

S. Dultz, S.K. Woche, R. Mikutta, M. Schrapel, G. Guggenberger (2019): Size and charge constraints in microaggregation: Model experiments with mineral particle size fractions. Applied Clay Science 170, 29-40.

A. Rupp and K. Totsche and A. Prechtel and N. Ray (2018): Discrete-continuum multiphase model for structure formation in soils including electrostatic effects. Frontiers in Environmental Science, 6, 96.

A. Rupp, T. Guhra, A. Meier, A. Prechtel, T. Ritschel, N. Ray, K.U. Totsche (2019): Application of a cellular automaton method to model the structure formation in soils under saturated conditions: A mechanistic approach. Frontiers in Environmental Science 7, 170.

How to cite: Prechtel, A., Zech, S., Dultz, S., Guggenberger, G., and Ray, N.: Microaggregation of goethite and illite: Linking mechanistic modeling and laboratory experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9493, https://doi.org/10.5194/egusphere-egu2020-9493, 2020.

Anecic and endogeic earthworm species are known as “ecosystem engineers” that significantly contribute to the porosity and coherence of soil as well as soil water infiltration and the turnover rates of organic matter (OM). Additionally, earthworms actively excrete nutrient rich mucus, release bacteria within casts and translocate litter into the subsurface. In this way, earthworms not only shape the structure of soils but also the chemical milieu of the drilosphere where mucus forms a prominent fraction of OM. Furthermore, other biogenic extracellular polymeric substances (EPS) are known to form organo-mineral associations, which suggests that earthworms also facilitate their further attachment into soil aggregates.

With this study, we investigated how earthworms contribute to soil aggregate formation and impact aggregate properties by OM translocation and incorporation in the drilosphere. At vertically sampled burrow walls predominantly formed by Lumbricus terrestris, a patchy and depth dependent distribution of hydrophobic and hydrophilic regions was found with the water drop penetration time (WDPT) test. In the hydrophobic regions, we identified an enrichment of carbon and aggregate surface coatings containing plant residuals, bacteria, OM screenings and enmeshments by scanning electron microscopy (SEM) and elemental analysis. These structures were further investigated by factor analysis of Fourier transform infrared (FTIR) spectra that permitted the FTIR-band extraction of earthworm typical aggregation agents as, e.g., bacterial EPS, earthworm mucus and plant components (leaves, roots and sprout). Furthermore, with sorption experiments to typical minerals of temperate soils (e.g. illite and goethite), we found a mineral-specific adsorption of earthworm cutaneous mucus (of Lumbricus terrestris and Aporrectodea caliginosa) and bacterial EPS (of Bacillus subtilis). Specifically, a preferential adsorption of phosphorus containing constituents of mucus and bacterial EPS to goethite has been observed. The resulting formation of organo-mineral associations characterized by screened mineral surface charges was shown by zeta potential measurements.

We show that besides the active incorporation of particulate OM, as e.g. plant residuals and microorganisms, the mineral specific adsorption of EPS formed by earthworms and bacteria induce the formation of organo-mineral associations and alteration of the physico-chemical properties of earthworm-formed structures and soil aggregates.

How to cite: Guhra, T., Stolze, K., Ritschel, T., and Totsche, K. U.: The contribution of earthworms to soil aggregate formation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17611, https://doi.org/10.5194/egusphere-egu2020-17611, 2020.

Nanosized mineral particles and organic matter (<100 nm) ,as well as their associations, belong to the most important ingredients for the formation of the soil aggregate structure being a hierarchically organized system. Colloids (< 220 nm) including nanoparticles can be occluded as primary building units of soil aggregates. Nevertheless, a large proportion of these colloids is mobile and presents in the solution phase (as “free”) within the soil matrix. However, the differences between “free” and occluded colloids remain unclear.

Here, both occluded and free colloids were isolated from soil samples of an arable field with different clay contents (19% and 34%) using wet sieving and centrifugation. The release of occluded colloids from soil macroaggregates (>250 µm) was carried out with ultrasonic treatment at 1000 J mL^-1. The free and occluded colloidal fractions were then characterized for their size-resolved elemental composition using flow field-flow fractionation inductively coupled plasma mass spectrometry and organic carbon detector (FFF-ICP-MS/OCD). In addition, selected samples were also subjected to transmission electron microscopy as well as pyrolysis field ionization mass spectrometry (Py-FIMS).

Both, free and occluded colloids were composed of three size fractions: nanoparticles <20 nm, medium-sized nanoparticles (20 nm–60 nm), and, fine colloids (60 nm–220 nm). The fine colloid fraction was the dominant size fraction in both free and occluded colloids, which mainly consist of organic carbon, Al, Si, and Fe, probably present as phyllosilicates and associations of Fe- and Al- (hydr)oxides and organic matter. However, the organic matter contents for all three size fractions were higher for the occluded colloids than for the free ones. The role of OM concentration and composition in these colloids will be discussed in the paper.

How to cite: Tang, N., Siebers, N., and Klumpp, E.: Free soil colloids and colloidal building units of soil aggregates, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16496, https://doi.org/10.5194/egusphere-egu2020-16496, 2020.

Interactions of organic matter with mineral surfaces are seen as one of the important mechanisms to increase carbon preservation in soils. Often, the mineral associated organic matter is assumed to consist of microbial derived material, because of its small C/N ratio and its isotopic signature.

We sampled sediments and surface water flocs from a small creek (pH 6.4) to obtain natural samples with a much higher microbial versus plant derived organic matter input than expected for soils. The bulk material was investigated by CNS analysis, X-ray diffraction (XRD), and infrared spectroscopy (FTIR). Organo-mineral associations were imaged by a combination of atomic force microscopy (AFM), scanning electron microscopy (SEM), and X-ray fluorescence spectroscopy (STXM-XRF) at 2550 eV (S, P, Si, Al, Fe) and 320 eV (C) at a spatial resolution of 50 nm. The speciation of C and P was addressed by near edge X-ray absorption fine structure spectroscopy (STXM-NEXAFS). Synchrotron measurements were performed at the PO4 beamline at PETRA III using the Animax STXM endstation with a 4-channel fluorescence detector with a solid angle of detection of up to 1.1 sr.

Organic matter was mainly found on Fe oxides (ferrihydrite). However, the C concentration on the Fe oxides varied and some Fe oxides were not covered by organic matter. Clay minerals (mainly illite) were either free of organic matter or showed a lower concentration of organic matter than the Fe oxides. Phosphorus was only observed on some of the Fe oxides surfaces and its P K-edge NEXAFS spectrum usually showed a small pre-edge peak at ~2150 eV, which can be taken as evidence for inner-sphere Fe-O-P bonds. Although Fe oxides were often found in close proximity of bacterial cells, the Fe oxide-associated organic matter was rich in carbonyl C and O-alkyl C, but showed higher contributions of aryl C and/or alkyl C than pure extracellular polymeric substances (EPS) or bacterial cells.

Our observations confirm a high reactivity of Fe oxides towards organic matter and phosphate. However, the Fe oxides were not fully coated, i.e. saturated with organic matter. The mineral associated organic matter was not similar to EPS or bacterial cells.

How to cite: Eusterhues, K., Thieme, J., Lühl, L., Haidl, A., Heym, S. J., Adrianov, K., Dehlinger, A., Rehbein, S., Wilhein, T., Kanngießer, B., and Totsche, K. U.: Imaging organo-mineral associations of creek sediments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10404, https://doi.org/10.5194/egusphere-egu2020-10404, 2020.

Organic matter makes up less than 5% of soil and sediment and is often linked to the particulate mineral fraction, forming an organo-mineral microsystem that certainly contributes to its stability in the natural environment. However, most of the techniques used to study and quantify organic matter necessarily require the separation of the organic phase from the mineral phase by chemical or physical extraction and, subsequently fractionation methods that destroy and/or degrade the original aggregate morphological structure. The present work demonstrates that Scanning Confocal Microscopy (MC) can be used as a non-destructive fluorescence technique capable of characterizing organic matter (OM) interacting with the surface of the mineral fraction in soils and sediments without prior sample preparation and use of extraction or chemical fractionation of its components. Organic matter (OM) interacts with the mineral surface through molecular stacking in the form of stable molecular aggregates. Besides that, aggregate states also favor energy transfer processes between aggregated molecules which strongly affects the dynamics of excited state producing spectral shifts to the red and changes in life-time that can be correlated to aggregate morphology and to molecular amount deposited on the surface. These features confer a high spectral and intensity contrast of the confocal images. Here, infrared 2-photon (2P) excitation proved to be adequate to selectively excite OM aggregate states in the visible region between 400 and 700 nm, which allows a direct access to the fundamental aspects of the organic matter-mineral interactions.

We will show that the use of confocal methodologies, together with image analysis, provide helpful tools to understand the complex OM interactions at a molecular level. Here, we studied the interaction of OM with sodium bicarbonate and sodium hydroxide surfaces that form fractal crystals. When a drop of water containing both soil and solubilized bicarbonate or hydroxide salts is dried on a glass surface, dendritic-type salt crystals are first formed on the glass surface within the water droplet. In a second step of the droplet drying process, suspended organic molecules deposit on the surface of these fractal crystals. We will show that the morphology and molecular packaging substantially change spectral and life-time properties which strongly depend on the amount of OM on the crystal surface. Special features can be obtained from linear unmixing of spectral images using 1P and 2P excitation at 375 nm and 750 nm respectively for OM interacting with powder bicarbonate. Therefore, molecular aggregates of interacting fluorescence-emitting species can be used to characterize OM regarding the morphological, molecular structure and interactions with inorganic surfaces. These properties determine the stability of the original OM packing and the limits for the molecular stacking on different active surfaces in nature.

How to cite: Guimaraes, F. E. G., Morais, C. P., Tadini, A., Falvo, M., Bruno, O., Housam, H., Redon, R., Martin Neto, L., Mounier, S., and Milori, D. M. B. P.: Investigation on organic matter-mineral interaction by confocal multispectral and time-resolved microscopy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11509, https://doi.org/10.5194/egusphere-egu2020-11509, 2020.

A strong link exists between iron oxides and soil organic carbon (SOC). However, the role of iron oxides in the preservation of SOC in agricultural soil remains poorly understood. In this study we comprehensively examined the concentration, molecular composition and biological sources of iron oxide-bound organic carbon (Fe-bound OC) in arable soils collected from 12 sites in central and east China. The effect of elevated temperatures on Fe-bound OC in two contrasting soils was also investigated. The results indicated that 6.2 ~ 31.2% of the SOC was bound to iron oxides in agricultural soil, and that the binding mechanisms varied from adsorption in most soils to coprecipitation in those with a large content of organic carbon. The distribution of Fe-bound OC showed no clear variation in relation to site, but Fe-bound OC reached a peak in soils with an annual mean temperature of 16.4°C. Correlation analysis demonstrated that TOC might be the main determinant for the amount of Fe-bound OC, and that the binding mechanism is influenced by both TOC and the active Fe ratio. Analysis of C/N, ¹³C isotope, and synchrotron radiation-based Fourier transform infrared (SR-FTIR) spectroscopy showed that iron oxides selectively protected plant-derived aliphatic compounds and polysaccharides in agricultural soil. Warming decreased the content of Fe-bound OC from 3.46 g kg⁻¹ to 1.99 g kg⁻¹ in Ultisol, while enhanced that from 4.04 g kg⁻¹ to 5.12 g kg⁻¹ in Histosol. NMR results suggested that warming could alter the composition of soil organic matter by accelerating O-alkyl C degradation and increasing the sequestration of recalcitrant alkyl C and carboxyl C. It is supposed that warming promoted the association of iron oxides with microbial-derived polysaccharides and aliphatic compounds. This study revealed the quantitative characterization, biological sources and molecular composition of Fe-bound OC in arable soils, which provides useful information for evaluating and managing the global C cycle under the framework of climate change.

How to cite: Huang, Q., Chen, W., and Liu, Y.: Biological sources and molecular composition of iron oxides bound organic carbon in agricultural soils, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11295, https://doi.org/10.5194/egusphere-egu2020-11295, 2020.

Organo-mineral associations drive organic matter (OM) stabilization in soils, but mechanisms controlling their dynamics are still not fully known at micro and nanoscale. Adsorption of OM on minerals’ surfaces is a prevalent viewpoint of OM stabilization processes (Kleber et al., 2007), but Basile-Doelsch et al., (2015) suggested that mineral alteration generating amorphous nanophases and cationic oligomers on minerals’ surfaces is also a driver of OM stabilization through coprecipitation processes. Lab experiments which mimic these processes showed that the nanosized co-precipitates (Nanosized Coprecipitates of inorganic oLIgomers with organiCs: nanoCLICs) are made of inorganic Fe, Al, Si oligomers associated with organic molecules (Tamrat et al., 2019). Andosols are known to have a high OM-stabilization capacity, mostly attributed to associations of OM with nanominerals (imogolite, allophane, proto-imogolite) (Basile-Doelsch et al., 2007; Levard et al., 2012). In the present study, we investigated the presence of nanoCLICs in Andosol fractions from La Martinique (French West Indies). We used Transmission Electron Microscopy (TEM, FEI Tecnai Osiris 200kV) coupled with 4 EDX detectors and EELS to semi-quantify and map major elements. TEM analyzed zones of interest ranged from 5 µm to 10 nm with pixel size from 500 to 1 nm. Few crystallized minerals, particulate OM and amorphous thin fibers that could not be definitively attributed to imogolite nanotubes were observed. However, we mainly observed totally amorphous phases to electron diffraction. Al, Si, C, Fe and O were the main component of the latter amorphous phases. Al, Si and Fe were systematically associated to C even at a size resolution down to 1 nm (semi-quantifications ranged from 11 to 41% of C, 4 to 7% of Fe, 34 to 36% of Al and 22 to 46% of Si). Similar high-resolution images were obtained for the andosol organo-mineral associations and the synthetic nanoCLICs. At the working TEM resolution, the nanoCLICs model proposed by Tamrat et al., (2019) is consistent with the structures observed on the andosol. Based on these results, the majority of C appears to be in nanoCLICs form in these Andosol fractions and confirms the hypothesis puts forward by Basile Doelsch et al., (2015).

Basile Doelsch et al., 2007. Mineral control of carbon pools in a volcanic soil horizon. Geoderma, 137 (3-4), 477-489. ISSN 0016-706.

Basile-Doelsch et al., 2015. Are Interactions between Organic Compounds and Nanoscale Weathering Minerals the Key Drivers of Carbon Storage in Soils? Environ. Sci. Technol. 49, 3997–3998.

Kleber et al., 2007. A Conceptual Model of Organo-Mineral Interactions in Soils: Self-Assembly of Organic Molecular Fragments into Zonal Structures on Mineral Surfaces. Biogeochemistry 85, nᵒ 1 (1 août 2007): 9‑24.

Levard et al., 2012. « Structure and distribution of allophanes, imogolite and proto-imogolite in volcanic soils ». Geoderma 183‑184 (1 août 2012): 100‑108. 

Tamrat et al., 2019. « Soil organo-mineral associations formed by co-precipitation of Fe, Si and Al in presence of organic ligands ». Geochimica et Cosmochimica Acta, 10 juin 2019. 

How to cite: Jamoteau, F., Cam, N., Levard, C., Woignier, T., Boulineau, A., Doelsch, E., Rose, J., and Basile-Doelsch, I.: Nanoscale chemical imaging of soil organo-mineral associations., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4678, https://doi.org/10.5194/egusphere-egu2020-4678, 2020.

Organo-mineral interactions stabilize soil organic matter (SOM) by protecting from microbial enzymatic attack. Soil water content affects aggregation, mineral weathering, and microbial respiration, thus influencing the relative importance of SOM stabilization mechanisms. While the response of microbial respiration to momentary changes in water content is well established, it is unclear how microbial activity will impact stabilization mechanisms under different long-term moisture contents.

To understand how long-term soil moisture affects SOM stabilization mechanisms we studied fallow soils from upstate New York situated on a naturally occurring water content gradient. Wetter (but not saturated) soils contained more exchangeable Ca and had more strongly stabilized SOM, resulting in SOM accumulation. But it was not clear whether Ca-driven surface interactions or occlusion in micro-aggregates was more important, and if interactions with Fe and Al played a role in the Ca-poor soils. Also, the role of biotic drivers in SOM stabilization at different water contents was unknown.

We tested which mechanisms governed SOM stabilization by determining C and N contents and natural isotope abundances in particulate and mineral-associated organic matter fractions. We also extracted the C bound to Ca and to reactive Fe+Al phases. Wetter, Ca-rich soils had higher oPOM content, and in the heavy mineral fraction, higher relative concentrations of Ca-bound C, lower C:N values, and more oxidized C forms. In addition, wetter soils had greater microbial biomass. Together, these results showed that high long-term soil moisture increased microbial SOM cycling, and that processed SOM was better stabilized, in agreement with the recent notion that stable SOM consists of processed labile C. Additionally, higher soil moisture augmented the role of Ca in SOM stabilization over that of Al+Fe phases. We then manipulated the exchangeable Ca content and incubated soils with ¹³C¹⁵N labeled plant litter. Ca-amended soils emitted less CO₂ while incubated with litter, confirming that Ca is instrumental in SOM stabilization. Tracing the labeled isotopes in the gaseous phase and soil fractions will allow us to gain a clearer understanding of how water content and soil Ca interact to stabilize SOM.  

How to cite: Shabtai, I., Das, S., Inagaki, T., Kogel-Knabner, I., and Lehmann, J.: Long-term soil water content and exchangeable Ca interact to stabilize organic matter, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12377, https://doi.org/10.5194/egusphere-egu2020-12377, 2020.

Protective mineral-organic associations are the quantitatively most important soil carbon storage mechanism, but their vulnerability to environmental change is largely uncertain. While it is well established that root growth can promote (or “prime”) the microbial decomposition of organic matter (OM), our mechanistic knowledge of the ability of roots to destabilize OM protected within mineral-organic associations remains limited. Here we examined how the composition of root-derived compounds (rhizodeposits) affects the stability of mineral-organic associations.

In model systems, we first tested the ability of functionally distinct low-molecular weight compounds (ligands, reductants, simple sugars) commonly observed in the rhizosphere to cause the mobilization and mineralization of isotopically labeled OM from different mineral types (Fe and Al hydroxides). Our results showed that all compounds stimulated mobilization and mineralization of previously mineral-associated OM. However, OM bound to Al hydroxide was less susceptible to mobilization than OM bound to Fe hydroxide. Further, sugars and reductants revealed a greater mobilization potential than ligands for both mineral types, suggesting that OM mobilization in soils may be microbially mediated, rather than driven by direct mineral dissolution. In complementary pot experiments, we investigated the effect of rhizodeposition on the mobilization of mineral-associated OM. We grew Avena sativa in soils amended with isotopically-labeled mineral-organic associations and followed mobilization dynamics over four weeks. First results indicated that rhizodeposition dynamics dictate the mobilization and mineralization of mineral-associated OM. Together, our results suggest a strong mechanistic linkage between the composition and functionality of rhizodeposits and their ability to destabilize mineral-associated OM.

How to cite: Bölscher, T., Li, H., Garcia Arredondo, M., Cardon, Z. G., Malmstrom, C. M., Winnick, M., and Keiluweit, M.: Chemical complexity matters: differential mobilization of mineral-associated organic matter driven by functionally distinct rhizodeposits, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4559, https://doi.org/10.5194/egusphere-egu2020-4559, 2020.

In redoximorphic soils, iron (Fe) and manganese (Mn) oxides undergo reduction with subsequent oxidation of their reduced counterparts (Fe²⁺ and Mn²⁺) impacting nutrient sorption and the stability of soil organic matter (SOM). One tool to investigate the soil redox status is the indicator of reduction in soils (IRIS) method. Thereby, synthetic Fe and Mn oxides are coated onto polyvinyl chloride (PVC) bars, which are typically installed for an operator-defined period in the soil. After removal of the bars we studied organo-mineral associations, which have been formed under field conditions on the surface of the coated bars.

In this study, each one Mn and Fe oxide-coated redox bar were installed for 30 days in a Mollic Gleysol. A previous study revealed, that the Mn oxide coating facilitated a non-enzymatic redox reaction under anoxic conditions, while Fe²⁺ from the soil solution is oxidized to Fe³⁺ along the Mn oxide coating and Mn²⁺ is removed from the PVC surface [1]. In consequence, in situ Fe oxides formed along the Mn oxide coatings and were further considered as ‘natural’ Fe oxides. This enables us to differentiate between sorption occurring onto the surfaces of ‘synthetic’ Fe oxides from the Fe bar versus ‘natural’ formed Fe oxides along the Mn bar. They were analysed by nanoscale secondary ion mass spectrometry (NanoSIMS) to study the distribution of Fe (⁵⁶Fe¹⁶O⁻), SOM (¹²C¹⁴N⁻), and phosphorus (³¹P¹⁶O₂⁻). NanoSIMS is a spectromicroscopic technique offering a high lateral resolution of about 100 nm, while having a great sensitivity for light elements. In contrast to classic bulk analysis, it offers the possibility to examine the spatial distribution of SOM and phosphorous at the microscale within the intact organo-mineral matrix. 

Image analysis of individual Fe oxide particles revealed a close association of Fe, SOM, and P resulting in coverage values up to 71% for synthetic and natural iron oxides. Furthermore, ion ratios between sorbent (⁵⁶Fe¹⁶O⁻) and sorbate (¹²C¹⁴N⁻; ³¹P¹⁶O₂⁻) were smaller along the natural oxides when compared with those for synthetic Fe oxides. We conclude that both natural and synthetic Fe oxides rapidly sequestered SOM and P (i.e., within 30 days) but that newly, natural formed Fe oxides sorbed more SOM and P than synthetic Fe oxides.

[1] Dorau, K.; Eickmeier, M.; Mansfeldt, T. Comparison of Manganese and Iron Oxide-Coated Redox Bars for Characterization of the Redox Status in Wetland Soils. Wetlands 2016, 36, 133–144.

How to cite: Pohl, L., Dorau, K., Just, C., Höschen, C., Ufer, K., Mansfeldt, T., and Mueller, C. W.: Soil organic matter and phosphate sorption on natural and synthetic Fe oxides under in situ conditions , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-171, https://doi.org/10.5194/egusphere-egu2020-171, 2020.

Rice paddy soils are known to represent a large proportion of global terrestrial carbon (C) stocks (ca.10 Pg), accumulating organic C in the topsoil due to cultivation under submerged conditions. Apart from the limited mineralization under anoxic soil conditions resulting from frequent field flooding, other mechanisms involving the dynamic interactions between organic C and redox-active minerals particularly Fe (oxy)hydroxides, together with the transport of organic C to deep mineral horizons, can lead to long-term C stabilization. Our previous studies have shown that up to 30-50 g m^-2 of dissolved organic C (DOC, defined as <450 nm) and 25-40 g m^-2 of Fe²⁺ may be mobilized and translocated into the subsoil over a rice cropping season in temperate rice paddies, contributing to an increase in belowground C stocks. However, little is yet know on influence of frequent redox fluctuations on the contribution of colloidal organo-mineral associations to C mobilization and accrual in paddy subsoils.

We hypothesized that (i) redox fluctuations may lead to an overall increase in colloid dispersion (via reductive dissolution of Fe oxides, changes in soil pH, as well as neoformation of colloidal organo-mineral associations), and that (ii) colloidal mobility may represent an important C input to paddy subsoils. In order to evaluate the effects of redox fluctuations on colloid dynamics in situ, water-dispersible fine colloids (WDFC) were isolated from soils collected from different horizons along two profiles opened in adjacent plots under long-term paddy (P) and non-paddy (NP) management in NW Italy. Moreover, WDFC were also isolated from anaerobically-incubated topsoil samples to evaluate the changes in colloid dispersion under reducing conditions as a function of management. Colloidal size-fractionation and their elemental compositions were evaluated by asymmetric flow field-flow fractionation (AF4) coupled with OCD or ICP-MS. 

Our results evidenced that redox cycling favours colloidal stability in the topsoils, with a preferential dispersion of the smallest-sized colloidal C (<30 nm and 30-240 nm fractions), even though larger-sized colloidal C (>240 nm) contributes predominantly to the WDFC. Consequently, under long-term paddy management colloidal dispersion and transport along the soil profile were probably responsible for the lower amounts of colloidal C (and Fe) observed in the Ap topsoil horizons of P with respect to NP, as well as for the significant accumulation of colloidal C in correspondence with the Brd subsoil horizons just beneath the plough pan. These illuvial horizons were also particularly rich in small-sized (30-240 nm) colloidal Fe, Al and Si possibly due to mineral phase changes induced by redox fluctuations.

Our findings therefore indicate that downward mobilization of colloidal C associated with Fe (hydr)oxides (e.g. coprecipitates) or small aluminosilicate minerals, rather than dissolved organic C, may represent an important process driving organic C accrual in paddy subsoils. However, further insights are still required to entangle the contribution of the different mechanisms involved.

How to cite: Said-Pullicino, D., Giannetta, B., Demeglio, B., Missong, A., Gottselig, N., Bol, R., Klumpp, E., and Celi, L.: Redox-driven colloidal mobility and its effects on carbon cycling in temperate paddy soils, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18840, https://doi.org/10.5194/egusphere-egu2020-18840, 2020.

Quantifying the upper limit of stable soil carbon storage and relative saturation is essential for guiding policies designed to increase soil carbon storage, such as ‘4 per 1000’ sequestration initiative. Carbon stabilization processes are diverse, but one particular pool of carbon that is considered stable across climate zones and soil types is the mineral-associated fraction, measured using density or size fractionation. Some soil carbon decomposition models assume sorption to minerals is the main form of stabilization in this fraction. We estimate the global capacity of mineral soils in six soil orders to sorb additional dissolved organic carbon (DOC). We gathered data from 400 DOC sorption experiments representing 133 soil profiles across six soil orders. We used the relationship between DOC added and DOC sorbed to calibrate a modified Langmuir sorption equation, from which we quantified the DOC sorption potential in each soil. We found that the sorption potential is empirically related to climate variables (including mean annual temperature and mean annual precipitation) and soil geochemical variables (chiefly, percent clay, pH, and soil order). From this relationship, we then estimated the DOC sorption potential for 14631 profiles distributed globally. This amount was 1.4 (global median; 95% CI: 0.50, 2.8) kg C m^-3, totaling 102 Pg C globally across six soil orders, representing up to a 7% increase in the existing total C stock. We show that there is greater capacity for additional DOC sorption in subsoils (30cm-1m) compared to top-soils (0-30cm). The gap between the modest potential of mineral sorption processes found in this study and the large total capacity of long-term organic matter stabilization (2541 Pg C for the six soil orders of this study) indicates that other mechanisms such as aggregation, the sorption of microbial necromass, layering, and co-precipitation also play a critical role in stable organic matter formation and persistence.

How to cite: Abramoff, R., Georgiou, K., Guenet, B., Torn, M., Huang, Y., Zhang, H., Feng, W., Jagadamma, S., Kaiser, K., Kothawala, D., Mayes, M., and Ciais, P.: How much more carbon can be sorbed to soil?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1107, https://doi.org/10.5194/egusphere-egu2020-1107, 2020.

Recent studies suggest significant control on pedogenic iron (Fe) and aluminum (Al) on organic matter (OM) storage and stability across a wide range of soils around the world. This information would be useful to improve or replace existing SOM models. On the other hand, metal extraction studies have shown that only minor portions of soil OM are directly bound to pedogenic Fe and Al. How can these metals control OM storage and stability without direct binding with bulk of OM? To answer this, an important step is to understand the location of the metals and OM within bulk soils. Sequential density fractionation is useful to examine their localizations because pure OM (e.g., plant detritus) and pure mineral particles (e.g., quartz, clay, Fe oxide) are the two endmembers along particle density gradient. We tested if Fe and Al released by chemical weathering are mainly present in association with OM using 22 soil samples from 11 sites spanning 5 climate zones, 5 soil orders (Andisols, Spodosols, Inceptisols, Mollisols, Ultisols), and including several subsurface horizons and both natural and managed (upland and paddy) soils. Across all the studied soil samples, meso-density fractions (1.8-2.4 g cm-3) accounted for major portions of OM and the metals extractable by pyrophosphate, acid oxalate, and dithionite. We also found a strong stoichiometric relationship between the extractable metals and co-dissolved OM. We discuss the biogeochemical processes that may cause the co-localization of the metals and OM at the mesodensity across the soils from a wide range of pedogenic environments.

How to cite: Wagai, R., Kajiura, M., and Asano, M.: Co-localization of iron and aluminum with organic matter across a range of soils: a density-based approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22089, https://doi.org/10.5194/egusphere-egu2020-22089, 2020.

Formation of Microaggregates and organo-mineral composite building units: Novel pathways in the soil-parent rock continuum

Summary

Microaggregates and organo-mineral composite building seem to be unique structural features of natural permeable media like soils, rocks and aquifers. Thee develop in response to various aggregation processes and mechanisms that result in a non-random spatial arrangement of the solid phase already at the submicron scale. Soil microaggregates are defined as compound structures smaller <0.25mm, comprising the colloidal-sized and nanoparticulate composite building units and the organo-mineral composites (Totsche et al. 2018). Noteworthy, microaggregates, may be present as suspended or colloidally-dispersed components of the mobile phase. As such, they are prone to transport with the seepage and may affect the surface and pore-space properties. Surface alteration by interactions of seepage components with immobile surfaces is likely an important, yet essentially unexplored pathway triggering formation of microaggregates in the soil-parent rock continuum. In matured soils, the commonly found associations of clays with other, often poorly crystallized but highly reactive minerals and organic matter is the consequence of nucleation in the chemically heterogeneous soil suspension. Both pathways coexist and can be studied in the soil-parent-rock transition zone were weathering and formation/alteration of secondary mineral phases are still in the early stage. The stability of microaggregates and their interactions are dependent on wetting-drying and in turn by hydration-dehydration cycles. Such moisture-related dynamics regularly take place in soils of the temperate regions even down to the soil-parent-rock transition zone and suggests that the hydraulic and osmotic stress and their history results in attachment, detachment, translocation and accumulation. The presentation will We focus on two so far vastly ignored formation pathways of microaggregates and composite building units, i.e., the “geochemical inheritance” and “heteroaggregation from suspension”, thereby considering the role of dynamic relocation of composite building units and microaggregate forming materials from upstream compartments.

Totsche K.U., Amelung W., Gerzabek M.H., Guggenberger G., Klumpp E., Knief C., Lehndorff E., Mikutta R., Peth S., Prechtel A., Ray N., Kögel-Knabner I. (2018) Microaggregates in soils. Journal of Plant Nutrition and Soil Science 181(1), 104-136.

How to cite: Totsche, K. U.: Formation of Microaggregates and organo-mineral composite building units: Novel pathways in the soil-parent rock continuum, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10220, https://doi.org/10.5194/egusphere-egu2020-10220, 2020.

During pedogenesis, the development of the soil aggregate system may be strongly dependent on weathering of bedrock in the vadose zone. There, periodic drying and flushing by meteoric waters provides alternating hydro- and biogeochemical conditions for fluid-rock interactions and provokes the dissolution, displacement and placement of minerals as well as the release or adsorption of colloids. As a result, the seepage suspension is enriched with mobile mineral and organic matter that infiltrates into the bedrock void system thereby fueling aggregate forming materials and composite building units to exposed surfaces of the bedrock. We aim to elucidate related bedrock alteration processes in dependence on water composition, seepage vs. saturation and the fracture network during weathering.

Our study combines the investigation of the weathering rim of natural bedrocks (outcrop analogue) with the simulation of the natural conditions by column experiments in the laboratory. Study object are Triassic limestones (Upper Muschelkalk) of the Hainich area in Thuringia (central Germany). The columns were filled with fresh, unaltered material, crushed into coarse gravel fraction size and percolated with artificial rainwater or soil litter extract over a runtime of 6 months. In order to mimic natural conditions percolation periods changed with periods of drying. Geochemical data of the liquid phase resemble very well the alternating periods of drying and flushing by systematic changes of the element concentration and milieu parameters. Generally, dissolved elements in the seepage are higher in concentration when litter extract is used pointing towards a significant impact on dissolution kinetics, especially after periods of longer water-rock interaction. Weathered natural bedrock surfaces (bedrock clasts in the covering soil and fractures and voids in the bedrock) exhibit carbonate dissolution (edge pits and dissolution vugs) and the formation of clay mineral coatings, in part with iron oxides. The same holds true for rock clasts after the column experiments. The alteration is macroscopically visible by brownish and beige coatings on formerly greyish pristine surfaces. This feature seems more pronounced on clasts percolated with liquids from soil litter extracts than on clasts treated with artificial rainwater indicating the formation of organo-mineral associations during solid-liquid interaction.

Generally in both, nature and experiment, facial aging features include the dissolution of carbonates, the formation of clay minerals as well as oxides and hydroxides of iron, but also the appearance of organic constituents. Our results contribute to a better mechanistic understanding of the role of bedrock alteration during weathering for (a) the provision of microaggregate forming materials and (b) the formation of composite building units and microaggregates from pristine environments.

How to cite: Aehnelt, M., Ritschel, T., and Totsche, K. U.: Role of bedrock weathering in microaggregate formation - limestone alteration in the aeration zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8349, https://doi.org/10.5194/egusphere-egu2020-8349, 2020.

The organic matter stability is regulated by the different protection mechanisms of the soil matrix and soil minerals. In spite of that, beyond the determination of the amount of fine fractions, relatively little research studied the mineralogical composition of these fractions and their organic matter stabilizing effects. Therefore, the aim of my work was to investigate the influence of the soil mineral phases on the decomposition of soil organic carbon pools of soils under forest vegetation.

Maize residues were added to the 13 soil samples (depth of 0−20 cm) collected from Hungary. The samples were incubated at 20°C and 70% field capacity during 163 days. The soil respiration was measured at specified intervals (on day 3, 8, 15, 30, 51, 79, 107, 135 and 163) and trapped in 2M NaOH and quantified by titration with 1M HCl. Another aliquot of NaOH was mixed with 2MSrCl₂ to get SrCO₃ for δ¹³C analysis.

The samples were analysed with an X-ray diffractometer (Rigaku Miniflex 600), a microwave plasma-atomic emission spectrometer (4200, Agilent Technologies) and an isotope ratio mass spectrometer (Delta plus XP, Thermo Finnigan). Carbon mineralization kinetics was modelled by fitting a first-order two pools model.

The results showed that 1−6% and 2−18% of the organic carbon content of the soils was mineralized in the control and amended samples during the incubation, respectively. Carbon mineralization was mostly reduced by the illite content (R²=0,797; p<0,001), Al-oxide content (R²=0,708; p<0,001) and clay content (R²=0,475; p<0,05) of the soils. The decomposition rates of the two carbon pools were found to be influenced to the greatest extent by the illite and total Al-oxide content of the soils investigated. Whereas the decomposition rate constant of the slowly mineralizable C pool was only affected by the Al-oxide and illite content, the decomposition rate constant of the easily mineralizable carbon pool was also sensitive to the other soil parameters (aromaticity, Fe-oxide content, C/N ratio, pH and clay content).

The priming effect was found to be influenced to the greatest extent by the pH (R²=0,715; p<0,05), whereas weaker negative relationship with the content of non-swelling clay minerals (R²=0,396; p<0,05), illite content (R²=0,389; p<0,05) and the C/N ratio (R²=0,345; p<0,05) of the soils was also detected.

This work was supported by the Development and Innovation Fund of Hungary [Nr. NKFIH 123953].

How to cite: Zacháry, D., Jakab, G., Filep, T., Balázs, R., and Szalai, Z.: The role of mineral composition regulating the turnover of organic matter in 13 forest soils from Hungary, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5314, https://doi.org/10.5194/egusphere-egu2020-5314, 2020.

In this work, soil organic matter (SOM) content and composition was investigated in a Brazilian Red Oxisol submitted to ICLS during 15 years. The experiment was conducted in a clayey Oxisol in South Brazil in randomized blocks (n=3) with different grazing intensities according to pasture height: 10 (P10), 20 (P20) and 40 cm (P40) cm). The ICLS system consisted of black oat (Avena strigosa) and ryegrass (Lolium multiflorum) in winter and soybean (Glycine max) in summer (C3 plants). Litter and soil samples were collected within 1 m depth from treatments and from secondary bush forest (SF) soil. Previous to experiment, the whole area had been used for conventional agriculture for about 45 years. Before that (circa 60 years ago), the area was under native pasture (Aristida pallens, C4 plant). C and N contents and isotopic signature δ¹³C were determined and SOM chemical composition was investigated in HF-concentrated samples by ¹³C NMR CP/MAS spectroscopy. δ¹³C from a modal profile under native vegetation was determined as well. Grazing intensity did not affect C contents, that varied from 31.5 g kg^-1 in surface to 7.6 g kg^-1 at 1 m depth in ICLS. The greatest C contents were observed under SF down to 20 cm depth (47.8 to 20 g kg^-1), evidencing the more relevant contribution of forest vegetation on C sequestration in this area. C/N ratio under ICLS tended to increase with depth from 11 to 16. In contrast, no such trend was observed under SF, where C/N ratio was smaller than under ICLS in all analyzed layers. For SF and P40 soils, δ¹³C values were around -22 ‰ at 0-5 cm (typical for C3 plants SOM), changing abruptly to -17 ‰ at 5-10 cm layer and increasing steadily downward the profile to -14 ‰. In the modal profile, δ¹³C values were typical for C4 plants and varied from –12.4 ‰ in A horizon to -9.7 ‰ at Bw horizon. It follows that residues from past and recent agriculture use contributed relevantly to surface layer (0-5 cm) SOM, whereas below 5 cm, endogenous SOM was the main constituent. SOM chemical composition at 0-5 cm layer was dominated by O alkyl C groups (45-110 ppm) that contributed with 52 to 54 %, followed by alkyl C groups (0-45 ppm) with 18 to 26 % and aromatic C groups (110-160 ppm) with 9 to 16%. In all analyzed sites, alkyl C decreased and aromatic C proportions increased with depth, reaching 14 to 17 % and 19 to 23 %, respectively, at 80-100 cm layer. Nevertheless, at this depth, O alkyl C proportion tended to remain high (49 to 52%). Under P10, alkyl C/O alkyl C ratio at surface was lower than in P40, indicating that grazing affected SOM composition mainly at the surface layer. Our results suggest that in the studied Oxisol profile, endogenous SOM resists to be exchanged by “recent vegetation” SOM from ICLS due to stabilization via interactions with Fe-Oxides that also preserve biochemically labile C groups.

How to cite: Pinheiro Dick, D., Garcia, G., Anghinoni, I., and de Faccio Cravalho, P. C.: Impact of integrated crop-livestock system (ICLS) on endogenous organic matter in a subtropical Oxisol, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6068, https://doi.org/10.5194/egusphere-egu2020-6068, 2020.

Biochar is a material created through the pyrolysis of different kind of biomass. On the basis of last research reports, it was found, that the kind of used biomass influences on surface properties of biochar. These properties are of key importance in effectiveness of biochar as a soil sorbent.

The experiments were carried out using three biochars derived from: tobacco, sweet corn cobs and vineyard which are produced by the "double-barrel" method.

The aim of the research was an investigation the effects of feedstock kind on surface properties of biochar. The characteristics of selected surface properties of biochar included the determination of: functional group content by Boehm's acid-base titration method; determination of the variable surface charge and distribution of surface functional groups from potentiometric titration.

Biochar samples exhibit a high variable surface charge (Q) and content of surface functional groups. Tobacco has the highest Q and content of acidic groups, while sweet corn cobs - the lowest one. Variable surface charge provides information about the quantity of surface functional groups. The distribution of surface functional groups exhibits the existence of 3 peaks, which indicate the presence of acidic groups (carboxylic, lactonic and phenolic).

The biochar from tobacco, vineyard or sweet corn cobs possessed merit for the improvement pollution removal from soil.

Research was conducted under the project "Water in soil - satellite monitoring and improving the retention using biochar" no. BIOSTRATEG3/345940/7/NCBR/2017 which was financed by Polish National Centre for Research and Development in the framework of "Environment, agriculture and forestry" - BIOSTRATEG strategic R&D programme

How to cite: Tomczyk, A., Szewczuk-Karpisz, K., Sokołowska, Z., Kercheva, M., and Dimitrov, E.: Impact of Feedstock on Biochar Surface Properties: Practical Application of Boehm’s and Potentiometric Titration, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7299, https://doi.org/10.5194/egusphere-egu2020-7299, 2020.

Aggregate tensile strength is a significant parameter of soil structure. Adequate mechanical stability of aggregates promotes long-term crop productivity due to, inter alia, maintaining gas diffusion, facilitating root penetration and improving water infiltration. Soil aggregates characterized by high tensile strength are also resistant to erosion. Nowadays, intensive agriculture and environmental pollution contribute to clear deterioration of soil condition. The soil structure is often destroyed. In order to limit the negative phenomena, various soil additives are used, e.g. biochar.

In this paper, the effect of wood waste biochar on tensile strength and porosity of Dystric Cambisol artificial aggregates was examined. The experiments were performed on dry-air and wet soil aggregates non-containing and containing 0.1% or 5% dose of biochar. Tensile strength of the probes was determined using strength testing device (Zwick/Roell), whereas porosity – by mercury intrusion porosimetry (Micrometrics). The obtained results indicated that the biochar addition decreases tensile strength of all examined aggregates. This effect was more significant for higher biochar dose – 5%. This phenomenon is probably connected with formation of macropores of larger sizes within aggregates after the biochar addition.

Research was conducted under the project "Water in soil - satellite monitoring and improving the retention using biochar" no. BIOSTRATEG3/345940/7/NCBR/2017 which was financed by Polish National Centre for Research and Development in the framework of "Environment, agriculture and forestry" - BIOSTRATEG strategic R&D programme.

How to cite: Szewczuk-Karpisz, K., Tomczyk, A., Sokołowska, Z., Turski, M., Cybulak, M., and Skic, K.: Biochar impact on tensile strength of Dystric Cambisol aggregates – a model study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7351, https://doi.org/10.5194/egusphere-egu2020-7351, 2020.

Organo mineral associations intermediated by Fe and Al are considered one of the most important mechanisms for soil organic carbon (SOC) stabilization. However, since Fe and Al are normally mentioned together as stabilizing agents, we still lack knowledge about their relative role. In addition, this stabilization mechanism can be profoundly affected by climate differences, but the magnitude of this influence whether as a direct effect or an indirect consequence due to changes in soil mineralogy is not yet fully understood. In this study, we evaluated a series of subsoil samples throughout a climate gradient (1800–2400 mm precipitation year^-1 and 15–24º C) on Kohala Mountain, Hawaii to understand the impact of climate differences on organic matter protection. We have used a combined approach of analyses at the bulk soil and microscale using NanoSIMS. At the bulk soil scale, we have observed a concurrent decline of subsoil Fe, Al (i.e., dithionite citrate and ammonium oxalate extractions) and SOC above a precipitation level of 2000 to 2200 mm year^-1_. This decline co-occurred with more reduced forms of Fe s (evaluated by Fe K-edge XANES) and declines in carboxyl-C (evaluated by CP-MAS ¹³C NMR). We found significant positive correlations between SOC with Fe and Al in the bulk soil throughout the gradient, and we could discern the relative role of Fe and Al in promoting organo-mineral associations in contrasting climate conditions (e.g., ~1800 and ~2300 mm year^-1) using NanoSIMS. While Fe contributed to approximately 40% of the microscale organo-mineral associations in the lower precipitation site (assessed by co-localizations with OM segments), this contribution at the higher rainfall regime was only 5%. In contrast, the contribution of Al was approximately the same in both rainfall levels (approximately 30%). This fact indicates that Al may be more important than Fe in stabilizing SOC especially under high precipitation levels. The normalized CN:C ratio was higher when associated with Fe and Al especially in the high precipitation level, which demonstrates the importance of Fe and Al in stabilizing N-rich organic matter. Here we demonstrate that spatial relationships between Fe and Al with SOC at the microscale display a shift towards Al-dominated SOC associations at higher precipitation that could not be ascertained from bulk measurements alone. Thus, they are of great importance to understand the impact of climatic differences on SOC sequestration in organo-mineral associations.

How to cite: Inagaki, T. M., Possinger, A. R., Grant, K. E., Schweizer, S. A., Mueller, C. W., Derry, L. A., Lehmann, J., and Kögel-Knabner, I.: Subsoil organo-mineral associations under contrasting climate conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7673, https://doi.org/10.5194/egusphere-egu2020-7673, 2020.

Several beneficial soil functions are linked to aggregates, but how the formation and stability depend on the presence of colloidal building blocks is still understood poorly. Here, we sampled subsites from an arable toposequence with 190 and 340 g kg^-1 clay, and isolated small soil microaggregates (small SMA; < 20 µm) from larger macroaggregate units (> 250 µm) using an ultrasonic dispersion energy of 60, 250, and 440 J mL^-1 , respectively. We then allowed these small SMA to reaggregate after chemical removal of organic carbon (OC) as well as of Fe- and Al (hydr)oxides, respectively. The size distribution of the reaggregated small SMA and fine colloids (< 0.45 µm) was analyzed via laser diffraction and asymmetric flow field-flow fractionation coupled to inductively coupled plasma mass spectrometry and OC detection, respectively. We found elevated amount of both fine colloids and stable SMA at subsites with larger clay contents. The size distribution of small SMA was composed of two distinct fractions including colloids (< 1 µm) and SMA with an average size of 5 µm. The removal of Fe with Dithionite-Citrate-Bicarbonate (DCB) shifted the size of the small SMA to a larger equivalent diameter, while destruction of OC with NaOCl reduced it. After three wetting and drying cycles, the concentration of colloids declined, whereas the small SMA without chemical pre-treatments reaggregated to particles with larger average diameters up to 10 µm, with the size depending on the clay content. Intriguingly, the gain in size was more pronounced after Fe removal, but it was not affected by OC removal. We suggest that Fe (hydr)oxides impact the stability of small SMA primarily via cementing the aggregates to smaller size. In contrast, the effect of OC was restricted to the size of colloids, gluing them together to small SMAs within defined size ranges when OC was present but releasing these colloids when OC was absent.

How to cite: Wang, L., Krause, L., Klumpp, E., Nofz, I., Missong, A., Amelung, W., and Siebers, N.: Colloidal iron and organic carbon control soil aggregate formation and stability, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8660, https://doi.org/10.5194/egusphere-egu2020-8660, 2020.

Understanding the mechanisms governing the composition and stability of organo-mineral associations is critical to predicting the dynamics of soil organic matter (SOC) and the related global carbon cycling. Redox-induced biogeochemical transformations are the key processes that control the stabilization of SOC via association with metal oxides in terrestrial environments such as wetlands. Despite its high C content (20-30% of terrestrial C), size-dependent organo-mineral associations and their dynamic changes in the redox-dynamic wetlands are poorly understood. Here we present size distribution, concentration, and composition of organo-mineral associations in pore water samples from a depressional wetland located at the Delmarva Bay in Delaware, USA, as influenced by seasonal fluctuations in water table level. The samples were collected from piezometers installed at multiple depths (50 cm, 100 cm, and 200 cm) and in three zones (upland, transitional, and wetland), respectively. Four size fractions were analyzed: dissolved (<2.3 nm), natural nanoparticle (2.3-100 nm, NNP), fine colloid (100-450 nm), and particulate (450-100 nm). Our results revealed that dissolved, NNP, fine colloid and particulate fractions comprised 47 ± 4%, 37 ± 4%, 8 ± 3% and 8 ± 3% of  the bulk organic C (<1000 nm) concentration, respectively. Relative percentages of respective Al, Mn, and Fe were 47 ± 24%, 30 ± 22%, 50 ± 18% at 2.3-450 nm and 22 ± 16%, 17 ± 12%, 25 ± 19% at 450-1000 nm size fraction. The main finding from this study are 1) dissolved and NNP fractions contain higher amount of C than colloidal and particulate fractions and 2) organo-mineral associations have significant differences in their elemental concentrations among different size fractions within colloidal size range. Additionally, the results clearly indicate that the commonly used operational definition for dissolved organic matter (DOM, <450 nm) significantly overestimates the dissolved phase C concentration by including the NNP and colloidal fractions, which contain mineral-associated C. This has important implications in the estimation of SOC decomposition rate in soils, particularly in redox sensitive wetlands, thus in assessing terrestrial C cycling and the transport of OC as well as the associated elements.

How to cite: Afsar, M., Vasilas, B., and Jin, Y.: Size-dependent organo-mineral interactions and dynamics in a seasonally-flooded wetland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12457, https://doi.org/10.5194/egusphere-egu2020-12457, 2020.

Soil microaggregates (SMA) are characterized by a pronounced small-scale structural heterogeneity, with recognizable chemical differences between the aggregate`s interior and its surface. Latter suggests a deterministic spatial pattern with respect to C stabilization, element exchange, and habitat function for microorganisms. Here, a detailed characterization of the pore space is crucial for the understanding of element transfer and microbial colonization in SMA. In our study, the 53-250 µm size fraction of SMA isolated along a soil clay content gradient (19-35%) were investigated in terms of their pore space characteristics. For the visualization of connected “open” pore structures as well as “closed” pores, a modified Hg-porosimetry technique utilizing Wood´s metal was used (WIP). The molten alloy was pressed into accessible connective pores by applying an argon pressure of 55 MPa, filling up pores with a diameter down to ≈20 nm. After solidification of the alloy, polished sections of SMA were analyzed by laser scanning confocal microscopy (Keyence, VK-9700). To image and quantify open and closed pores, grayscale-histograms were segmented and three pore size classes (<10, 10-100, and >100 µm²) were distinguished for open and closed pore systems. Additionally, we scanned 27 samples with high-resolution X-ray tomography (CT, Zeiss Xradia 520 versa) to characterize the 3D pore features at resolutions between 480 and 928 nm. SMA typically consist of two different sections, where particle arrangements are loose or dense. Relatively coarse-sized aggregate-forming materials were observed in sections with loose particle arrangements, where pores appear well connected. To some extent, these coarse aggregate-forming materials are arranged in larger circular structures. In contrast, dense particle arrangements consist primarily of fine aggregate-forming materials. The total porosity of the SMA derived by WIP was highly variable with a maximum of 40 area-%. While CT aggregate volume and CT aggregate surface area did not change with clay content, CT-porosity (vol.-%) increased with increasing clay content. Maximum CT porosity of 27 % was found in the samples with the highest clay content. Maximum pore diameter was similar across all clay contents, but the share of macropores with diameters >10 µm increased with increasing clay content. The Euler number decreased with increasing clay content, which indicates an increased connectivity of the pore space. Another parameter that increased with increasing clay content was the CT aggregate volume / CT internal pore surface area ratio, signifying more accessible surfaces for element exchange and/or C storage. While pores exceeding 100 µm² had the highest share within the open pores, it was the pore system <10 µm² for the closed pores. The proportion of closed pores of total porosity was smaller for the finer SMA sizes within the 53-250 µm fraction, which confirms the CT results (increasing Euler number). Our WIP data reveal that higher shares of clay minerals in SMA cause a narrower pore size distribution with smaller average diameters and increased tortuosity. Consequently, element transport and habitation by microorganisms might be slowed down in smaller, more clay-rich SMA, potentially resulting in larger C conservation within the interior of smaller SMA.

How to cite: Dultz, S., Felde, V., Woche, S. K., Mikutta, R., Uteau, D., Peth, S., and Guggenberger, G.: Pore space characteristics of soil microaggregates – Possible implications for functioning, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13589, https://doi.org/10.5194/egusphere-egu2020-13589, 2020.

Soil microaggregates are considered to play an important role in soil functioning and soil organic carbon (SOC) is of great importance for the formation and stabilization of these aggregates. The loss of SOC can occur, for example, after a change in land use and may lead to a decreased aggregate stability, which makes soils vulnerable to various threats, such as erosion or compaction. It is therefore important to understand the effect of SOC loss on aggregate stability in order to better understand and preserve the functioning of healthy soils.

We sampled two adjacent plots from a loess soil in Selhausen (North Rhine-Westphalia, Germany) in November of 2019 and measured aggregate stability and architecture of soil microaggregates. One plot was kept free from vegetation by the application of herbicides and by tillage (to a depth of 5 cm) from 2005 on, while the other plot was used for agriculture (conventional tillage). Over the course of 11 years, the SOC concentration in the bulk soil was reduced from 12.2 to 10.1 g SOC kg^-1 soil. We took 10 undisturbed soil cores from two depths of each plot (Ap and Bt horizons).

The stability of aggregates against hydraulic and mechanical stresses was tested using the widespread wet sieving approach and a newly developed dry crushing approach. Isolated microaggregates gained from the latter procedure were tested against tensile stress by adapting a crushing test in a load frame to the microaggregate scale. To shed light on the effect of a decreased SOC content on microaggregate structure, we scanned several microaggregates with a high-resolution computed tomography scanner (Zeiss Xradia 520 versa) at sub-micron resolutions and analyzed the features of their pore systems.

This will give us valuable insights into the interplay of mechanical and physicochemical stability, as well as the structural properties of microaggregates with regard to SOC depletion. The consequences for various soil functions provided by microaggregates, like the habitat function for microorganisms or their capacity to store and transport gas, water and nutrients, are discussed.

How to cite: Roosch, S., Felde, V., Uteau, D., and Peth, S.: Effect of soil organic carbon loss on the stability and structure of microaggregates: First insights from an organic carbon depletion field trial in a loess soil, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17358, https://doi.org/10.5194/egusphere-egu2020-17358, 2020.

The physical arrangement of soil compounds in microaggregates is important in many ways, e.g. by controlling soil stability and C sequestration. However, little is known about the spatial arrangement of organic and inorganic compounds in soil microaggregates, due to the lack of in-situ analyses in undisturbed material. Here we hypothesize that microaggregates are spatially organized, resulting in deterministic, predictable spatial patterns of different organic matter and mineral phases and that this organization depends on the abundance of specific phases such as on clay mineral content. We separated the water stable, occluded large and small microaggregate fractions from Ap horizons of a sequence of sandy to loamy Luvisols (19 to 35% clay, Scheyern, Germany) and subjected in total 60 individual aggregates to elemental mapping by electron probe micro analysis (EPMA), which recorded C, N, P, Al, Fe, Ca, K, Cl, and Si contents at µm scale resolution. Spatial arrangements of soil organic matter and soil minerals were extracted using cluster analyses. We found a pronounced heterogeneity in aggregate structure and composition, which was not reproducible and largely independent from clay content in soil. However, neighborhood analyses revealed close spatial correlations between organic matter debris (C:N app. 100:10) and microbial organic matter (C:N app. 10:1) indicating a spatial relationship between source and consumer. There was no systematic relationship between soil minerals and organic matter, suggesting that well-established macroscale correlations between contents of pedogenic oxides and clay minerals with soil organic matter storage do not apply to soil microaggregates.

How to cite: Lehndorff, E., Meyer, N., Radionov, A., Plümmer, L., Rottmann, P., Spiering, B., Amelung, W., and Dultz, S.: Spatial organization of soil microaggregates, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22405, https://doi.org/10.5194/egusphere-egu2020-22405, 2020.

Fluid flow and reactive transport in natural porous media take place in a three-dimensional, hierarchically organized network of voids and pores in the size range of sub-micrometers inside small aggregates to several millimeters in, e.g., earthworm burrows or cracks. Thus, fluid flow regimes are manifold with consequences not only for the transport of solutes, but also for the displacement of colloidal particles and organic matter and thus, for their inclusion into soil aggregates. Therefore, we incorporated the simulation of three-dimensional fluid flow in pore networks typical for natural porous media into our recent approach to model soil aggregate formation using DLVO theory and diffusion-limited aggregation to overcome its previous limitation to suspensions at rest. To visualize the model capabilities, we simulated aggregation in pore networks that were either synthetically designed to represent certain structural features such as pore junctions and dead-end pores, or taken directly from X-ray µ-CT measurements of undisturbed soil cores. We explored the development of structural aggregated features that evolve in response to flow, transport and the topology of the soil pore space. The resulting three-dimensional arrangement of compounds and the entire aggregates were classified according to their morphological metrics, e.g. the pore space distribution, and functional properties, e.g. the water retention capacity, that are provided by these structures. By this fusion of complementary modeling approaches, we significantly contribute to the fundamental mechanistic understanding of the complex interplay and feedback of structure, interactions and functions on the scale of (micro-)aggregates.

How to cite: Ritschel, T. and Totsche, K.: Aggregate formation dynamics driven by 3D fluid flow in natural porous media, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13488, https://doi.org/10.5194/egusphere-egu2020-13488, 2020.

Subsurface sediments usually show limited microbial activity, however, inputs of nutrients due to anthropogenic spills or infiltration from the surface may quickly activate the native microbial communities, thereby changing composition, structure and properties of sediments. We studied the effect of glucose addition, an easily available carbon source, on mineralogy, microstructure and properties of several (0.5-35 m) loamy and sandy sediments over 30 days in laboratory experiments. We followed the time changes in biomass by direct cell count; respiratory activity by CO₂ emission; clay mineralogy by X-ray diffraction (XRD); microaggregate size distribution by pipette analysis; and observed microbial binding via scanning electron microscopy (SEM).

Glucose addition caused transient buildup of respiratory activity and biomass with maximal values 3-10 times more than in control (water-treated) samples appearing around the 7^th day after the treatment. The biomass of bacteria, archaea, actinomycetes and fungi increased. After that the biomass and the CO₂ emission declined sharply and reached stable values about twice as much as in control samples.

On day 7, we noted an increase in the proportion of smectite layers in the disordered mixed layer illite-smectite minerals (MLM), yet no changes in content and composition of other clay and non-clay minerals. After 30 days of observation, XRD showed further transformation of MLM composition, as well as partial destruction of other clay minerals. We hypothesize that with abundant external nutrition, microbes mined the lacking K from illite layers of the MLM. After the consumption of glucose, all clay minerals were a source of essential elements.

The content of microaggregates of 0.1-0.05 mm in size increased in loams on the 7^th day after the treatment, presumably due to microbial binding and gluing of aggregates by cells and EPS. With the decline of the biomass, the previously-formed microaggregates partially disintegrated. We assume that after the consumption of glucose, the microorganisms lived on biomass and EPS, thereby removing previously formed glue and meshes from the aggregates.

SEM performed on air dried sands collected during maximal microbial activity revealed biofilms consisting of microbial cells and EPS, attaching to the fine clay coatings around sand grains. SEM on lyophilized loams showed filamentous structures, which we interpret to be actinomycete mycelium that enmeshes particles into microaggregates.

Irreversible changes in clay mineralogy and transient aggregation caused temporary alteration of stress-strain properties: increased cohesion, and decreased friction and compressive strength.

Our data show that ongoing/continued microbial activity is crucial for the formation of aggregates as well as for the clay mineral paragenesis in sediments. Both processes affect sediment quality, e.g. in terms of soil organic matter stabilization or with respect to the overall mechanical properties.

How to cite: Ivanov, P., Manucharova, N., Nikolaeva, S., Safonov, A., Krupskaya, V., Chernov, M., Eusterhues, K., and Totsche, K. U.: Glucose-stimulation of natural microbial activity causes transient aggregation and alteration of clay mineralogy in sandy and loamy sediments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7642, https://doi.org/10.5194/egusphere-egu2020-7642, 2020.

Dissolved organic matter (DOM) is one of the most mobile components of the global carbon cycle. Corresponding transport processes in the environment have received plenty of attention in the context of carbon sequestration as well as the mobility of DOM-associated contaminants.

However, most previous transport studies have been conducted exclusively under continuous flow conditions, which are not comparable to real water flow characteristics in soil. The present study aims to address that gap in knowledge by systematically assessing the effect of defined flow interruption phases on the retention of DOM.

For that, the breakthrough behavior of DOM as affected by phases of flow interruption was investigated in an increasingly complex system of solid matrices rich in oxide mineral coatings: goethite coated quartz sand, disturbed Cambisol subsoil, and undisturbed Cambisol subsoil. The classic DLVO and extended DLVO (XDLVO) models including Lewis acid—base parameters were applied based on measurements of sessile drop contact angles and zeta potentials.  

DOM retention was increasing with the duration of flow interruption, and retention was considerably higher in the soils than in goethite coated sand. After 112 hours of flow stagnation, DOM release from the soils was reduced to 16 to 22 % as compared to continuous flow conditions. The retention in the different solid matrix materials was well correlated with the respective amounts of oxalate and dithionite extractable oxide mineral phases. The DLVO model was capable of correctly predicting the mobility of DOM in goethite coated sand, but not in the soils, due to the fact that soil surface charge heterogeneities could not be measured. The XDVLO model predicted short-range hydrophilic repulsive interactions that may have contributed to the distinct tailing of the DOM breakthrough curves.

We conclude that the significant DOM retention during phases of flow stagnation phases shows that more complex flow regimes need to be considered in order to assess the mobility of DOM in soils. In fact, many previous studies excluding phases of flow stagnation likely overestimated the mobility of DOM in the environment.

How to cite: Carstens, J. F., Guggenberger, G., and Bachmann, J.: Effects of Flow Regime on DOM Retention in Soils: Continuous Flow vs. Flow Interruption , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13629, https://doi.org/10.5194/egusphere-egu2020-13629, 2020.

Land use change can significantly influence both mineralogy and chemical soil properties. This conversion, particularly from forest to agricultural system occurs often in volcanic soils due to their favorable properties for food production. Under agriculture, minerals can weather faster than in natural vegetation and this also impacts soil functioning. We aim to assess the impact of land use on geochemical soil properties and soil organic carbon across soils of different age. This study was conducted in Mt. Tangkuban Perahu and Mt. Burangrang where the soils were derived from similar andesitic parent material and have different ages based on their lithology. Five sites were selected representing land uses that have been converted (pine forest and agricultural land) and one site of natural forest as the origin of land use. The results showed that land use management enhances the mineral transformation. Pine forest and agricultural sites displayed higher weathering degree than natural forests as indicated by higher clay content, iron crystallinity index and the presence of gibbsite. The weathering degree of soils in agricultural sites might result from the length of cultivation period and soil age. Land use conversion also altered chemical properties such as pH, CEC, basic cations, and the proportion of amorphous materials. Non-crystalline Al and Fe minerals as indicated by Al_o+1/2Fe_o were highly correlated with organic carbon and specific surface area (SSA) in the subsoils of all land uses. However, we did not see the accumulation of organic carbon in subsoils compared to topsoils as the amount of non-crystalline Al and Fe minerals increases with depth, especially in agricultural lands where the organic fertilizer input is very high. In addition, a significant proportion of carbon was stored in sand aggregate fractions in agricultural land which have longer cultivation period, while it was more readily found in silt and clay fractions in the site with shorter period.

How to cite: Anindita, S., Finke, P., and Sleutel, S.: Land use effects on the geochemical soil properties and their control on organic carbon in volcanic soils, near Bandung area (Indonesia), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5516, https://doi.org/10.5194/egusphere-egu2020-5516, 2020.

Soil organic matter (OM) interacts with cations like Ca by using C=O and OH functional groups. Such interactions are known to protect soil OM against decomposition. This process affects the bonding strength of functional groups. Changes in bonding strength are assumed to shift the wavenumber region of OH and C=O absorption band maxima in Fourier transform infrared (FTIR) spectra. The aim is to analyze the extent of such shifts to determine presence and strength of OM–cation interaction. Solutions of PGA and Chia seed mucilage were mixed at different ratios with CaCl₂ solution. The mixtures were freeze dried. FTIR spectra of PGA–Ca, and mucilage-Ca mixtures indicate that the OH band is affected by the presence of Ca. However, the C=O band maximum and the CH/C=O ratio were not affected. For the PGA–Ca and mucilage-Ca mixtures the shift in OH band maxima relative to PGA and mucilage, respectively, increases with Ca content. Such shifts in OH band maxima are in a similar range as the ones observed for the outer compared to inner regions of an intact chia seed mucilage droplet. The results suggest that it is necessary to know the relation between OM-cation interactions and band shifts for the correct interpretation of the FTIR spectra of soil and rhizosphere samples.

How to cite: Ellerbrock, R. and Gerke, H. H.: FTIR spectral properties affected by OM-cation interactions , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18375, https://doi.org/10.5194/egusphere-egu2020-18375, 2020.