Climate and land use change affect weathering and pedogenesis with potential consequences for the fate of Al-bearing minerals and the potential export of Aluminum to groundwater resources. These changes might result in strong acidification, originally known for “acid rain” affecting these areas until the second but last decade of the past century. To explore the fate of Al in areas now affected by climate and land use change, we investigated two sites of different geology in North-Bavaria. Site 1 is located on granitic rocks under a reforested 6-year-old Norway spruce forest. Site 2 is a hilltop site located on metamorphic rocks under a 60-80-year-old spruce forest. Soil samples (< 2mm) and clay fractions were analyzed by hydrochemical and spectroscopic techniques. Zero tension controlled lysimeter and automated tension controlled lysimeters were installed for monitoring the soil solution volume and composition at the topsoil-subsoil and the subsoil-regolith boundary. Monitoring started in June 2018. Since then, 85 sampling campaigns have been completed that amounted to 1500 individual lysimeter samples. Analysis comprised among others EC, pH, elemental composition major anions and cations, and carbon sum parameters (DOC, TOC, DIC, TIC). Recent climate at the sites differs markedly from the 1961-1990 period, indicating a transient climate at the sites. Mean soil pH ranged from 3.2 to 4.7 at both sites and was comparable to values published in 1995 by Franken et al. (3.4 to 4.2). Thus, recent soil pH is as low as used to be under the conditions of strong acid precipitation of the last century. Soils developed from magmatic rock showed higher contents of variable Al phases than those developed from metamorphic rocks. At both sites pyrophosphate extractable Al is the dominant Al pool accounting 19.4% of total Al in site 1(14.1 g/kg in Bs horizon), and 6.9% of total Al in site 2 (4.9 g/kg in Bs horizon). Noteworthy, hydrological summer was more important for seepage generation than the hydrologic winter: Roughly 68% of the total annual seepage volume was found in the hydrological summer. As a result, the TOC flux from the subsoil in summer is 35.66 ± 20 mg/year, and only 13.88 ± 13.8 mg/year in winter. Similarly, the Al flux in summer is 1.02 ± 0.7 mg/year and only 0.43 ± 0.4 mg/year in winter. Variation partitioning analysis showed that the seasonal variation and the difference between topsoil and subsoil combined explained less than 5 % of the particle-related soil solution properties ((pH, ∑LMWO, TOC, Al and Si(mg/L)) and less than 1% of the hydrochemical properties (TIC, Cl−, SO42−, Ca, Mg, Na (mg/L)). Difference between the two sites explained 13.84% and 6.48% of the two sets, respectively and the sampling year explained 4.52% and 4.74%. We conclude that the Al system at our sites is controlled by climatic conditions and site properties (lithology, slope, vegetation..). There are no indications that the released Al is immobilized in any secondary immobile Al-phase in the subsoil or downstream, pointing to the potential transport of Al and other unwanted substances to the aquifers.

At numerous open pits worldwide, carcinogenic and geno‐toxic tar oil is still exposed to the environment. To understand ongoing tar degradation under different environmental conditions we studied soil structure, water retention, tar composition, and microbial biomass of a technosol under a small tar-oil spill at a former brown coal processing site. We observed that microbial biomass increased with pore volume on our study site: Generally, contaminated layers of the technosol were more porous than uncontaminated control soils and accommodated more microbes. However, the relationship was not linear. We therefore wondered whether the redox regimes within the aggregates of the different layers provide comparable conditions for microbial degradation. We used the chemical state of S as a proxy for the prevailing redox-conditions and µXANES on thin sections (5 µm spatial resolution) to analyze the S speciation in relation to soil structure. First results show that the tar is not homogeneously composed and that the proportion of reduced S compounds increases with soil depth: Particularly S-rich domains within the tar are often roundish, up to 200 µm in size and composed of varying proportions of inorganic sulfide-S, organic monosulfide-S or thiol-S, sulfoxide-S, and sulfonate-S. The topmost layer (0-5 cm) of the technosol is very porous. Here, the tar matrix is dominated by sulfonate-S. At more than 5 cm depth, the soil also has a high porosity due to large pores > 50 µm but at the same time includes mm-sized, compact aggregates with only few small pores (1-10 µm and 10-50 µm). The tar matrix within these aggregates contains sulfidic S in addition to the sulfonate-rich component. However, adjacent to pore surfaces we observe 5-15 µm thick (oxidized) rims with only sulfonate-S. Our data show that the tar is not only chemically complex, but also heterogeneous in composition at the µm scale. Below a soil depth of 5 cm, we can assume that microbial tar degradation is slowed down because of the anoxic conditions within the aggregates, although pores > 50 µm are abundant and bacterial cell counts are high.

The functions of soils are intimately linked to their three-dimensional pore space and the associated biogeochemical interfaces, mirrored in the complex structure that developed during pedogenesis. Under stress overload, soil disintegrates into smaller compound structures, conventionally named aggregates. Microaggregates (<250 mu m) are recognized as the most stable soil structural units. They are built of mineral, organic, and biotic materials, provide habitats for a vast diversity of microorganisms, and are closely involved in the cycling of matter and energy. However, exploring the architecture of soil microaggregates and their linkage to soil functions remains a challenging but demanding scientific endeavor. With the advent of complementary spectromicroscopic and tomographic techniques, we can now assess and visualize the size, composition, and porosity of microaggregates and the spatial arrangement of their interior building units. Their combinations with advanced experimental pedology, multi-isotope labeling experiments, and computational approaches pave the way to investigate microaggregate turnover and stability, explore their role in element cycling, and unravel the intricate linkage between structure and function. However, spectromicroscopic techniques operate at different scales and resolutions, and have specific requirements for sample preparation and microaggregate isolation; hence, special attention must be paid to both the separation of microaggregates in a reproducible manner and the synopsis of the geography of information that originates from the diverse complementary instrumental techniques. The latter calls for further development of strategies for synlocation and synscaling beyond the present state of correlative analysis. Here, we present examples of recent scientific progress and review both options and challenges of the joint application of cutting-edge techniques to achieve a sophisticated picture of the properties and functions of soil microaggregates.

Sorption of organic molecules on mineral surfaces can occur through several binding mechanisms of varying strength. Here, we investigated the importance of inner-sphere P-O-Fe bonds in synthetic and natural mineral-organic associations. Natural organic matter such as water extracted soil organic matter (WESOM) and extracellular polymeric substances (EPS) from liquid bacterial cultures were adsorbed to goethite and examined by FTIR spectroscopy and P K-edge NEXAFS spectroscopy. Natural particles from a Bg soil horizon (Gleysol) were subjected to X-ray fluorescence (XRF) mapping, NanoSIMS imaging, and NEXAFS spectro-microscopy at the P K-edge. Inner-sphere P-O-Fe bonds were identified for both, adsorbed EPS extracts and adsorbed WESOMs. Characteristic infrared peaks for P-O-Fe stretching vibrations are present but cannot unambiguously be interpreted due to possible interferences with mono- and polysaccharides. For the Bg horizon, P was only found on Fe oxides, covering the entire surface at different concentrations, but not on clay minerals. Linear combination fitting of NEXAFS spectra indicates that this adsorbed P is mainly a mixture of orthophosphate and organic P compounds. By combining atomic force microscopy (AFM) images with STXM-generated C and Fe distribution maps, we show that the Fe oxide surfaces were fully coated with organic matter. In contrast, clay minerals revealed a much lower C signal. The C NEXAFS spectra taken on the Fe oxides had a substantial contribution of carboxylic C, aliphatic C, and O-alkyl C, which is a composition clearly different from pure adsorbed EPS or aromatic-rich lignin-derived compounds. Our data show that inner-sphere P-O-Fe bonds are important for the association of Fe oxides with soil organic matter. In the Bg horizon, carboxyl groups and orthophosphate compete with the organic P compounds for adsorption sites.

The speciation of aluminum (Al) in soils is important for Al cycling and Al toxicity in terrestrial ecosystems. Current soil Al speciation methods are mostly wet-chemical, discriminating among operationally defined frac-tions rather than distinct Al species. We performed synchrotron-based X-ray absorption near edge structure (XANES) spectroscopy at the Al K-edge (1560 eV) for Al speciation in different horizons of three forest soil profiles and O layer density fractions. We deconvoluted the spectra by linear combination fitting (LCF; 1550-1690 eV), using reference spectra of 22 diluted inorganic and organic Al-bearing soil constituents. Our first time application of Al XANES spectroscopy + LCF for soil Al speciation revealed methodological challenges and difficulties. Thus, a minimum sample Al content of 5 mg g-1 was necessary to yield spectra with satisfactory signal-to-noise ratio in reasonable acquisition time, and Al contents > 15 mg g-1 resulted in spectrum distortion by self absorption, requiring adequate sample dilution prior to XANES analysis. Nevertheless, Al K-edge XANES spectroscopy allowed for estimating the relative contribution of different Al species to total Al in our soil and density fraction samples. The Al XANES spectra of different Al carboxylates were very similar. The same was true for different Al phenol species, different 2:1 clay minerals (illite, montmorillonite, chlorite), different 1:1 clay minerals (kaolinite, halloysite), and different feldspars (orthoclase, anorthite), jeopardizing a reliable differen-tiation and quantification of these specific compounds. We therefore combined these compounds into the groups SOM carboxyl(ate)-bound Al, SOM phenol-bound Al, 1:1 clay mineral-bound Al, 2:1 clay mineral-bound Al, and feldspar-bound Al. XANES results showed a decreasing contribution of organically bound Al to total soil Al with soil depth (O layers 20-43 %; Ah horizons 10-22 %; B horizons 0-12 %). Major inorganic Al-bearing constituents in the fine earth of all soils were 2:1 clay minerals and feldspars, followed by 1:1 clay minerals (kaolinite), and Fe oxyhydroxides with partial Al substitution. Gibbsite or allophane did contribute only marginally to fine-earth Al in our soils. Different Oa layer density fractions with different SOM content and SOM decomposition status also differed in Al content and Al speciation, indicating the existence of spatially separated forest floor constituents with different Al speciation. Aluminum in the mineral-dominated density fraction > 1.6 g cm-3 with advanced SOM decomposition, comprising about 50 % of total Oa mass, was entirely bound in clay minerals (65-70 % of total Al) and feldspars (30-35 % of total Al). In contrast, Al in the mineral-poor density fraction < 1.0 g cm-3 was entirely bound to SOM as Al-organo complex, with Al bound to SOM phenol groups (>75 % of total Al) being more relevant than Al bound to SOM carboxyl(ate) groups. However, the latter density fraction comprised only 4 % of total Oa mass in both soils. Aluminum K-edge XANES spectroscopy, particularly when combined with total element analysis and XRD, is a promising novel tool for speciation of Al in soils and in SOM-mineral associations, with a great potential to promote our understanding of Al biogeochemistry in terrestrial ecosystems.

A table-top near-edge X-ray absorption fine structure (NEXAFS) spectroscopy system consisting of a soft X-ray source and an integrated spectrometer with a significantly improved resolution is presented. The soft X-ray source is based on a long-term stable and nearly debris-free picosecond laser-induced plasma generated in a pulsed krypton gas jet target. Photon energies ranging from 250 to 1000 eV can be used for the absorption spectroscopy of thin samples. The newly designed spectrometer accomplishes a spectral resolution of E/ΔE = 1535 at 430 eV, being close to typical synchrotron setups. Moreover, a simultaneous multi-edge analysis is possible. The performance of the new system is demonstrated by investigating the fine structure of the K- and L-absorption edges of various elements (carbon, calcium, oxygen, iron, nickel, and copper) for different types of samples. An excellent agreement with synchrotron spectra is achieved.

Understanding of ongoing biogeochemical processes (natural attenuation) within contaminated soils is crucial for the development of plausible remediation strategies. We studied a tar oil contaminated soil with weak grass vegetation at a former manufactured gas plant site in Germany. Despite of the apparent toxicity (the soil contained up to 120 g kg-1 petroleum hydrocarbons, 26 g kg-1 toxic metals, and 100 mg kg-1 polycyclic aromatic hydrocarbons), the contaminated layers have 3-5 times as much cell counts as an uncontaminated control soil nearby. To test, if the geometry of the pore space provides favourable living space for microorganisms, we applied scanning electron microscopy to the thin sections and calculated on sets of 15 images per layer three specific Minkowski functionals, connected to soil total porosity, interface, and hydraulic parameters.Our investigation showed that the uncontaminated control soil has a relatively low porosity of 15-20 %, of which 50-70 % is comprised of small (< 15 µm) pores. These pores are poorly connected and show high distances between them (mean distance to the next pore 10 µm). The dominating habitats in the control soil are therefore created by small pores. They provide good protection from predators and desiccation, but input of dissolved organic C and removal of metabolic products are diffusion limited. Coarser pores (>15 µm) provide less space (< 50 % of total porosity) and solid surface area (< 20 %), are prone to desiccation and offer less protection from predators. However, they serve as preferential flow paths for the soil solution (input of nutrients) and are well aerated, therefore we expect the microbial activity in them to appear in “hot moments”, i.e. after rain events.All layers of the contaminated profile have higher porosities (20-70 %) than the control. Coarse pores comprise 83-90 % of total pore area and create 34-52 % of total interface. Pores are also more connected and tortuous than in the control soil, which implies a better aeration and circulation of soil solution. The loops of pore channels may retain soil solution and be therefore preferably populated with microorganisms. The small (< 15 µm) pores comprise less than 17 % of total porosity but represent a substantial proportion of the interface (48-66 % vs 82-91 % in control). In the uppermost layer of the contaminated profile, such pores occur in plant residues, are close to the largest pores (mean distance to the next pore 4 µm) and therefore, along with good protection, are supplied with air, water, and non-tar C. In the middle of the profile, the small pores, presumably constantly filled with water, are located within dense tar pieces remote from the neighbouring pores (mean distance to the next pore 22 µm), and therefore, with hindered aeration and no supply of non-tar C, may create anaerobic domains of tar attenuation.Our results show that the contaminated soil offers more favourable conditions for microorganisms than the control soil, probably because the hydrocarbons provide suitable energy and nutrition sources and a beneficial pore space geometry.

The development of effective remediation strategies for soils contaminated by aged non-aqueous phase liquids like tars requires detailed investigation of composition, microstructure and microbial communities. We studied an aged tar spill with an overgrowing grass vegetation at a former manufactured gas plant site in Germany. The soil contained 10-120 g kg-1 petroleum hydrocarbons, up to 26 g kg-1 potentially toxic metals, and up to 100 mg kg-1 polycyclic aromatic hydrocarbons. Although these substances are considered toxic and recalcitrant, the microbial biomass was up to twice as much in contaminated layers than in uncontaminated layers of the control soil. We assume the high content of vital elements, such as C (up to 500 g kg-1), S (5 g kg-1), P (4.8 g kg-1), Fe (65 g kg-1), and N in plant residues, compensates possible toxicity. Investigation of the 2D soil microstructure on thin sections with digital light and scanning electron microscopy showed increased total porosity (2-3 times more than in control) and the share of coarse wide pores (> 50 µm, root channels and large cracks) in contaminated layers. Within the root channels aerobic conditions persist, with free inflow of soil solution and supply of root exudates. Tar dominated particles between the coarse pores had small isolated pores, and the average distance to the next pore within the particles (assessed by Euclidian distance) was about 3 times higher than for the control soil. This highlights anaerobic conditions within the pores, where tar borne compounds are the source of nutrition and energy. FTIR microspectroscopy showed oxidized tar on root coatings and near some isolated pores. Natural attenuation of the contaminant proceeds both under aerobic and anaerobic conditions. Positive matrix factorization analysis of EDX spectra allowed us to map the spatial distribution of different components (quartz, feldspars, secondary minerals, metal-rich particles, tar and the embedding resin). We found presumably authigenic Fe minerals within small isolated pores and along root channels. Based on XANES spectroscopy and the difference between total Fe and Fe in Fe oxides (FeDCB), they contained Fe2+ and Fe3+ in different proportions, which suggests Fe reduction to be an accompanying process during tar attenuation. The 16S rRNA analysis showed similar microbial communities on the rooted rim of the spill and the control soil. The community in the centre of the spill was less diverse and remarkably different. The contaminated profiles contained specific functional groups of bacteria (e.g. Fe-reducing Geobacteraceae or N-fixing Rhizobiales). Microfluidic droplet cultivation facilitated abundant microbial growth from tar layers under both aerobic and anaerobic conditions. We conclude that aged tar is used as a substrate by the microbial communities, especially in the presence of grass vegetation. Natural attenuation of tar occurs in hotspots under either oxic (root channels and large connected voids) and anoxic (small isolated pores) conditions and is coupled with reduction of Fe.

High inputs of easily available organic matter to the subsurface may quickly activate the native microbial communities, thereby changing soil engineering properties. We studied the effect of glucose addition, an easily available carbon source, on stress-strain properties, mineralogy, and microstructure of several loamy and sandy soils over 30 days in laboratory experiments. During the period of high microbial activity, direct shear tests revealed a reduction of the friction angle of 15-30 % and a raise of cohesion in sands. Unconfined compression tests showed a 20-30 % decrease in the compressive strength of loamy soils. With the decline of microbial activity, the stress-strain properties recovered partially. The alterations of the stress-strain properties with increasing microbial activity were linked to changes in mineralogy and composition. X-ray diffraction showed that the proportion of smectite layers in illite-smectite mixed layer minerals increased, as well as the overall imperfection of clay minerals. Glucose addition resulted in a temporary increase in the content of microaggregates (0.1-0.05 mm). Newly formed linkages of organic matter between solid particles, biofilm formation and direct interaction of cells with mineral surfaces were observed by scanning electron microscopy. Our data show that microbial-mediated processes may adversely influence stress-strain properties of soils and endanger the safety of buildings.

The association of organic molecules with mineral surfaces is a major mechanism to stabilize soil organic matter against biodegradation. Strong inner-sphere iron phosphate complexes are of particular importance for the attachment of microorganisms to iron oxides, either by exudation of extracellular polymeric substances or by interactions of cell membrane molecules with the mineral surface. These P-O-Fe bonds are not easily detected because of the difficulty to detect individual bonding mechanisms in a complex matrix of organic matter and minerals in soils. To investigate for these bonds, P K-edge XANES spectroscopy measurements with a spatial resolution < 100 nm on secondary minerals from soils and sediments were performed to determine whether P-O-Fe bonds exist on pedogenic iron oxides and whether C-O-Fe bonds from carboxylic groups are overrated with respect to mineral binding. XRF maps allow to differentiate between clay minerals and Fe oxides and help to locate C-rich and P-rich regions on these minerals. As the spatial distribution of P in soils is heterogeneous (Fig. 1), element specific techniques with a high spatial resolution and a high energy resolution are the ideal tools to resolve such research questions. XRF maps and XANES image stacks at the K-absorption edges of carbon and phosphorus have been measured on sediments using the AnImaX STXM-endstation set up at beamline P04 of PETRA-III at DESY. Results will be presented.

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.

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 7th 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 7th 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.

We used SEM-EDX, X-ray fluorescence imaging, and X-ray absorption spectromicroscopy (STXM-NEXFAS) at the C1s absorption edge to quantify extent and chemical composition of organic coatings on natural Fe oxides and clay minerals from a Podzol Bs and a Gley Bg horizon. For these subsoils we assumed that organic coatings develop by adsorption from dissolved organic matter of the soil solution. We compared these samples with synthetic goethites, that had been in contact with biofilms and with Fe oxides and clay minerals from surface water flocs of a spring, i.e. to samples for which we assume that most of the organic matter is microbially derived.