In Mediterranean regions, olive plantations are commonly located on steep 
slopes, leading to significant erosion. To mitigate soil loss, sustainable management 
practices often involve allowing natural vegetation cover to grow in the inter-tree 
spaces. 

To provide insights into the impact of different management forms on soil quality, we 
collected topsoil samples (0-15 cm) from an olive orchard with an 11% slope located in 
Southern Spain (Benacazón, Seville). Samples were taken along the slope of plots
managed with conventional tillage (CT) and with natural cover (NC). Additionally, soils
from the tree line, treated with herbicide (TL-Herb.), were included. To account for 
seasonality effects, sampling campaigns were conducted in autumn 2022 (following the 
dry season) and spring 2023 (after the rainy season). 

Soil organic matter content will be correlated with the management practices, slope 
location and microorganism abundance, determined through phospholipid fatty acid 
analysis (PLFA). Microbial respiration analysis will be also performed, using MicroResp
to assess microbial activity. The main hypothesis is that plots with vegetation cover will 
have higher SOM and total microbial biomass contents. We also expect an impact of 
slope location not only on the size of the microbial pool but also on the microbial 
biodiversity.

How to cite: Gismero Rodríguez, L., Aguilar-Romero, I., Gómez, J. A., Valverde, Á., and Knicker, H.: The effect of slope on soil organic matter and on microbial communities under different soil management practices., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-627, https://doi.org/10.5194/egusphere-egu24-627, 2024.

Landsliding creates new substrates upon which ecosystems develop. From a recovery perspective, these substrates may limit plant growth due their harsh conditions. Yet, from a soil formation perspective, these substrates may enhance weathering rates due to a combination of abiotic and biotic factors. Central to both is the role that mycorrhiza may play in ecosystem development, including biogeochemical cycles. Given that at mountainscape scales landslide populations are diverse due to variation in underlying climatic and edaphic conditions, we hypothesize that plant-mycorrhizal associations and their functions will mirror this diversity. To test this hypothesis, we focus on the Sierra de Las Minas in Guatemala (SLM), a mountain range that offers contrasting climatic characteristics associated with aspect (south aspect is dry to mesic and north aspect wet) and steep elevation gradients (400 – 2600 m a.s.l.). Leveraging plant and soil inventories conducted in forest and landslide habitats in the SLM we ask 1) How do plant-mycorrhizal associations vary with aspect, elevation, and habitat? 2) What is the contribution of soil chemistry to the observed variation in plant-mycorrhizal associations? And 3) How do plant-mycorrhizal associations and environmental conditions explain variation in weathering rates? To answer these questions, we integrated our plant inventories with a global database on plant-mycorrhizal associations. The former contains species composition and soil elemental analyses that we used to estimate a variety of weathering indices. The latter was used to assign plants from our field inventories to one or more mycorrhizal type.

Plant-mycorrhizal associations were diverse and greatly contributed to the separation of plant communities in multivariate space. Aspect followed by habitat explained a large fraction of the observed variability. Subsets of soil variables correlated with different dimensions derived from principal component analyses suggesting that plant-mycorrhizal associations contribute diverse functions. In general, weathering indexes differed with aspect and habitat. For example, Vogt Residual Index and Chemical Index of Alteration, were higher in landslide than forest and this difference was more pronounced in the wet than dry to mesic aspects. Further analyses will allow us to examine the contribution of plant-mycorrhizal associations to ecosystem development and soil formation in tropical mountainscapes influenced by landslide activity.

How to cite: Restrepo, C. and Ortiz, Y.: Plant-mycorrhizal associations vary with climate and soil in tropical mountainscapes influenced by landslides: Implications for ecosystem development and soil formation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4322, https://doi.org/10.5194/egusphere-egu24-4322, 2024.

High concentrations of Fe-rich colloids have been detected in the anoxic zones of our Slate River floodplain field site (CO, USA) since 2018. We speculated that the composition and abundance of the colloids is controlled by the seasonal dynamics and spatial heterogeneities of the subsurface. Therefore, our goals were to 1) decipher the structure and chemical composition of Fe-rich colloids, 2) identify mechanisms of colloid transformation and 3) understand their biogeochemical function.

TEM analysis revealed nano-spheres and nano-assemblages that consist mainly of Fe, O, Si, and C, with lower contributions from Al, S and Ca. Based on this elemental distribution, we hypothesized that the colloids are composed of Fe minerals that are associated with organic matter and Si. This was further confirmed through Mössbauer spectroscopy and Fe-EXAFS that indicated the colloids consist ferrihydrite associated with organic matter and Si. NanoSIMS imaging detected co-localities of Fe, S, Si, and O, as well as C and N, which demonstrate once more that these are ferrihydrite-based colloids that are embedded in an organic matter-Si matrix.

 These findings are intriguing as they demonstrate high abundance of ferrihydrite-based colloids in anoxic depths of our field site. The stability of the colloids is likely attributed to the coating of organic matter and Si that serves as a protective layer against the reducing conditions. Nevertheless, our data also showed that colloids collected during snowmelt (Spring 2021) contained a higher proportion of Fe(II) than colloids collected during baseflow conditions (Summer 2021). XPS analysis measured higher atomic percentages of C and Si compared to Fe and O in baseflow versus snowmelt colloids, indicating a decrease in the organic-Si protective layer under baseflow conditions, allowing for Fe(II) oxidation and an increase in Fe(III)/ferrihydrite content. The fact that there is an occurrence of S species only in the more reduced snowmelt colloids illustrates the dynamic and delicate composition of these environmental colloids during seasonal changes in hydrology and porewater chemistry.

We are also in the process of interpreting our seasonal data that will shed light on the controls over the seasonal dynamics of the colloids. We have already linked Fe(II) colloid abundance to lower vertical porewater velocities and higher organic matter levels, and are working on additional analyses including data acquired from FT-ICR-MS analysis of the organic composition of the colloids.

How to cite: Engel, M., Noel, V., Pierce, S., Babey, T., Kovarik, L., Kukkadapu, R., Zhao, Q., Chu, R., Boye, K., and Bargar, J.: Seasonal floodplain dynamics control Fe-rich colloid characteristics, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4570, https://doi.org/10.5194/egusphere-egu24-4570, 2024.

Understanding the formation and stability of soil aggregates is crucial for sustaining soil functions. This study investigates the impact of organic matter (OM), pedogenic Fe (oxyhydr)oxides, and aggregate size on aggregate stability in an arable soil. Samples were collected from the Ap and Bt horizons of a Luvisol after 14 years of bare fallow, and results were compared with a control field under permanent cropping. In the Ap horizon, bare fallow led to a 26% reduction in the median diameter of the 53-250 µm size fraction, indicating decreased stability of larger microaggregates. Simultaneously, the mass of the 20-53 µm size fraction increased by 65%, suggesting reduced stability, particularly of larger soil microaggregates, due to the absence of fresh OM input. The ¹⁴carbon (¹⁴C) fraction of modern C (F14C) under bare fallow ranged from 0.50 to 0.90, lower than the cropped site (F14C between 0.75 and 1.01). This difference was most pronounced in the smallest size fraction, indicating the presence of older C. Higher stability and reduced C turnover in <20 µm aggregates were attributed to their elevated content of poorly crystalline Fe (oxy)hydroxides, acting as cementing agents. Colloid transport from the Ap to the Bt horizon was observed under bare fallow treatment, highlighting the release of mobile colloids. This transport may initiate elemental fluxes with potential unknown environmental consequences. In conclusion, the absence of OM input decreased microaggregate stability, releasing mobile colloids and initiating colloid transport.

How to cite: Siebers, N., Voggenreiter, E., Joshi, P., Rethemeyer, J., and Wang, L.: Synergistic relationships between the age of soil organic matter, Fe speciation, and aggregate stability, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5851, https://doi.org/10.5194/egusphere-egu24-5851, 2024.

Perennial bioenergy crops like Silphium perfoliatum (cup plant) are a promising alternative to currently used energy crops such as maize because of their positive feedbacks on various soil properties including carbon sequestration, edaphon activity and erosion control. This study investigates the long-term impact (over 10 years) of the cup plant on soil organic carbon and soil structural parameters in comparisons to a to a nearby ploughed reference site.

We employed tension infiltrometers to measure water infiltration rates at two soil depths (5 and 45 cm). Following this, 100 cm³ aluminum soil cores were extracted for X-ray computed tomography at a resolution of 35 µm. The image analysis, enhanced by machine learning, classified structures including roots, particulate organic matter (POM), biopores, and two types of soil matrix: dense and loose, the latter indicating higher carbon content or more pores slightly below image resolution. The dataset was complimented by the determination of total carbon content and the root length distribution with RhizoVisionExplorer.

The results indicate significant differences in pore structure, primarily in the topsoil, where the cup plant site showed a greater volume of biopores than the reference site. In contrast, the subsoil differences were less marked. Organic carbon content analysis demonstrated a notable increase in the upper soil layer (10-15 cm) at the cup plant site, contributing to a higher soil organic carbon stock than the reference site. However, this effect diminished with depth, becoming negligible at 50-55 cm. In the topsoil, extensive bioturbation/biomixing was observed, as indicated by the darker, more loosely structured soil matrix, which often had the shape of biopores. This bioturbation, which mixed particulate organic matter (POM) into the soil, significantly enhanced soil organic carbon, as evidenced by linear regression analysis.

These findings underscore the substantial impact of the perennial cup plant in enhancing soil structure and carbon content, particularly in the topsoil.

How to cite: Lucas, M., Rohlmann, L., and Deiglmayr, K.: Long-Term Effects of Silphium perfoliatum on Soil Pore Dynamics and Organic Carbon Accumulation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6272, https://doi.org/10.5194/egusphere-egu24-6272, 2024.

Landslides are an important erosion mechanism in mountainous terrain. They strip large quantities of organic material from ecosystems and expose bedrock to weathering. At the west side of New Zealand’s Southern Alps, super-humid climate and slope instabilities create ideal conditions for frequent landslides, allowing studies of soil formation processes in short-term chronosequences. Here, we investigated soils from three landslides that occurred in 2019 (“young”), 1997 (“intermediate”), and 1965±5y (“old”), respectively. Additionally, reference forest soils located outside of landslide areas were sampled at each location. Closed PVC chambers were installed at different positions on landslide surfaces and reference soils, and gas samples were collected for flux analysis. Soil samples were collected at the same positions from the surface and 20 cm depth and investigated for physico-chemical parameters and microbial community composition.

Net CO₂ emissions reached 830 mg h^-1m^-2 at the “old” site in summer and remained below 356 mg h^-1m^-2 in winter. Net N₂O emissions showed a patchy spatial pattern, reaching rates of 0.2 mg h^-1m^-2 at the “old” site in summer. At most locations, N₂O flux was below the detection limit during winter.

Soils were dominated by bacterial phyla Acidobacteria, Bacteroidota, Chloroflexi, Gemmatimonadota, Planctomycetota, Proteobacteria and Verrucomicrobiota. Dominant archaeal phyla comprised Thermoplasmatota and Crenarchaeota. Beta diversity analysis revealed distinct community composition patterns with the “young” site forming a separate cluster from the older landslides and reference soils. Surface- and depth-associated microbial communities showed high similarity at the “young” site, but they became increasingly distinct at the “intermediate”, “old” and reference soil sites. Community composition at the “old” site showed the least difference to reference sites, indicating that ecosystem development rapidly reached a state similar to older mature forests.

In general, the three landslide sites showed a gradient in the development of soil chemical parameters, microbial community composition and soil respiration rates, with the “old” site being closest to reference sites. Soil respiration rates showed strong seasonal dependence and soil temperature sensitivity.

Our results indicate that respiration rates and microbial community composition of landslide soils reach those of older mature forest soils within a few decades after mass wasting events, if no reactivation occurs and soil development can proceed without disturbance.

How to cite: Rasigraf, O., Riedel, A., Bartholomäus, A., Clough, T., Friedl, T., and Wagner, D.: Seasonal greenhouse gas flux and microbial community dynamics along a landslide soil chronosequence at the west side of New Zealand's Southern Alps, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8956, https://doi.org/10.5194/egusphere-egu24-8956, 2024.

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.

How to cite: Eusterhues, K., Thieme, J., Ritschel, T., Ivanov, P., and Totsche, K. U.: Mapping redox conditions in a tar-oil contaminated technosol by S K-edge XANES at the micrometer scale, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9465, https://doi.org/10.5194/egusphere-egu24-9465, 2024.

Floodplains are dynamic ecosystems that cycle carbon, which is both delivered from upstream catchment sources and produced in-situ by pedogenesis. These landforms are being progressively acknowledged as important environments of carbon processing, with the capacity for both substantial carbon sequestration in addition to acting as hotspots of carbon mineralisation. The balance between storage and release is dependent on a number of controls including landscape position, environmental conditions and soil characteristics. This study focuses on three headwater floodplains in a single catchment (approximately 10km in length), downstream of a highly eroded blanket bog peatland in the Peak District, UK. Aged organic carbon of peatland origin has been found in floodplains in this area based on prior research, and therefore we aimed to understand whether the allochthonous carbon was being mineralised in this context. We examined sediment cores and analysed the radiocarbon (¹⁴C) content of CO₂ respired from the floodplain soils using a partitioning approach to scrutinise the depth and age relations of respiration in the individual floodplains and patterns of age distributions downstream. As such, we examined whether soil heterogeneity as a function of distance downstream and within individual floodplain profiles had an impact on age of respired CO₂.

Aged organic carbon was released from the upper and mid floodplain sites (¹⁴C ages of 682 and 232 years BP, respectively), whereas only modern dates were recorded at the lower site. The sedimentology was in accordance with the radiocarbon dates, suggesting primarily allochthonous deposition at the upper sites, but a dominance of in-situ soil development at the lower site. There was no age-depth relationship within individual floodplains, suggesting that the floodplain sediments were well-mixed and that aged organic matter was being processed both at the surface and at depth in the uppermost sites. An isotope mass balance mixing model indicated the control of two sources of CO₂; recently fixed C3 organic matter and CO₂ produced by methanogenesis. The results indicate that floodplains in a relatively narrow halo around eroding headwater peatlands could be important sites of aged carbon turnover originally derived from upstream sources, with those further downstream playing a different role. Reworked carbon does not transfer passively through the system and experiences periods of deposition where it can be subject to microbial action. In areas where organic carbon has previously been ‘locked up’ (e.g., permafrost regions) but is now under the threat of release due to climate change, this is an important consideration.

How to cite: Alderson, D., Evans, M., Garnett, M., and Worrall, F.: Aged carbon mineralisation from headwater peatland floodplains in the Peak District, UK, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11265, https://doi.org/10.5194/egusphere-egu24-11265, 2024.

Manganese (Mn) has been demonstrated to be a significant driver of litter decomposition in forest soils, regulating nutrient cycling, CO₂ production, and ultimately soil carbon storage across forest systems globally. Recent evidence suggests that Mn-driven litter decomposition is dependent on ubiquitous oxic-anoxic interfaces in soils, which act as potential hotspots for the formation of reactive Mn(III) oxidants. Here we will show how oxic-anoxic interfaces in forest soils, arising from spatiotemporal variations in moisture, affect Mn(III)-driven litter decomposition. To do this, we tested the effect of in-field Mn additions on litter decomposition along a deciduous forest upland-to-wetland transect exhibiting dynamics in oxic-anoxic transitions. Within Mn-amended litterbags incubated across the transect, we monitored spatiotemporal variations in Mn(III) formation and litter decomposition. Over the course of the experiment, increased Mn amendments in litter correlated with both enhanced CO₂ production as greater mass loss compared to untreated litter, particularly during periods of greater Mn(II) oxidation. Wet-chemical extractions revealed increasing Mn(II) oxidation over the growing season in elevated Mn treatments, resulting in enhanced Mn(III) formation. Additionally, the greater abundance of Mn(III) phases in Mn treated litter compared to untreated litter was significantly correlated to a greater degree of oxidation in the litter and greater water extractability of litter carbon, demonstrating enhanced litter decomposition in Mn amended litter. Across the forest transect, Mn oxidation and Mn(III) formation were greatest at sites with greater presence of oxic-anoxic transitions, which coincided with the sites exhibiting higher litter decomposition. Therefore, our results show that Mn(III)-mediated litter decomposition occur in hotspots present at oxic-anoxic interfaces across spatiotemporal gradients in forest soils. These findings provide first insights into the spatiotemporal links between Mn and carbon coupled redox cycling in forest ecosystems.

How to cite: Chin, N., Van Der Loo, E., DeAngelis, K., and Keiluweit, M.: Spatiotemporal variations of manganese-mediated litter decompositions across oxic-anoxic transitions in deciduous forest soils, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11474, https://doi.org/10.5194/egusphere-egu24-11474, 2024.

The way microbes interact in nature can vary widely depending on the spatial characteristics they are located in. This aspect of the microbial environment can determine whether processes such as organic matter turnover, community dynamics, or microbial speciation, among others, occur and their impact on soil functions. Investigating how the geometry of microhabitats influences microbes has been traditionally challenging due to methodological limitations. A major challenge in soil microbial ecology is to reveal the mechanisms that allow a wide diversity of microorganisms to co-exist. This study is directed towards answering the question of how spatial complexity affects bacterial competition, and how this can lead to organic matter turnover.

Using microfluidic chips that mimic the inner soil pore physical geometry, and fluorescence microscopy, we followed the effect of an increasing complexity in the growth and substrate degradation of two soil bacterial strains. The parameters used to define complexity were two: the turning angle and order of pore channels, and the fractal order of pore mazes. When we tested the effect of an increasing in turning angle sharpness on microbial growth, we found that in sharper angles, both species coexisted, but only until certain sharpness where both populations decreased. We also found that substrate degradation was highest in the same sharp angles that permitted the coexistence of both strains. Our next series of experiments, testing the effect of maze fractal complexity showed that both strains could coexist and degrade the most substrate in complex mazes that had dead ends as opposed to mazes that were highly connected. Our results demonstrate the relevance of microhabitat complexity in bacterial competition and substrate degradation, showing that complex habitats allow bacterial strains to coexist and perform functions with higher efficiency than in less complex ones.

How to cite: Arellano-Caicedo, C., Fadhel, A., Ohlsson, P., and Hammer, E. C.: The effect of habitat complexity on bacterial competition, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14407, https://doi.org/10.5194/egusphere-egu24-14407, 2024.

A large fraction of organic matter in soils is associated with mineral phases. This association is mediated by manifold processes (reactive transport, drying-rewetting cycles, biofilm formation, digestion of mineral particles, etc.) and ultimately leads to a stabilization of organic matter in soil [1]. Adsorption describes the mechanism for this association at the molecular level. Ultrahigh resolution mass spectrometry like FT-ICR-MS has gained interest in the study of adsorption processes because of its high sensitivity, resolving power and mass accuracy which allow to study the complex organic mixtures present in soils, and therefore provides molecular insight [2]. For that, supernatant composition can be compared before and after adsorption, but this indirect approach is often not sensitive enough to detect the sorption of small amounts of organic matter, i.e., during initial adsorption to pristine mineral surfaces. An alternative approach is to use laser desorption ionization (LDI) to directly analyze the adsorbed molecules on the mineral surfaces. The method theoretically allows laser spot size of ~ 20-50 µm and even allows imaging of thin sections; methodological advances could therefore improve our understanding of soil organic matter and its spatial heterogeneity [3].

We applied LDI-FT-ICR-MS to study the ionization of individual molecules with and without the presence of DOM (SRFA and pine/ beech litter extracts), and measured adsorption isotherms of individual molecular formulas in dissolved organic matter on quartz, illite and goethite after 24h of contact. Analog to organic matrices used in matrix-assisted LDI, detectability of model compounds improved by factor 2.5 – 40 when spiked into a DOM matrix. In case of sinapic acid, presence of DOM shifted the ionization towards the monomer ion [M-H]^- as compared to a mixture of mono-, di- [2M-H]^- and trimer [3M-H]^- species when analyzed in pure form. These results suggest that soil organic matter and its soluble analogs act as suitable matrices in LDI experiments that ensure proper ionization of the mixture as a whole. In a next step, ion abundance data from sorption experiments was used to model the adsorption process of individual molecular formulas by a Langmuir isotherm approach. We derived estimates of sorption capacity and sorption affinity for each molecular formula, and identified the molecular properties explaining differences in both estimates, as well as their differences between mineral phases. Our data highlight the benefits of LDI-FT-ICR-MS for the study of sorption phenomena in soils, and opens perspectives for resolution of spatial heterogeneity in soils.

References

[1] Kleber, M., Bourg, I. C., ... & Nunan, N. (2021): Dynamic interactions at the mineral–organic matter interface. Nature Reviews Earth & Environment 2: 402-421.

[2] Bahureksa, W., Tfaily, M. M., ... & Borch, T. (2021): Soil organic matter characterization by Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS): A critical review of sample preparation, analysis, and data interpretation. Environmental Science & Technology 55: 9637-9656.

[3] Lohse, M., Haag, R., ... & Lechtenfeld, O. J. (2021). Direct imaging of plant metabolites in the rhizosphere using laser desorption ionization ultra-high resolution mass spectrometry. Frontiers in Plant Science 12: 753812.

How to cite: Simon, C. and Lechtenfeld, O.: Adsorption of dissolved organic matter to mineral phases studied by laser desorption ionization mass spectrometry, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15127, https://doi.org/10.5194/egusphere-egu24-15127, 2024.

In the soil, mineral particles, organic matter, and soil organisms are arranged in a complex architecture which can influence organic matter dynamics. Nanoscale secondary ion mass spectrometry (NanoSIMS) provides insights into the microscale arrangement of soil minerals and organic compounds at a resolution of approximately 50 nm. However, current NanoSIMS image processing lacks methods to analyze large datasets automatically and requires manual intervention. We developed a two-step unsupervised clustering method for batch analyses of NanoSIMS images. Our two-step method consists of K-Means clustering as the first step generating around 100 clusters, followed by hierarchical agglomerative clustering (HAC) re-grouping the K clusters into less than 10 cluster groups. The elbow method, HAC linkage method and gap statistics are used to automatically select the optimal clustering numbers. Subsequently, soil minerals and organic matter can be spatially segmented and identified as different species. Further, this method could apply to the spatial arrangement of mineral-dominated and organic matter-dominated parts or different mineral types. Moreover, this method enables the analyses of larger datasets with >100 NanoSIMS images providing insights of organo-mineral interactive hotspots or even micro function domains involved in soil organic matter dynamics.

How to cite: Hu, Y., Zollner, J., Werner, M., Schweizer, S., and Höschen, C.: Two-step unsupervised clustering method on soil nanoscale secondary ion mass spectrometry (NanoSIMS) images to determine the spatial arrangement of minerals and organic matter, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16904, https://doi.org/10.5194/egusphere-egu24-16904, 2024.

Plant derived photosynthates are a component of the biotic organic matter involved in carbon cycling and storage due their mobility and formation of organo-mineral associations and (micro-)aggregates. Such photosynthates are typically found as components of mucilage or generate during the germination of seeds in the soil environments. While the effect of, e.g., mucilage, on soil physical properties has been intensively investigated, their role for carbon transport, adsorption, and aggregation process is rather unclear. Most of the knowledge available originates from studies that used single compounds, e.g., oxalic acid, glucose, polygalacturonic acid or mixtures of those in aqueous solutions. In our study, we utilized hydrogel-freed plant exuded photosynthates (PDE) extracted from four different seed types (Linum usitatissimum, Plantago ovata, Ocimum basilicum and Salvia hispanica) to investigate their interactions with minerals typically for temperate soil. In batch experiments, PDE adsorbed to both illite and goethite minerals with a preference of polysaccharide-rich PDE to goethite. During the adsorption, organo-mineral associations were formed leaving behind a less mineral-affine fraction of PDE prone to transport or degradation. Hence, PDE transport was also studied in column experiments using quartz functionalized with reactive minerals where we measured the breakthrough of PDE exploiting their distinct fluorescence. Furthermore, we followed the gravity-constrained aggregation dynamics of PDE-mineral associations using tensiometric measurements. We showed that low PDE concentrations facilitate aggregation via polymer bridges leading to a rapid sedimentation of aggregates while PDE available in excess prevents aggregation via steric repulsion and thus decelerates sedimentation. These results showed that PDE extracted from seeds might serve as better surrogates of natural plant exudates to study their role for the formation and transport of organo-mineral associations and aggregates in soils.

How to cite: Guhra, T., Schönherr, J., and Totsche, K. U.: Plant-derived organic exudates in soil: Mobility and aggregation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17573, https://doi.org/10.5194/egusphere-egu24-17573, 2024.

Nitrate (NO₃^-) is one of the most serious contaminants in groundwater, frequently deteriorating water quality and groundwater use as drinking water. Nitrate can, at specific environmental conditions, be removed within the subsurface environment via natural biogeochemical processes, such as denitrification and dissimilatory nitrate reduction to ammonium (DNRA). Traditionally, research has concentrated on the NO₃^- attenuation potential and microbial activity in groundwater, largely overlooking an aquifers’ sediment matrix as a reaction contributor. We hypothesize that the sedimentary deposits host the major potential for NO₃^- reduction, carrying the majority of microorganisms as well as different sources of electron donors. Moreover, diverse hydrogeological settings harbor different sources and varying amounts of electron donors such as reduced iron minerals and organic matter. Therefore, subsurface physicochemical heterogeneity, determined by sedimentology, controls the potential of an aquifer to reduce nitrate. We hypothesize that locations where physicochemical conditions favor NO₃^- reduction, we may expect microbes involved in these processes to be more abundant and highly active, leading to a fast NO₃^- removal from groundwater.

Here, we present results from temperature-controlled (12°C) batch incubations and flow-through experiments conducted over several months using fresh sediments from an alluvial, tufaceous unconfined aquifer, with embedded peat layers, in an agricultural landscape in southwest Germany. Batch experiments received an addition of NO₃^- of 50 mg L^–1. Sediment columns were supplied with nitrate-spiked groundwater collected from the site at a maximum inlet concentration of 150 mg L^– and run in continuous injection mode. Batch experiments showed a gradual decrease in the initially high hydrogen sulfide (H₂S) concentration, becoming undetectable after Day 7, compared to an untreated control, which revealed a slow conversion of HS^-. These preliminary findings indicate a strong autotrophic denitrification with NO₃^- reduction coupled to aqueous HS^- oxidation. In contrast to nitrate, NH₄⁺ concentrations remained stable in all experiments. In the column experiments, the sediment exhibited a substantial NO₃^- reduction capacity in the early phase but rapidly declined with time. We hypothesize that a dual contribution to denitrification via easily bioavailable electron donors in the groundwater followed by slower denitrification coupled to organic matter in the sediment matrix is responsible for the observed dynamics. Our ongoing experiments, including groundwater and sediments from other, hydrogeologically different aquifers, will dissect the individual redox processes and key microorganisms involved in turnover of prominent nitrate and carbon species in the context of different hydrogeological settings. 

How to cite: Pide, J. L., Holdt, J., Cantarella, V. P., Mellage, A., Cirpka, O., Leven-Pfister, C., Duda, J.-P., Buchner, D., and Griebler, C.: Intrinsic potential and activity of nitrate turnover examined for different hydrogeological aquifer settings, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18568, https://doi.org/10.5194/egusphere-egu24-18568, 2024.