Microbial composition and functions in the rhizosphere – an important microbial hotspot – are among the most fascinating yet elusive topics in microbial ecology. Based on the similarity of rhizosphere properties with respect to carbon availability and nutrient depletion, we hypothesized that (i) rhizobacterial populations are recruited from the bulk soil, but are preselected by excess released root carbon, so that bacterial diversity is lower in the rhizosphere and bacterial networks are less stable, (ii) the rhizosphere is home to more abundant copiotrophic bacteria than the bulk soil, and iii) the functional capacity involved in the carbon and nitrogen transformation would be greater in the rhizosphere.

We used 557 pairs of published 16S rDNA amplicon sequences from the bulk soils and rhizosphere in natural and agricultural ecosystems (forests, grasslands, croplands) around the world to generalize bacterial characteristics with respect to community diversity, composition, and functions.

The rhizosphere selects microorganisms from bulk soil to function as a seed bank, reducing microbial diversity. The rhizosphere is enriched in Bacteroidetes, Proteobacteria, and other copiotrophs. Highly modular but unstable bacterial networks in the rhizosphere (common for r-strategists) reflect the interactions and adaptations of microorganisms to dynamic conditions. Dormancy strategies in the rhizosphere are dominated by toxin–antitoxin systems, while sporulation is common in bulk soils. Functional predictions showed that genes involved in organic compound conversion, nitrogen fixation, and denitrification were strongly enriched in the rhizosphere (11–182%), while genes involved in nitrification were strongly depleted. Thus, rhizosphere is the most powerful factor shaping the composition, structure and functions of the soil microbiome and of biogenic element’s cycling.

Reference

Ling N, Wang T, Kuzyakov Y 2022. Rhizosphere bacteriome structure and functions. Nature Communications 13, 836. https://doi.org/10.1038/s41467-022-28448-9

How to cite: Kuzyakov, Y., Ling, N., and Wang, T.: Microbiome of rhizosphere: from structure and functions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6404, https://doi.org/10.5194/egusphere-egu23-6404, 2023.

Microorganisms possess the ability to adapt to different environmental conditions through the use of various strategies. This diversity in strategies allows us to categorize them based on their functions in the ecosystem. Copiotrophs have a fast growth rate but a low carbon use efficiency (CUE), while oligotrophs have a slow growth rate but a high CUE. In the rhizosphere, the effect of root exudation on different functional microbial groups is not well understood. Process-based modeling is a useful tool to analyze the complex feedback between roots and soil in the rhizosphere. Here, we present a rhizosphere model that explicitly considers two different microbial groups (oligotrophs and copiotrophs) classified based on their microbial traits that correlates each other due to physiological trade-offs and organic carbon accessibility (dissolved organic carbon, mucilage and sorbed carbon). The model is one-dimensional axisymmetric, simulating a soil cylinder around individual root segments. The model was conditioned using a novel constraint-based Markov chain Monte Carlo parameter sampling method. Applying this approach enabled the identification of parameter sets that led to plausible model results in agreement with experimental findings from a comprehensive literature review. The conditioned model predicts organic matter concentration curves from the root surface into the soil driven by root exudation. Our simulations show a decreasing pattern of dissolved organic carbon, which is utilized by oligotrophs and copiotrophs, away from the root surface. Furthermore, we observe a slightly higher proportion of copiotrophs than oligotrophs near the root surface and dominance of copiotrophic biomass at very high nutrient availability conditions as expected from ecological theory and experimental evidence. However, the model predictions are still highly uncertain. Thus, further experimental data and observations are required for model conditioning.

How to cite: Sırcan, A., Streck, T., Schnepf, A., and Pagel, H.: Traitbased modeling of microbial distribution and carbon turnover in the rhizosphere, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12829, https://doi.org/10.5194/egusphere-egu23-12829, 2023.

The spatial arrangement of the soil surrounding the root can improve plant resource acquisition under drought and is closely related to the fate of soil organic carbon (SOC). Thus, the formation of soil structure and the establishment of a stable rhizosheath can potentially improve plant drought resistance and contribute to maintained crop yields during drought events. Yet, soil structure formation is a complex process determined by the interaction between various functional plant and soil properties, such as the soil (micro)biome, root exudation, or root morphological characteristics. To date, it is not understood how water scarcity affects soil aggregation in the vicinity of roots, by which functional traits these drought effects can be modified, and how this feedbacks on the cycling of SOC. 

Thus, we investigated drought effects on rhizosheath properties and their link with functional plant traits. We conducted a greenhouse experiment with 38 maize varieties where half of the plants were grown under optimum moisture, while the second half of replicates were subjected to drought stress after an initial establishment phase. For each plant, the rhizosheath soil was sampled and its aggregate size distribution, carbon (C) and nitrogen (N) content, and the proportion of newly maize-derived C were analysed via natural abundance ¹³C. In addition, we recorded functional plant and rhizosphere traits, such as morphological and chemical root properties, microbial enzyme activities, and plant biomass.

Drought-stressed plants formed lower amounts of rhizosheath, with a decreased physical aggregate stability and increased concentrations of SOC, N, and newly maize-derived C. Furthermore, under drought larger proportions of the elements were allocated into the microaggregate fractions. In particular, maize-derived C, along with N, accumulated under drought stress in the smaller aggregate size classes of the rhizosheath. Maize varieties forming larger amounts of roots under drought stress tended to maintain higher macroaggregate stability in the rhizosheath. In contrast, cultivars that invested little in root growth but promoted higher microbial enzyme activities in the rhizosheath and maintained root N contents under drought were associated with a strong accumulation of maize-C and N in the smaller aggregate size classes. 

Trait-based experimental approaches, such as the one presented here, are deepening our mechanistic understanding of drought effects in the crop rhizosheath and can thus help to guide future crop selection for improved drought resistance.

How to cite: Steiner, F., Wild, A. J., Tyborski, N., Tung, S.-Y., Köhler, T., Buegger, F., Carminati, A., Eder, B., Groth, J., Hesse, B. D., Pausch, J., Lüders, T., Vahl, W., Wolfrum, S., Mueller, C. W., and Vidal, A.: Functional traits of Zea mays L. varieties determine drought effects on soil structure and carbon allocation in the rhizosheath, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8238, https://doi.org/10.5194/egusphere-egu23-8238, 2023.

Climate change scenarios forecast increasing droughts in large areas globally with significant effects on food production. Nutrient availability is an imperative factor for plant growth and it is greatly modulated by water availability. Nitrogen (N) availability extensively constrains plant growth in most terrestrial ecosystems especially in sub-Saharan Africa, where soils are unfertile and often degraded. How rhizosphere traits at the plant soil-interface affect N uptake in response to drought in N poor tropical soils remains elusive. We used ¹⁵N, and ¹³C pulse labelling to trace and quantify N transport from a root-restricted compartment by AMF across an air gap to the host plants coupled with quantifying the allocation of carbon to below-ground pools. Three sorghum genotypes were grown under optimal and water deficit conditions. By tracer analysis in the plant tissues, we assessed that drought enlarged uptake and delivery ¹⁵N by arbuscular mycorrhizal fungi (AMF) from the root restricted compartment across the air gap to the host plant. In addition, drought induced enhanced below-ground incorporation of recently assimilated carbon (C) into the microbial biomass pool both in rhizo-hyphosphere and hyphosphere. Enzyme assays revealed that whereas potential enzymatic reaction (Vmax) of chitinase was reduced under drought, that of leucine amino peptidase (LAP) was upregulated by water scarcity suggesting that N input from protein mineralization was relatively enhanced to that of chitin following moisture limitation. Michaelis-Menten constant (Km) of LAP strongly increased by drought compared to that of chitinase which displayed genotype-specific shifts in rhizosphere enzyme systems. We conclude that in addition to AMF symbiosis, enzyme regulation and enhanced belowground C allocation are key strategies to enhance nitrogen uptake under adverse conditions of resource limitations.

How to cite: Munene, R., Mustafa, O., Loftus, S., Ahmed, M., and Dippold, M.: Drought increases the relative contribution of mycorrhiza-mediated mineral N uptake of Sorghum bicolor, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14382, https://doi.org/10.5194/egusphere-egu23-14382, 2023.

Vegetation patchiness is hypothesized to affect the spatial heterogeneity of resources and soil nutrient distribution in drylands. Nutrient accumulation under perennial vegetative patches leads to faster nitrogen (N) cycling in times of water availability. Compared to perennials, annual plant patches have a shorter life cycle, and labile nutrient buildup can occur more quickly in these patches due to faster nutrient turnover rates of litterfall and root death. The buildup of these labile nutrient pools, in surface soils under annual plant patches over time, may indirectly facilitate succession by other plants, thus aiding in the establishment of fertility islands. To understand how the establishment of annual plant patches affects soil nutrient dynamics, we planted replicated patches of a widespread local summer annual plant, saltwort (Salsola inermis Forssk.), and assessed how these patches influence the soil N cycle and soil N oxides (N₂O and NO) emissions. We also assessed rates of surface litter decomposition of the saltwort plant. We found that rates of soil N transformations and soil N oxides emissions were highest under the plant patch, while they decreased across the patch-to-bare-soil gradient. Water extractable organic carbon (WEOC) accumulation increased in the surface soil beneath the plants and was associated with a large burst in soil N oxides emissions within the patch, following dry soil wetting by the first winter rains. Soil N₂O emission pulse increased by 5.2 folds, whiles NO increased by 95.8 folds. Each N-oxide gas, however, had a different post-wetting pattern, with N₂O peaking a few hours after wetting and NO after one day. We measured a second pulse in soil N oxide emissions after the third rain event. This pulse occurred only with the plant patch and not outside the patch and was reduced by 54% and 31% for N₂O and NO respectively. However, the temporal (peaking) pattern of the second N-oxides pulse was similar to that of the first pulse. Suggesting a reduction in substrate availability as a cause of the reduced pulse. We also found 43% mass loss from the plant litter after 12 months of decomposition. Together, these results suggest that the establishment of saltwort plants affects soil nutrient dynamics and accumulation, thus creating nutrient-rich microsites for potential succession by other annuals and perennials, leading to fertility island establishment in the Negev Desert ecosystem.

How to cite: Yagle, I., Segoli, M., and Gelfand, I.: Patch establishment of the summer annual saltwort plant (Salsola inermis Forssk.) increases N cycling rates and soil N-oxide emissions in Israel’s Negev Desert, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3645, https://doi.org/10.5194/egusphere-egu23-3645, 2023.

The role of biochar-microbe interaction in plant rhizosphere mediating soil-borne disease suppression has been poorly understood for plant health in field conditions. Chinese ginseng (Panax ginseng C. A. Meyer) is widely cultivated in Alfisols across Northeast China, being often stressed severely by pathogenic diseases. In this study, topsoil of a continuously cropped ginseng farm was amended at 20 t ha^-1 respectively with manure biochar (PB), wood biochar (WB) and maize residue biochar (MB) in comparison to conventional manure compost (MC). Post-amendment changes in edaphic properties of bulk topsoil and the rhizosphere, in root growth and quality and in disease incidence were examined with field observations and physicochemical, molecular and biochemical assays. Three years following amendment, increases over MC in root biomass was parallel to the overall fertility improvement, being greater with MB and WB than with PB. Differently, survival rate of ginseng plants increased insignificantly with PB but significantly with WB (14%) and MB (21%) while ginseng root quality unchanged with WB but improved with PB (32%) and MB (56%). For the rhizosphere at harvest following three years growing, total content of phenolic acids from root exudate decreased by 56%, 35% and 45% with PB, WB and MB respectively over MC. For rhizosphere microbiome, total fungal and bacterial abundance was both unchanged under WB but significantly increased under MB (by 200% and 38%), respectively over MC. At phyla level, abundances of arbuscular mycorrhizal and Bryobacter as potentially beneficial microbes was elevated while those of Fusarium and Ilyonectria as potentially pathogenic microbes reduced, with WB and MB over MC. Moreover, rhizosphere fungal network complexity was enhanced insignificantly under PB but significantly under WB moderately and MB greatly, over MC. Overall, maize biochar exerted great impact rather on rhizosphere microbial community composition and networking of functional groups, particularly of fungi, and thus plant defense than on soil fertility and root growth.

How to cite: Liu, C. and Pan, G.: More microbial manipulation and plant defense than soil fertility for biochar in food production: A field experiment of replanted ginseng with different biochars, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4105, https://doi.org/10.5194/egusphere-egu23-4105, 2023.

The presence of oxygen in soil controls the occurrence and rates of biogeochemical processes underlying soil nutrient transformations and greenhouse gas dynamics.  Oxygen (O₂) levels within the rhizosphere are heavily modulated by both root and microbial respiration.  Thus, a microscopic environment near the root may be a microbial hotspot and not well represented by broader, non-rhizosphere soil.  Here we examine the millimeter-scale oxygen consumption or loss processes in the rhizosphere of sorghum, how they are influenced by irrigation practices, and the relationship between oxygen dynamics and nitrification in the rhizosphere.

In a field study at a research farm at the University of Illinois Urbana-Champaign, sorghum was grown under a rainout shelter with plants undergoing one of 2 irrigation treatments.  Soil O₂ concentration and isotopic ratios, nitrous oxide (N₂O), and carbon dioxide (CO₂) were measured in the rhizosphere of the sorghum via an array of novel microvolume probes coupled to an Aerodyne TILDAS (Tunable Infrared Laser Direct Absorption Spectrometer).  Probes were placed within the rhizosphere or outside the root zone with the aid of root windows installed at the site.

We collected continuous, real-time, in situ measurements of O₂, O₂ isotopes, CO₂ and N₂O over the 2022 sorghum growing season.  The high spatial and temporal resolution of the measurements allowed us to observe spatiotemporal heterogeneity of biogeochemical activity in the rhizosphere as a function of agricultural activity.  

We will also report on controlled laboratory incubations to quantify the impact of soil microbial oxygen consumption on ¹⁸O enrichment as compared to water displacement in the soil; and controlled greenhouse experiments to measure fine scale gradients of oxygen concentrations and isotopic composition near roots.

The novel microvolume sampling system coupled with the O₂ detection method can provide insights into fine scale gradients driven by higher microbial activity in microbial hotspots within the rhizosphere.  Measurements on this mm-scale have further applications for monitoring other trace soil gases and their spatial and temporal heterogeneity in soil systems. 

How to cite: Shorter, J., Roscioli, J. R., Lunny, E., Eddy, W., and Yang, W.: Exploring Real-time Oxygen Dynamics in the Rhizosphere of Sorghum with High Spatial and Temporal Resolution, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9746, https://doi.org/10.5194/egusphere-egu23-9746, 2023.

Hydroxyapatite nanoparticles (nHAs) deriving from by-products have gained increasing interest as novel phosphorus (P)-based fertilisers, since they can provide a slow P release, minimising P losses and adverse environmental side-effects, and reducing the dependency of agriculture on mineral fertiliser inputs. Phosphate solubilising bacteria (PSB) have proven to release P available for crop uptake from different inorganic sources (e.g. tricalcium phosphate, TCP, hydroxyapatite, HA). In the present study, nHAs were prepared from salmon (S-nHAs) and tuna (T-nHAs) bones by a calcination process, followed by a high energy ball milling. The obtained fine powders were characterised by scanning electron microscopy (SEM) for size and shape and by X-ray diffraction (XRD) for crystal phase composition. The phosphate solubilisation activity of seven selected PSB strains belonging to Pseudomonas and Paraburkholderia genera was in vitro investigated under acidic (pH = 5.5) and alkaline (pH = 7.5) conditions by a quantitative assessment of the solubilised PO₄^3- from TCP, S-nHAs and T-nHAs over time. Moreover, time trend of pH and organic acids in the liquid media were investigated. Characterization of S-nHAs by XRD and SEM revealed a biphasic composition of the material consisting of TCP and HA – about 50 wt% of each phase - and a heterogeneous rounded-shape (Ø < 50 nm) material. By contrast, XRD pattern of T-nHAs showed a single-phase composition mainly made of pure HA (> 95 wt%) and SEM micrographs exhibiting an elongated shape uniform in size (200 x 30 nm). At day seven, Pseudomonas graminis PG0319 solubilised the highest proportion of the total PO₄^3- in the TCP substrate under acidic pH (83%), followed by Pseudomonas rhodesiae PR0393 and P. graminis PG1211 (79% and 72%, respectively). In S-nHAs under alkaline pH, Paraburkholderia terricola PT0405, PR0393, PG0319 and PG1211 solubilised from 53% to 57% of the total PO₄^3-, whereas in T-nHAs under acidic pH the maximum solubilisation efficiency was 27% by PT0405 at day seven. The difference in the solubilisation of S-nHAs and T-nHAs is due to the lower solubility of HA in comparison with TCP. Values of pH in in the liquid media decreased over the time along with an increasing PO₄^3- solubilisation activity, suggesting an extracellular secretion of organic acids by PSB. Accordingly, differential patterns of organic acids were detected among strains with TCP as well as S-nHAs and T-nHAs. Notably, gluconic, propionic, fumaric and acetic acids played key roles during P solubilisation with all the tested strains, substrates, and pH conditions. Our results indicate that the use of microbial inocula together with P-based nanofertilisers is a promising option for a sustainable agricultural transition.

How to cite: Quattrocelli, P., Pellegrino, E., Piccirillo, C., Pullar, R. C., and Ercoli, L.: Designing of novel hydroxyapatite nanoparticles from fish by-products to be coupled with highly efficient phosphate solubilising bacteria, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7293, https://doi.org/10.5194/egusphere-egu23-7293, 2023.

Phosphorus (P) availability to rice plants is influenced by its strong interaction with iron (Fe). In the rhizosphere microenvironment, the soil-plant interactions cause the formation of Fe-plaques that can retain porewater components, such as P. The Fe-P processes have been extensively described in paddy soils managed under continuous flooding, although, due to the increasing water scarcity, new water-saving techniques have been adopted. However, their effects on P retention/release mechanisms are largely unknown.  

In order to assess the impacts of water-saving techniques on the rhizosphere Fe-P dynamics and P availability to rice, a macrocosm experiment was conducted to compare the effects of three different water management practices: continuous water flooding (WFL), alternated wet and dry (AWD), and delayed flooding (DFL). Three P fertilization levels were tested for each water management strategy. The concentrations of Fe and P in porewater were monitored until rice harvesting. The plant tissues were analyzed for P concentration, and the content of amorphous and crystalline Fe (hydr)oxides in root plaque was estimated via oxalate and dithionite extractions at mid-tillering, stem elongation, heading and harvesting.

The molar P/Fe ratio in porewater and the formation of Fe plaques differed as a result of the combined effect of water management and P fertilization.  The WFL and DFL treatments led to a higher Fe plaque formation with respect to AWD, while in all water management treatments, Fe plaque formation was higher without P fertilization. The early rice development stages were characterized by a greater amount of amorphous Fe (hydr)oxides in root plaques. The proportion of crystalline Fe (hydr)oxides increased with plant development, despite the lower amount of total Fe plaques, suggesting a reduction of the poorly ordered fraction, especially when no P was supplied. Rice plants could be supposed to respond to P-limited conditions, exuding protons and/or organic acid anions that increase P availability through Fe plaque dissolution. This was confirmed by the negative correlation between porewater P concentration and the content of crystalline Fe in the plaques. These results indicate the complex spatio-temporal interconnection between P and Fe cycling at the root-soil interface. The amount of Fe plaques formed on the root surface and their crystallinity degree can explain the mechanisms that regulate their potential in P retention/release and the consequent effects on plant uptake.

This study was funded by the PSR Lombardia 2014-2020 (“P-rice Fosforo in risaia: equilibrio tra produttività e ambiente nell'ottica delle nuove pratiche agronomiche”)

How to cite: Celi, L., Martinengo, S., Schiavon, M., Romani, M., Said-Pullicino, D., Seyfferth, A., and Martin, M.: The role of water management technologies in regulating iron-phosphorus interaction in rice rhizosphere, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16135, https://doi.org/10.5194/egusphere-egu23-16135, 2023.

Background. High plant diversity is known to increase carbon inputs to soils, impact soil microbial community composition and promote soil microbial activity. Large pores are likely to hold more roots residues, provide more efficient oxygen supply, and have more dissolved nutrients and carbon carried by water fluxes. Soil pore structure also impacts the activities of soil microbial communities. The aim of this study was to investigate the effects of 1) plant systems, representing a 9-year gradient of plant diversity (no plants, monoculture switchgrass (Panicum virgatum L.), and high diversity prairie), 2) soil pore size (small (4-10 µm Ø) and large (30-150 µm Ø)), and 3) incubation time (24 hr (short-term) and 30 days (long-term)) on the microbial communities involved in the utilization of a newly added carbon (glucose). This is the first work to explore the influence of soil micro-habitat, as presented by pores of different sizes ranges, on the microbial communities’ responses to new carbon inputs.

Methods. The intact soil cores (5 cm Ø) from the three systems were supplied with either 50 μM C g^-1 soil of ¹³C labeled glucose, unlabeled glucose, or no glucose. Glucose was added to small or large pores based on matrix potential approach. After 24 hr or 30 day incubations stable isotope probing (SIP) was used to identify the phylotypes actively responsible for glucose assimilation in the small and large pore micro-habitats. Both extracted DNA and the fractions separated by SIP were subject to 16S rRNA gene sequencing. PICRUST2 was used to predict the microbial functions of the sequencing data from KEGG orthologs.

Results. The overall microbial communities were affected by multiple years of contrasting vegetation, but not by pore sizes or incubation times. Pseudomonas (Proteobacteria) played an important role in carbon uptake from glucose in all short-term incubations and in the long-term incubations within large pores. In the long-term incubations of both switchgrass and prairie systems’ soils, the community compositions of carbon consumers acting within the small and large pore micro-habitats differed and could be linked to disparate carbon assimilation strategies (r- vs. K-strategists) and to disparate carbon acquisition ecological strategies (plant polymer decomposers, microbial necromass decomposers, predators, and passive consumers). The predicted enriched functional genes indicated the dominance of glucokinase in the soil of the prairie, but not switchgrass system, suggesting a competitive advantage for consuming glucose.

How to cite: Li, Z., Cupples, A., Guber, A., and Kravchenko, A.: Soil Microorganisms Involved in Glucose Assimilation in Small and Large Pore Micro-habitats of Different Plant Systems, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9613, https://doi.org/10.5194/egusphere-egu23-9613, 2023.

Agricultural soils are increasingly polluted by antibiotics, and this makes them a source of antimicrobial resistance (AMR). However, antibiotics may also change microbial communities in soils, and so alter microbiological processes. Given the knowledge gap on how antibiotics affect soil functioning, in particular soil organic carbon (C) cycling, we conducted an experiment to investigate how different concentrations of the model antibiotic sulfamethoxazole (SMX) alter soil heterotrophic respiration (C mineralization) and priming of soil C.

  We collected Austrian soils rich and poor in soil organic C from Seibersdorf and Grabenegg, respectively. After the samples were sieved at 2 mm, we incubated 80 g of soil in 100 mL jars at room temperature for 30 days. SMX was added at day 1, at six rates (0; 0.01; 0.1; 1; 10 and 100 mg.kg^-1) in water solution. Soil moisture was kept constant at 45% of the soil water-filled pore space throughout the incubation experiment. The flux of CO₂ and isotopic composition of C in respired CO₂ were determined with a Picarro 2201-i laser isotope analyzer using Keeling plots.

In general, SMX negatively affected the CO₂ production rate. The negative effect was larger with a higher SMX concentration. The inhibitory effect of SMX followed a logarithmic function, after excluding outliers. The fitted equations may be used to predict how much the microbial activity is inhibited if the concentration of SMX in soil is known. However, it was also observed in our study that when the antibiotic concentration increases, the marginal toxic effect declines at some specific concentrations, and even a stimulation of CO₂ production could be found. This observed increase can be related to the following processes: it may be concentration-dependent AMR, or SMX may act as a C source, but the most likely explanation is that bacterial growth is inhibited. This last suggested process may then reduce competition, so that other microbial groups may proliferate and actively decompose soil organic matter.    

The estimated priming of soil C was positively related to SMX concentration. When readily available C source (glucose) was added, mineralization of soil C increased and this effect was accelerated with an increase in SMX concentration. Overall, the incubation experiment with different concentrations of SMX provided important insights on the toxicological effects of SMX on soil microbial life and the soil C cycle in agricultural soils. The SMX was shown to inhibit soil heterotrophic activity, but would increase losses of soil C in the presence of readily available C.

How to cite: Menyailo, O., Deroo, H., Eichinger, C., and Dercon, G.: Biogeochemical consequences of agricultural soil contamination with Sulfamethoxazole (SMX), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4926, https://doi.org/10.5194/egusphere-egu23-4926, 2023.

Thermodynamic characterization of soils is a developing field that involves the calculation of the enthalpies, Gibbs energy, and entropy of the soil organic matter, SOM. Its achievement would contribute to the development of the bioenergetics of soil systems beyond the existing theoretical models.

This work shows different experimental procedures and theoretical models for the complete thermodynamic characterization of SOM. It was applied to a total of 31 samples representing different soil horizons from different locations.

Thermodynamic characterization of SOM was achieved through the calculation of empirical formulae for SOM from the SOM elemental composition, application of Patel-Erickson, Sandler-Orbey, and Battley methods, as well as direct measurements of the energy content by simultaneous TG-DSC.

The used computational methods belong to a group of approaches modeling thermodynamic properties of SOM as a sum of contributions from its constituent elements. The first computational approaches were those from the Patel-Erickson and Battley equations. Patel-Erickson equation was used to find the standard enthalpy of combustion, Δ_CH⁰_PE, of SOM based on its elemental composition:

Δ_CH⁰_PE(SOM) = –111.14 kJ/mol ∙ (4n_C + n_H – 2n_O – 0n_N + 5n_P + 6n_S)

where n_J is the number of atoms of element J in the empirical formula of SOM. The Battley equation gives the standard molar entropy, S⁰_m, of SOM:

S⁰_m(SOM) = 0.187 ∑_J [ S⁰_m(J) / a_J ] n_J

where S⁰_m(J) and a_J are standard molar entropy and the number of atoms of element J in its standard state elemental form. The enthalpy from the Patel-Erickson equation is combined with entropy from the Battley equation, to find the Gibbs energy of SOM.

The second computational approach handled equations proposed by Sandler and Orbey that allow finding standard enthalpy of combustion Δ_CH⁰_SO and standard Gibbs energy of combustion, Δ_CG⁰, of SOM:

Δ_CH⁰_SO(SOM) = –109.04 kJ/C-mol ∙ (4n_C + n_H – 2n_O – 0n_N + 5n_P + 6n_S)

Δ_CG⁰(SOM) = –110.23 kJ/C-mol ∙ (4n_C + n_H – 2n_O – 0n_N + 5n_P + 6n_S)

The enthalpy and Gibbs energy obtained using the Sandler-Orbey method were combined to find entropy.  

Results obtained by the application of Patel-Erickson and Sandler-Orbey methods to calculate the enthalpy of SOM combustion did not significantly differ when comparing data given by the TG-DSC with those obtained from the SOM empirical formulation. The same results were obtained when comparing the Gibbs energy. These results enabled the calculation of the entropy of SOM and the comparison of those values among different soil layers and sampling sites.

How to cite: Barros, N., Popovic, M., and Pérez-Cruzado, C.: Complete thermodynamic characterization of the soil organic matter from forest ecosystems., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1389, https://doi.org/10.5194/egusphere-egu23-1389, 2023.

Predicting soil organic carbon (SOC) mineralization under changing climatic conditions is complicated by the diversity of SOC composition. We combined incubation experiments with continuous respiration measurements and thermal analysis to investigate the informativeness of SOC thermal stability. Thermogravimetry is used in studies to determine soil properties such as total organic C, nitrogen, and clay content and to investigate the relationships between thermally labile and stable SOC and biodegradability. Soil respiration (SR) was measured in forest soils with added organic materials (wood, litter, and weeds with C contents of 49, 10, and 35%, respectively, and N contents of 0.1, 0.6, and 3%, respectively) at 20°C and 10°C and different soil moisture contents (5, 10, 20, 40, and 75% of field capacity). Wood amendments were further subdivided into pine (Pinus sylvestris) and beech (Fagus sylvatica) with three different particle sizes. We used topsoil samples from a pine forest, a beech forest, and a long-term agricultural experiment with different properties (C in %: 1.5, 2, and 4; clay in %: 5, 9, and 25, respectively). Basal respiration increased with soil C content, while Q10 levels decreased with field capacity 10 > 40 > 75% in forest and agricultural soils. This order changed depending on the sampling location when organic material was added. Decreasing wood particle size significantly increased SR. Weed additions caused the highest increase in soil respiration. After 10 weeks of incubation at different moisture and temperature conditions, organic amendments were mineralized faster in beech forest soils than in soils under pine forests. A multifactorial analysis of variance showed a significant influence (p < 0.01) of the interactions between temperature, moisture, site, and wood particle size on SR. Preliminary results from analysis of changes in thermal mass loss (TML) between 200 and 550 °C (reflecting SOC thermal stability) due to added organic material and incubation will be presented. Approaches to determine relationships between TML and carbon mineralization will be discussed.

How to cite: Krahl, I., Kalbitz, K., and Siewert, C.: Influence of organic amendments, moisture content and temperature on carbon mineralization of forest soils, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16129, https://doi.org/10.5194/egusphere-egu23-16129, 2023.

Soil organic matter (SOM) represents a continuum of progressively decomposing organic compounds mainly provided by primary producers and predominantly metabolized by adapted dynamic microbial communities. The carbon (C) in SOM flows through the microbial biomass, which needs – beside C and nutrients – Gibbs energy for growth and maintenance. The microbial metabolism and thus the degradation and stabilization of SOM follow thermodynamic laws. The thermodynamic perspective on soil systems is increasingly becoming the focus of research and has the potential to take us a substantial step towards a mechanistic understanding of SOM turnover and stabilization. An integral part of new bioenergetic concepts and models is the energy content of SOM, but the number of empirical studies dealing with soil C cycling or storage in relation to energy contents and flux is small.

In this study, topsoil profiles (comprising organic forest floor horizons OL, OF, OH and the mineral soil layer 0-5 cm) at an afforested post-mining site were investigated to evaluate the influence of (i) soil depth – representing different stages of organic matter (OM) turnover – and (ii) litter quantity and quality (litterfall and fine root tissues) provided by different tree species (Douglas fir – Pseudotsuga menziesii, black pine – Pinus nigra, European beech – Fagus sylvatica, red oak – Quercus rubra) on the energy contents of SOM. The total energy content stored in soils and plant litter was determined using two calorimetric approaches: bomb calorimetry and differential scanning calorimetry combined with thermogravimetry (DSC-TG).

The results of the litter inputs obtained with both methods showed the same trends: the C cycle in the soil was fueled by aboveground and belowground litter inputs, with energy-richer litterfall tissues (needles > leaves) compared to fine root tissues. However, with bomb calorimetry higher energy contents were generally observed in plant litter but also in the upper two forest floor horizons (OL, OF) of the soil profiles. The energy content per unit C (calorific value) changed with increasing depth due to the progressive turnover and stabilization of organic compounds but surprisingly, we identified opposite depth trends with both methods: bomb calorimetry revealed decreasing calorific values, while with DSC-TG increasing calorific values were determined. The few existing studies reported either the one trend or the other with ongoing decomposition, leading to different interpretations of the energetic driven microbial modulated formation and turnover of SOM.

It is mandatory to overcome this fundamental challenge to achieve a reliable integration of the promising bioenergetic approaches into conceptual and modelling frameworks to assess SOC turnover and persistence based on robust empirical data.

How to cite: Lorenz, M., Diehl, D., Maskow, T., and Thiele-Bruhn, S.: Energy content of soil organic matter in soil profiles investigated by bomb calorimetry and DSC-TG, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6542, https://doi.org/10.5194/egusphere-egu23-6542, 2023.

Isothermal microcalorespirometry is a non-destructive technique widely used to study terrestrial activity in ecosystems by measuring the heat and the carbon dioxide (CO₂) released by metabolic reactions of soil organisms. Therefore, microbial communities naturally present in the soil play a key role in the C and N cycle thereby releasing heat and CO₂ which are quantitatively related to the matter fluxes via the law of Hess. In order to measure both quantities simultaneously, current methods follow mainly a purely calorimetric approach (absorbent method or GC analysis) ^[1]. In the absorbent method, CO₂ is measured indirectly via the heat released during the absorption reaction in a NaOH-solution (CO₂-trap), which is placed in the sample vessel together with the soil sample. This approach presents a few disadvantages, e.g. indirect CO₂ measurement, small sample size, low sample throughput, low CO₂ partial pressure and oxygen limitation. 

To overcome the drawbacks of the current calorespirometric approach, a newly designed isothermal macrocalorespirometer (IMCR) was developed by combining a classic respirometer and the proven concept of isothermal microcalorimetry. The IMCR is composed of 10 mobile channels placed in a thermally isolated box, water-thermostated at 20°C. Each channel is composed of a heat sink and a heat sensor directly in contact with the sample vessel (calorimetric unit), plus a vessel with a KOH-solution (CO₂-trap) in which a pair of electrodes is immersed (respirometric unit) connected to the channel’s lid. The spatial separation between the two units, the use of electrodes and the size of the channel, make it possible to overcome the disadvantages of the absorbent method (NaOH-solution) mentioned above. The new approach has been successfully tested with glucose-induced microbial metabolic activity in soil samples, allowing the quantification of the calorespirometric ratio . Additionally, TGA-DSC-MS and GC-MS analysis will be performed, necessary to close balances of mass and energy fluxes.

This newly designed IMCR will be applied in the wider frame of calorimetric environmental soil studies, aiming at understanding the carbon dynamics in soil, the latter being known as the biggest carbon pool among the natural matrix. New knowledge in this area support potential solutions for climate change, intimately connected to the global carbon fluxes.

^[1] Wadsö L., A method for time-resolved calorespirometry of terrestrial samples, Methods 76 (2015) 20–26

How to cite: Di Lodovico, E., Meyer, M., Maskow, T., Schaumann, G., and Fricke, C.: Isothermal Macrocalorespirometry – Novel Instrument Design to Analysis Microbial Metabolism in Soil Systems, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9096, https://doi.org/10.5194/egusphere-egu23-9096, 2023.

Soil structure is a key feature in controlling microbial access to organic matter in soils. The spatial arrangement of solids and pores in agricultural soils is shaped by the used tillage and crop system. However, spatial heterogeneities make it difficult to determine relationships between soil biology and soil structure, and often homogenized, sieved soils are used to evaluate organic matter turnover in soils. In this study, we used heat dissipation as an indicator for biological activity in soils taken from two different tillage systems (conventional vs. reduced tillage) and two different cropping systems (crop rotations with either maize or winter wheat as main crop) running for 12 years. In order to evaluate the impact of soil structure, we investigated the response of both repacked and undisturbed soil cores (3 cm in height, 2.7 cm in diameter) to water and glucose addition. Pore structure indicators and particulate organic matter content were quantified by X-ray computer tomography at a resolution of 15 µm.

We will show that calorimetry is a suitable tool to monitor the biodegradation of C sources in undisturbed soil cores and that both tillage system and crop rotation effect biological activity in soil. In summary, soil under maize cultivation dissipated more heat compared to the wheat crop rotation. In both, repacked and undisturbed samples, conventional tillage promoted heat dissipation in response to water addition, likely due to the annual incorporation of labile organic matter. However, structural and organic matter indicators could only explain the variance in heat dissipation to some extent. Thus, the usage of undisturbed soil cores provides new challenges to evaluate the link between soil structure and microbial activity due to increased variability.

How to cite: Leuther, F., Fischer, D., Nunan, N., and Herrmann, A.: Evaluating soil structure and biological activity in soil cores under different management systems, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8157, https://doi.org/10.5194/egusphere-egu23-8157, 2023.

Soil organic matter plays important roles in soil reactivity and fertility as well as soil physics. Nevertheless, we know relatively little about the individual molecules that make up soil organic matter but ultimately determine its properties. Ultrahigh-resolution mass spectrometry like FT-ICR-MS has revealed an enormous molecular diversity yet it often remains limited to the water-soluble fractions (i.e., dissolved organic matter) analyzed with electrospray ionization (ESI) that represent only a small fraction of the total organic matter contained in soils. To extend the analytical window and leverage the value of non-targeted mass spectrometry, parallel analyses of soluble (via ESI) and particle-associated organic matter (PAOM) via laser-desorption ionization (LDI) and FT-ICR-MS detection is a promising approach, that has yet to prove its full potential. Here, we studied the sensitivity and robustness of the LDI technique based on a combination of dried arable soils, their aqueous DOM extracts, reference DOM samples (Suwannee River Fulvic Acid, SRFA), model compounds (syringic acid, sinapic acid, syringaldehyde, vanillic acid and tannic acid) and model mineral phases (goethite, illite). DOM samples were used to study the effects of a mineral matrix and dilution, while model compounds and SRFA were used to test the effects of laser strength and presence of an organic matrix on intact ionization of analytes. Lastly, non-extracted and extracted soil samples were used to assess if DOM composition trends observed in solution are reproduced in PAOM composition. In general, ESI ionized a very different fraction of the DOM mixture, being more polar and more saturated, while LDI ionized rather small, low-to-mid polar, and less saturated ions. Besides clear differences in PAOM and DOM analytical windows, molecular trends such as aromaticity or nominal oxidation state were well-aligned. Although most insight was gained by combining both types of analyses, our results therefore suggest that direct analysis of soil particles is a fast, reproducible, sensitive and less invasive alternative to routine protocols employing FT-ICR-MS detection, and avoids additional extraction or purification steps.

How to cite: Simon, C., Pietsch, P., Stumpf, K., Kaiser, K., and Lechtenfeld, O.: Development of ultrahigh resolution mass spectrometry techniques to extend the molecular view of soil organic matter in solution and on mineral particles, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3113, https://doi.org/10.5194/egusphere-egu23-3113, 2023.