SSS5.6

Soil organic matter (SOM) is made up of different components that are contributed by plant residues and living microbial biomass. The quantitative estimation of the above-ground biomass is mostly done using the molecular proxies (n-alkane) and δ¹³C values of SOM. However, the estimations can be site-specific and can vary depending on the contribution from the C3 and C4 plants. In a need to understand the transfer of biomass signals from the vegetation to soil, sampling sites (1mX1m) were chosen which comprises a pure grassland (C4), forest land (C3), and a mixed vegetation ecosystem from the lower-Gangetic floodplain. The  δ¹³C values of the above-ground biomass in grassland, forestland, and mixed ecosystem show a variation of 4‰, 7‰, and 20‰, respectively. In the associated soil, however, the incorporation of organic matter from the vegetation was not straightforward and showed a variation between +1‰ and -8‰ in three different sites. The values were ¹³C-enriched in soil underlying the Grassland and depleted in forest soils. The n-alkane molecular proxies in the soil such as CPI, ACL, show a decrease (about 50%) and increase in LMW/HMW concentrations in values among different sites. The decrease in molecular proxies was evident due to differences in organic matter contribution from different species. In C3 forest, the difference in degradation from different components of trees (twigs, leaves, flowers, fruits) reduces the molecular proxies and also the  δ¹³C values in the soil. On the other hand, grasses acting as a whole, impart limited modification during incorporation into the soil. 

How to cite: Baidya, D., Roy, B., and Sanyal, P.: Organic matter distribution from the biomass to the soil in grassland, forest, and mixed ecosystem., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-321, https://doi.org/10.5194/egusphere-egu22-321, 2022.

In alpine areas of the European Alps, many of the pastures that are no longer economically profitable are being converted into forests (Bolli et al., 2007). Afforestation of former pasture has been acknowledged as a contribution to mitigate CO₂ emissions by an increased storage of soil carbon in soil and biomass (Smal et al., 2019). So far, several studies indicated that afforestation of former pastures does not always lead to an increase or decrease of soil organic carbon stocks after 30 to 40 years of afforestation. The loss of soil organic carbon in the mineral soil, however, can be rebalanced by the increased accumulation of soil organic carbon in the organic forest floor (Thuille and Schulze, 2006). Nevertheless, studies concerning the changes as well as restoration of SOM following afforestation are limited.

In this study, we aimed to trace the source and transformation of SOM in a subalpine afforestation sequence (0-130 years) with Norway spruce (Picea abies L.) on a former pasture in Jaun, Switzerland. Soil and root samples were taken with volumetry cylinders to a depth of 45cm at 5cm increments. To trace the source and transformation of SOM, soil samples were analysed for plant-and microorganism-derived SOM by combining multiple compound classes as free extractable lipids, such as n-alkanes and free fatty acids.

Preliminary results show a higher (p<0.05) fine roots biomass in pasture (8.2±4.4gm^-2) compared to forested areas. The highest (p=0.92) fine root biomass was observed in the youngest forest (40yr; 2.3±0.7gm^-2), followed by the 130yr (0.7±0.2gm^-2) and 55yr (0.6±0.2gm^-2) old forest. Highest carbon stocks (14.0±0.8 kgm^-2) were observed in the youngest forest followed by the 130yr (11.0±0.3 kgm^-2) old and 55yr (9.6±1.1kgm^-2) old forest. In summary, afforestation of former pasture (11.2±0.0 kgm^-2) does not result in changes (p=0.37) in the total (0-45cm) organic carbon stock over a period of decades. However, there is a significant (p<0.001) higher C concentration in the organic forest floor in all forested areas compared to the mineral soil of both pasture and forest. To conclude, the change in the SOM sources and quality following afforestation may not lead to stock changes, but the stability of SOM might be modified by this change. The changes in SOM dynamics following afforestation are further analysed by the use of phospholipid fatty acids as well as free extractable fatty acids and alkanes to improve our understanding of aboveground and belowground litter incorporation and cycling.

References

Bolli, J. C., Rigling, A., and Bugmann, H. (2007). The influence of changes in climate and land-use on regeneration dynamics of Norway spruce at the treeline in the Swiss Alps. Silva Fennica, 41, 55.

Smal, H., Ligęza, S., Pranagal, J., Urban, D., and Pietruczyk-Popławska, D. (2019). Changes in the stocks of soil organic carbon, total nitrogen and phosphorus following afforestation of post-arable soils: A chronosequence study. Forest Ecology and Management, 451, 117536.

Thuille, A., and Schulze, E. D. (2006). Carbon dynamics in successional and afforested spruce stands in Thuringia and the Alps. Global Change Biology, 12, 325-342

How to cite: Speckert, T. C., Gavazov, K., and Wiesenberg, G. L. B.: 130 Years of afforestation does not result in changes in soil organic carbon stocks in the Swiss Alps, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9923, https://doi.org/10.5194/egusphere-egu22-9923, 2022.

Microbial biomass and necromass are increasingly considered to be the main source of organic carbon (C) formation in soils. However, quantitative information on the contribution of microbial necromass to soil organic carbon (SOC) formation and the factors driving microbial necromass accumulation, decomposition and stabilization during initial soil formation in biological crusts (biocrusts) have remained elusive. To address this knowledge gap, we investigated the composition of microbial necromass and its contributions to SOC sequestration in a biocrust formation sequence consisting of five stages: bare sand stage, cyanobacteria stage, cyanobacteria-moss stage, moss-cyanobacteria stage, and moss stage on sandy parent material on the Loess Plateau. The fungal and bacterial necromass C content was analyzed based on cell wall biomarkers, i.e. amino sugars. Microbial necromass was an important source of SOC, and was incorporated into the particulate and mineral-associated organic C (MAOC). Because bacteria have smaller and thinner cell wall fragments as well as more proteins than fungi, bacterial necromass mainly contributed to the MAOC pool, while fungal residues contributed more to the particulate organic C (POC) pool. MAOC saturation by microbial necromass and the fact that POC accumulated more rapidly than MAOC during initial soil formation suggest that the clay content was the limiting factor for stable C accumulation in this sandy soil. Microbial necromass exceeding the MAOC saturation level was further stored in the labile POC pool (especially necromass from fungi). Activities of four enzymes (i.e., β-1,4-glucosidase, β-1,4-N-acetyl-glucosaminidase, leucine aminopeptidase, and alkaline phosphatase) increased with fungal and bacterial necromass, suggesting that the increasing activity of living microorganisms led to an accelerated turnover and formation of necromass. Microbial N limitation raised the production of N acquisition enzymes (e.g., β-1,4-N-acetyl-glucosaminidase and leucine aminopeptidase) to break down necromass compounds, leading to further increases of bio-available N in soil solution. The decrease of microbial N limitation along the biocrust formation chronosequence is an important factor triggering microbial necromass accumulation during initial soil development. High microbial N demands and insufficient clay protection led to fast necromass reutilization by microorganisms and thus, resulted in a low necromass accumulation coefficient, that is, the ratio of microbial necromass to living microbial biomass (on average, 9.6). Consequently, microbial necromass contribution to SOC during initial soil formation by biocrusts was lower (12-25%) than commonly found in fully developed soils (33%-60%, literature data). Nitrogen limitation of microorganisms and increased ratios between N-acquiring enzyme activities and microbial biomass N, as well as limited clay protection and MAOC saturation resulted in a low contribution of microbial necromass to SOC during initial development of this biocrust-covered sandy soil. in summary, soil development led not only to SOC accumulation, but also to increased contribution of microbial necromass to SOC, while the plant biomass contribution to SOC decreased.

How to cite: Wang, B., Wanek, W., Huang, Y., Kuzyakov, Y., and An, S.: Initial soil formation by biocrusts: nitrogen demand and clay protection control microbial necromass accrual and recycling, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1354, https://doi.org/10.5194/egusphere-egu22-1354, 2022.

Mediterranean savannahs (dehesas) are typical agro-sylvo-pastoral systems, characterized by the scattered presence of oak trees (Quercus ilex, Quercus suber), and the integration of livestock, forest, and agricultural practices. These Mediterranean ecosystems are subjected to a marked seasonality that imposes a sever summer drought after a favourable rainy autumn and spring, that is reflected in soil microbial dynamics. Under such conditions, the relative importance of a-biotic constraints such as temperature warming, irreversible dehydration favoured by intense solar radiation and drastic drying cycles, are important factors in soil organic matter (SOM) dynamics and the formation of stable forms in soil. The interplay of driving factors on the microbial dynamics - climate, vegetation and soil is key to understand biogeochemical cycles in Mediterranean forests that, in-turn is expected to be reflected in SOM structure.

In this communication analytical pyrolysis coupled with gas chromatography-mass spectrometry (Py-GC/MS) was used for the molecular characterization of SOM in a field manipulative experiment of rainfall exclusion and increased temperature aimed to evaluate the impact of forecasted warming and drying. The experimental trial is located in Sierra Morena (Pozoblanco, Córdoba, SW-Spain). Composite soil samples (0-10 cm) were taken from four forced climatic treatment plots: warming (W); drought (D); combination of both (W+D); untreated control (C). The plots were installed in 2016 under two distinct habitats: evergreen oak canopy (‘tree’) and in the open pasture (‘open’). Data presented correspond to sampling conducted in 2017 (a year after the installation of field trials) and five years later in 2021.

A total of 116 compounds were identified, and composition differences were detected between ‘tree’ and ‘open’ habitats both in 2017 and 2021, for the main compound classes: nitrogen compounds (N), aromatics (ARO), lignin methoxyphenols (LIG), isoprenoids (ISO), fatty acids (FA), lipids (LIP) and polysaccharide-derived (PS). Such chemical differences were found to be derived from the biomass composition of the predominant vegetation type incorporated into the soil. The FA and LIP (n-alkanes) were found most responsive to climatic treatments, showing less abundance under D and W plots. This trend is more pronounced in ‘open’ habitat and remains significant after 5 years of experiment. Moreover, the proportion between PS and LIG moieties increased over time especially in the ‘tree’ habitat, with a preferential degradation of PS due to increasing microbial activity. Finally, the proportion of ARO and short and mid-chain LIP increased during the trial, pointing to non-favourable SOM decomposition conditions.

Here, a short-term field experiment indicates that Mediterranean dehesa soils can buffer climate change effects over time. The results suggest that SOM molecular composition encompasses information on soil environmental shifts having biomarker value for monitoring climate change in Mediterranean soils. The technique can also help to monitor SOM turnover rates attending to the progressive transformation of different compound families.

Acknowledgement: EU-EJC 2nd Call Projects MIXROOT-C and MAXROOT-C. L.M. San Emeterio thanks Ministerio de Ciencia Innovación y Universidades (MICIU) FPI research grant (BES-2017-07968) for funding. A.M. Carmona, M.D. Hidalgo and P. Campos are acknowledged for technical assistance.

How to cite: M. San Emeterio, L., Pérez Ramos, I., Domínguez Núñez, M. T., and González Pérez, J. A.: Changes in soil organic matter after 5-year field experiment of rainfall exclusion and increased temperature in a Mediterranean savannah, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12418, https://doi.org/10.5194/egusphere-egu22-12418, 2022.

Carbohydrates are a ubiquitous constituent of living organisms and an important contributor to global sedimentary carbon pools. Yet, the factors that control the pool size and degradation of sedimentary carbohydrates are not well understood. Here, we investigated carbohydrate cycling over a complete Holocene sedimentary succession in high-altitude, meromictic Lake Cadagno (Switzerland). This succession includes sedimentary records of a stepwise evolving lake redox history from oxic, hypoxic to euxinic anoxic, as well as intercalated layers of lacustrine and event deposits. Event deposits differ from lacustrine layers in organic carbon content, stable isotope signatures, carbohydrate contents, and carbohydrate macromolecular compositions (pyrolysis gas/mass spectrometry) indicating significant terrestrial inputs. However, past changes in redox conditions, implied by redox-sensitive elements (e.g. Mn, Fe, Mo, and S), are not reflected in carbon isotope and carbohydrate data. Carbohydrate contributions are stable, or even increase, with sediment age and show compound-specific variations. Certain carbohydrate pyrolysis products of likely aquatic origin, such as levoglucosan, increase in percentage with sediment age, whereas others, such as furaldehyde, decrease with age. On the other hand, pyrolysis products that are of likely terrestrial origin, such as 3-acetamido-methylfuran, decrease strongly with age. In contrast to carbohydrate macromolecules, gas chromatography with flame-ionization detection analyses showed no clear changes in total hydrolyzed carbohydrate monomer compositions in relation to sediment age or between aquatic- and terrestrial-dominated layers. Our results indicate that carbohydrate degradation in sediments is mainly controlled at the macromolecular level. Notably, our findings suggest that carbohydrates can be well-preserved over the entire Holocene in lake sediments and that aquatic carbohydrates are in some cases selectively preserved over their terrestrial counterparts in these sediments.  

How to cite: Gajendra, N., Berg, J., Vogel, H., Schubert, C., and Lever, M.: What controls the fate of carbohydrates in meromictic lake sediments throughout the Holocene?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8145, https://doi.org/10.5194/egusphere-egu22-8145, 2022.

Peatlands occupy ~3% of the land surface, yet they store more than one-third of global terrestrial carbon. However, there is growing concern that the decomposition of this vast carbon bank in the face of climate change could alter peatlands from a carbon sink to a carbon source, but experimental data is scarce. Here, we examine peatland carbon stability after four years of whole-ecosystem warming (+0, +2.25, +4.5, +6.75 and +9 °C) and two years of elevated CO₂ manipulation (500 ppm above ambient). We use solvent-extractable (alkanoic acids, alkanols and alkanes) and hydrolysable lipids (cutin and suberin) and benzene polycarboxylic acids (BPCA) as tracers for fire-derived organic matter and investigate their degree of decomposition in a boreal forested peatland.

We found fire-derived organic matter stemming from past fires, either nearby or long-distance atmospheric transport. Warming alone or when combined with elevated CO₂ did not affect the quantity and quality of fire-derived organic matter stemming from past fires, as indicate by the molecular markers BPCA. The wet conditions probably helped to preserve these slowly degrading aromatic compounds. Molecular markers for leaf- (cutin) and root‐derived biomass (suberin), showed that with warming more new plant biomass came from roots, at the expense of leaf-derived compounds under both, ambient and elevated CO₂ treatments, implying dynamic alterations to leaf and root carbon incorporation and sequestration with environmental changes. These responses were more pronounced in the surface aerobic acrotelm, highlighting that the aerobic layer responded surprisingly fast, within a few seasons to changing environmental conditions.

How to cite: Nicholas, O., Cyrill, Z., Paul, H., Guido, W., and Michael, S.: Divergent response of plant-derived lipids and fire-derived organic matter to warming and elevated CO2 in a boreal peatland., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1470, https://doi.org/10.5194/egusphere-egu22-1470, 2022.

Peatlands are an important ecosystem for many reasons, including their function as carbon sinks essential for mitigating climate change. Additionally, due to the anaerobic conditions in which peatlands form, decomposition of their constituent plant material is inhibited, making peatlands valuable archives for paleoenvironmental reconstructions. In this study, we examined a new 3.4 m core from the ombrotrophic Beerberg peatland located in the Vessertal-Thuringian Forest Biosphere Reserve in Germany. While paleo-archives at this peatland have been studied in the past (e.g., Jahn, 1930; Lange, 1967), we aim to apply newer techniques at a higher resolution to obtain more detailed results. Radiocarbon dating indicates that the core spans approximately the last 2500 years. Samples from the core were analyzed for pollen, macrofossils, and biomarkers, in particular, free extractable lipids including n-alkanes, n-alcohols, and n-fatty acids. These proxy data were used both to perform a vegetation reconstruction as well as to compare the results of the different proxies to each other to determine accuracy as well as create a more complete picture of the environment over time at the Beerberg peatland. The current dominant vegetation at the moor are Sphagnum mosses as well as Calluna vulgaris. Additionally, Eriophorum vaginatum, Empetrum nigrum, Oxycoccus palustris, and various Vaccinium species were abundant. Preliminary results from the macrofossil and pollen analyses indicate thatthrough time, the peatland has been primarily dominated by Sphagnum mosses, particularly Sphagnum fuscum. However, there are also conflicting results of when transitions to other dominant vegetation, such as Eriophorum vaginatum, occurred, as well as the contributions of species, such as Calluna vulgaris, over time. We aim to clarify these results through the addition of the biomarker analysis to develop a robust picture of evolution of vegetation during the Holocene at Beerberg peatland. Data from this study will also be used to improve a future iteration of the VERHIB (VEgetation Reconstruction with the Help of Inverse modeling and Biomarkers) model (Jansen et al., 2010).

References:

Jahn, R. (1930). Pollenanalytische Untersuchungen an Hochmooren des Thüringer Waldes. Forstwissenschaftliches Centralblatt, 52, 761-774.

Jansen, B., van Loon, E. E., Hooghiemstra, H., & Verstraten, J. M. (2010). Improved reconstruction of palaeo-environments through unravelling of preserved vegetation biomarker patterns. Palaeogeography, Palaeoclimatology, Palaeoecology, 285(1-2), 119-130.

Lange, E. (1967). Zur Vegetationsgeschichte des Beerberggebietes im Thüringer Wald. Feddes Repertorium, 76(3), 205-219.

How to cite: Thomas, C. L., Galka, M., Czerwiński, S., Knorr, K.-H., Jansen, B., and Wiesenberg, G. L. B.: New insights into Central European environmental changes during the last 2500 years by multi-proxy analysis of the undisturbed Beerberg peatland, Thuringia, Germany., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12466, https://doi.org/10.5194/egusphere-egu22-12466, 2022.

Vegetated continental wetlands are responsible for 20% of global methane emissions. Wetlands encompass very diverse ecosystems, from ponds to peatlands. In mid-latitudes, temporary flooded lowlands are very common, due to the seasonal discharge of water from rivers or groundwater but their contribution to the global methane budget is unknown. These sites experience temporal successions of oxic and anoxic conditions and the impact of these dynamic redox conditions on the microbial communities is poorly understood. In particular, methanogenesis typically requires anoxic conditions while aerobic methanotrophy can oxidize large amounts of methane before it reaches the atmosphere. This study is carried out at the Ploemeur-Guidel hydrogeological observatory, where a wetland is created by the seasonal discharge of deep anoxic and iron-rich groundwater. Four one-meter-long cores were recovered along a transect from the inner to the outer side of the wetland and samples were collected every 20 to 30 cm along each core. Batch incubations revealed two potential hotspots of methane production at ca. 40 cm and 100 cm depth, reaching up to 3.5 µmol cm^-3 d^-1, in agreement with highest methane concentrations measured in the porewater collected at these depths. To shed light on the microbial processes involved in the methane cycling at this site, a metagenomic and metatranscriptomic analysis was conducted in combination with the analysis of the carbon isotopic composition of microbial intact polar lipids. Preliminary results of the omics study showed clear taxon stratification with depth. The proportion of metagenomic reads classified to archaeal taxa increased with depth and reached up to 15% of the total analyzed reads at one meter below the surface. Interestingly, the percentage of reads affiliated to known methanogens was highest at the surface (ca. 40% of the archaeal reads) and decreased with depth (ca. 13% at 100 cm depth). Archaeal communities appeared to be dominated by Candidatus Bathyarchaeota since up to 60% of the archaeal reads could be classified to this phylum. Carbon isotopic composition of the phytane and biphytanes (BP) derived from the major archaeal membrane lipids (archaeols and glycerol dibiphytanyl glycerol tetraethers, GDGTs) displayed a predominant heterotrophic signature. However, BP-2, derived from GDGTs with multiple rings was systematically depleted in ¹³C, suggesting autotrophic and/or methanogenic metabolism. Further statistical treatments will be performed to refine the taxonomic links between the detected membrane lipids and the archaeal populations along the cores.

How to cite: Coffinet, S., Dufresne, A., Quaiser, A., Hinrichs, K.-U., and Laverman, A.: Identification of microbial methane sources and sinks in an aquifer-fed continental wetland with a multi-omics approach, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9758, https://doi.org/10.5194/egusphere-egu22-9758, 2022.

Soil organic matter is composed to a large extent of microbial necromass, including fragmented cell wall residues and other cytoplasmic components from dead fungi and bacteria. These components accumulate in soil over long periods of time and have been used as biomarkers to trace microbial residues. Amino sugars are key components of microbial cell walls and can be found in polymeric forms as fungal chitin and bacterial peptidoglycan in soils. Among the most abundant amino sugars in soil are glucosamine, galactosamine, mannosamine and muramic acid. Glucosamine is used as a biomarker for both fungal and bacterial necromass, while muramic acid is exclusively found in bacterial peptidoglycan. Neutral sugars can also be used as biomarkers, where pentose:hexose ratios are used to determine the contribution of plant biomass relative to microbial necromass to soil organic matter (SOM). Potentially uronic acids can also be used as plant versus microbial biomarkers. Acid hydrolysis breaks apart polymers such as peptidoglycan, chitin, and plant matter into monomers, which can later be quantified to estimate the contribution of bacterial, fungal and plant necromass to stabilized SOM. Due to their structural similarity and complexity, high-throughput identification and quantification of these compounds has remained a challenge. Derivatization using 1-phenyl-3-methyl-5-pyrazolone (PMP) has been used to characterize carbohydrates because of its simple and rapid reaction mechanism and enhanced ionization efficiency with ESI-MS. Our aim was to develop a highly sensitive method to quantify sugar-containing compounds in a single rapid assay using pre-column PMP derivatization. Separation and quantification of the PMP-derivatives was carried out using reversed-phase ultra-high performance liquid chromatography (RP-UHPLC) coupled to high resolution-high accuracy Orbitrap mass spectrometry (MS). Our method allowed the simultaneous separation and quantification of >20 compounds, including hexosamines, muramic acid, N-acetylhexosamines, hexuronic acids, pentoses, hexoses, deoxyhexoses, cellobiose, chitobiose, and chitotriose. All PMP derivatives were separated within 20 minutes. This method provides high resolution and high sensitivity for the quantification of diverse sugar-related compounds in one single assay, which demonstrates its potential in measuring complex and heterogeneous mixtures. Likewise, this method can also be applied in isotope tracing studies, where the turnover and stabilization of ¹⁵N and ¹³C labeled compounds in necromass are traced through the different soil pools using UPLC-Orbitrap mass spectrometry.

How to cite: Salas, E., Kaiser, C., and Wanek, W.: Fast and sensitive detection of amino sugar, neutral sugar and uronic acid biomarkers using 1-phenyl-3-methyl-5-pyrazolone (PMP) derivatization and reversed phase liquid chromatography coupled to Orbitrap mass spectrometry, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2244, https://doi.org/10.5194/egusphere-egu22-2244, 2022.

A relatively small change in the balance of in- and outgoing fluxes between terrestrial Carbon (C_terr) and the atmosphere, sustained over centuries to millennia can change C_terr from a carbon source to a sink. The net carbon balance of any ecosystem is mainly determined by climate (temperature, humidity, seasonality) via its influence on primary productivity, respiration and preservation, and by geomorphology (erosion). More recently, human perturbance has increasingly also become a major factor. In particular, the slow cycling component of C_terr, with turnover times of centuries to millennia, is relevant for the long-term carbon balance on land. Build-up of this carbon pool is inherently slow, but loss can be rapid and thereby form a significant carbon source to the atmosphere. One way to gain insight in the dynamics of this slow cycling carbon pool is to interrogate sedimentary records that, through time, have stored snapshots of terrestrial carbon, the latter being a mixture of pre-aged, long-stored C_terr and fresh material. By downcore measurements of the radiocarbon age of specific plant-derived organic compounds, interferences by aquatically produced organic carbon or petrogenic organic carbon can be circumvented, and insights can be gained into the carbon cycle processes in the corresponding catchment area. This study presents compound-specific ¹⁴C data compiled from studies over the last 20 years of sedimentary records derived from small lake catchments to deltaic and submarine fan deposits near large river mouths. The main conclusions that can be drawn are: 1) Modern but also (pre)historic human perturbance through land-use change has released long-stored ecosystem carbon that otherwise would have escaped mobilization. 2) Both positive and negative correlations between millennial-scale hydroclimate change and C_terr dynamics are evident, and are attributed to the opposing effects on primary productivity, respiration and erosion rates.  3). Catchment size and geomorphology also influence the extent of net ecosystem carbon storage. 4). The Younger Dryas cold period promoted release of C_terr built up during the preceding warm Bølling-Allerød period, illustrating the role rapid climate change can play in carbon dynamics.

How to cite: Smittenberg, R., Galy, V., Bernasconi, S., Gierga, M., Birkholz, A., Hajdas, I., Wacker, L., Haghipour, N., Ponton, C., and Eglinton, T.: Terrestrial carbon dynamics through time - insights from downcore radiocarbon dating, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13158, https://doi.org/10.5194/egusphere-egu22-13158, 2022.

Soil organic matter (SOM) is the largest organic carbon (OC) pool on Earth’s surface after sedimentary rocks. Soil carbon storage is a key process that can mitigate climate change through the sequestration of CO₂ from the atmosphere. However, numerous uncertainties persist concerning how SOM reacts to changing environments due to the challenges in disentangling the effects and interplay between different climatic and physico-chemical controls on SOM stabilization. Radiocarbon has proven to be a useful tool to identify SOM sources and turnover times, yet comprehensive investigations of ¹⁴C dynamics of SOM across climatic and environmental gradients remain scarce.

Our study aimed at better understanding the drivers of carbon dynamics across different ecoregions in a large suite of 54 Swiss soils (0-20 cm depth) that span a broad range of climate and geological conditions. We measure radiocarbon signatures of different SOM fractions separated on the basis of density and chemical reactivity from both recently sampled (2014) and archived soils (collected in the 1990s) in order to estimate the evolution of ¹⁴C in the different soil fractions over two decades. Results are interpreted in the context of a comprehensive soil database in order to assess the impact of different drivers, such as climatic conditions, bedrock, altitude, land-use, soil biogeochemical properties on ¹⁴C signatures and turnover times of different SOM pools.

First results show a strong contrast between particulate organic matter (POM) and mineral associated organic matter (MAOM) fractions of the soils. The particulate organic matter ¹⁴C signature decreased between the two soil inventories, on average from 113 ‰ to 78 ‰, following the decline of ¹⁴C bomb spike in the atmosphere. This shows that thePOM is a fast cycling reactive pool. In contrast, MAOM finer than 20 µm showed an increase in Δ¹⁴C from -35‰ in the 1990s’ samples to 0.8 ‰ in 2014, indicating substantial C fluxes through MAOM cycling at decadal time scales. Further oxidation of MAOM using hydrogen peroxide, removing about 80 to 90% of its C, revealed that MAOM is composed of very old SOM with Δ¹⁴C values as low as -104.9 ± 0.8 ‰ and thus millennia old. By contrast, the removed SOC had high Δ¹⁴C values around 40 ‰. This finding implies that MAOM consists of a continuum from rather stable SOM to rather rapidly cycling components. First results also indicate a strong influence of pH on turnover times, suggesting slower OM processing in acidic soils. By linking our ¹⁴C data to auxiliary data, we will explore the factors driving turnover rates of fast and slower cycling OC pools and pinpoint their vulnerability to climate change.

How to cite: Moreno Duborgel, M., Minich, L. I., Haghipour, N., González-Domíngez, B., Abiven, S., Eglinton, T., and Hagedorn, F.: Radiocarbon based turnover rates of soil organic matter fractions along climatic and biogeochemical gradients across in Switzerland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5909, https://doi.org/10.5194/egusphere-egu22-5909, 2022.

An increase of nitrogen (N) deposition is predicted in the next 10 years by 50%, compared to the values observed 20 years ago. This, together with the increase of atmospheric temperatures can change the soil organic matter (SOM) to either stronger mineralization or offset effects can be observed, depending on the ecosystem, and initial characteristics of the soils at the sites. Thus, this experiment aimed to reveal the effect of N deposition on the temperature sensitivity of SOM from the P limit subtropical evergreen forest ecosystem. Soils were collected at the Heshan (HS) National Field Research Station of Forest Ecosystem located in Guangdong province, where an experiment with a complete randomized block design of N deposition or control was established in 2018 (100 kg N ha⁻¹ y⁻¹, during 2.5 years). The soil (0-20 cm, four replicates for each treatment) was incubated at 3 temperatures of 15, 25, and 35 °C during 112 d at 60 % of WHC. During the incubation, soil CO₂ efflux was constantly monitored, and three destructive samplings were done (at 10, 69 days, and at the end of the experiment). The soil was analyzed for the dissolved organic C, microbial biomass, available N pool, substrate use efficiency (with ¹⁴C-glucose), activities of C and P hydrolytic, and C oxidative enzymes, the content of microbial biomarkers, and functional gene abundances. The maximal mineralized SOC amount was found under 35 °C under N deposition and minimum at control 15 °C. The highest differences for the total CO₂ efflux were observed between N deposited and control plots at 35 °C (1.3 times), and smallest at 15 °C (0.8 times). Q₁₀ was higher for the temperature increase 15-25 °C (1.3 and 2.3 for the control and N, respectively) than for the 25-35 °C (1.16 and 1.6), and for the soil experienced N deposition. Microbial biomass was affected by both, N treatment and temperature. SUE was the highest at 15 °C in the middle of the incubation under N deposition, whereas no differences were found at the other sampling points or treatments. Activities of acid phosphatase decreased with the incubation for all temperatures, whereas b-glucosidase and xylanase had maximum in the middle of the incubation; all hydrolytic decreased activities by the end of the incubation under N treatment. Both oxidases were higher under N deposition than in control during the entire experiment, with the maximum found under 35 °C. Polyphenol oxidase activity increased at 35 °C with the incubation in both control and N deposition, however stayed constant for the other temperature treatments. In contrast, peroxidase activity was the same between the sampling times. Thus, the effect of N deposition on the activity of microbial biomass, expressed in SOM mineralization, appeared more clearly with the incubation temperature. Despite that, the complex of soil enzymes reacted differently to the studied impacts, with oxidative enzymes being more vulnerable than oxidative ones.

How to cite: Guo, Z., Qiang, W., Ludwig, B., Kuzyakov, Y., and Gunina, A.: Nitrogen deposition in subtropical forest: effect on temperature sensitivity of soil organic matter, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12545, https://doi.org/10.5194/egusphere-egu22-12545, 2022.

Drying-rewetting cycles (DRC) affect litter and soil organic carbon (SOC) decomposition and mineralization, especially in Mediterranean ecosystems. Global climate change is expected to increase drought periods as well as heavy precipitation frequency, which in turn will increase soils DRC. However, the effects of DRC on the functioning of microbial communities and dynamics of dissolved organic carbon (DOC) remain elusive. Here, we investigate the effects of climate-change on organic carbon turnover rates based on a DRC approach.

Composite dehesa soil samples (0-10 cm) (Pozoblanco, Córdoba, Spain) were taken from three forced climatic treatment plots (W: warming (heat increase); D: drought (water restriction); C: Control). The plots were installed 4 yrs ago under two distinct habitats: evergreen oak canopy (designated as ‘tree’) and in the open pasture (‘open’). The soil samples were incubated for 26-days at a constant moisture (40% of water-holding capacity, WHC) and labelled ¹⁴C-glucose (150 % of C from microbial biomass). Afterwards, to simulate drought in nature, ¾ of each sample were dried and further four rewetting treatments were established: 1) constant-moisture at 40% WHC, 2) slow DRC with 5-days water addition to 40% WHC, 3) fast DRC with all water added during the first day of the experiment, and 4) dry DRC with 7-days drying and no rewetting. Following DRC period, there was an extended incubation (26 d in total), where samples were taken at three times after rewetting (4, 7 and 26 days) for further analyses. Total and ¹⁴C-glucose-derived dissolved organic carbon (DOC), microbial biomass (MBC), C, N and P related enzymatic activities, and other parameters of microbial growth were measured. During the incubation period total and ¹⁴C-CO₂ were also monitored.

The results obtained and the discussion of the DRC effects detected and main threads regarding climate change in Mediterranean dehesa agroforestal system such as increasing temperatures and drought events on microbial biomass, respiration and C turnover, will be detailed. Changes in DRC can alter organic C mineralization, in turn such effect can strongly depend on previous field-induced conditions in Mediterranean savannas. In addition, our results will help to understand the responses of soil MBC and DOC to DWC in Mediterranean ecosystems and could improve the prediction of CO₂ emission under a changing environment in the future.

Acknowledgment: EU-EJC 2^nd Call Projects MIXROOT-C and MAXROOT-C. L.M. San Emeterio thanks Ministerio de Ciencia Innovación y Universidades (MICIU) FPI research grant (BES-2017-07968) and the German Academic Exchange Service (DAAD) for funding. A.M. Carmona, M.D. Hidalgo, P. Campos and K. Schmidt are acknowledged for technical assistance.

How to cite: M. San Emeterio, L., González Pérez, J. A., Chen, J., Pérez Ramos, I., Domínguez, M. T., Kuzyakov, Y., and Gunina, A.: Mediterranean soils under climate change: a drying-rewetting experiment with 14C-labelled glucose, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8758, https://doi.org/10.5194/egusphere-egu22-8758, 2022.

Drought due to climate change will be more frequent and severe and expected to cause a loss of agricultural productivity in the next 30 years. Drought affects soil water availability, leading to several effects: reduction of crop growth, increases shoot/root ratio, decreases the uptake of nutrients by roots and translocation from roots to shoots, and availability of nutrients (N, P, S, Ca, Mg) in soil. The plant community regulates the composition and quantity of rhizodeposition, however, due to the impact of drought on plants functioning, allocation and fate of assimilated carbon (C) in the plant-soil-microorganisms system is changed.  Especially strong effects of drought on the functioning of soil microbial communities are seen in the crop monocultures, whereas the introduction of intercropping, especially with legumes, can potentially mitigate the drought impact. Thus, the present study aimed i) to trace the fate of rhizodeposits in the maize-soybean intercropping and compare with that in monoculture under drought vs optimal moisture conditions, ii) to reveal the differences in the rhizodeposits-C utilization by microorganisms under pulse water regime (wet to drought vs drought to rewetting).

The greenhouse experiment was conducted in June-August 2021. The soils were taken from the 10-old abandoned agricultural land in Witzenhausen, sieved through 4 mm, and 3 kg were placed into the incubation pots (15x15x20 cm). Maize (Zea mays L.) and soybean (Glycine max (L.) Merr.) were grown as monoculture (4 plants per pot) or intercropping (2+2 plants) until the vegetative stage (6-8 leaves), soil moisture was kept at 60% of WHC. Further, plants were divided into four treatments (each had four replicates), and each was labeled with K¹⁵NO₃ and followed by ¹³C-CO₂ (one day later). The treatments were: i) constant wet (60% of WHC), ii) constant drought (30% of WHC), iii) wet to drought (labeling was done under 60% of WHC and soil dried till 30% of WHC and further kept for 14d), and iv) drought to wet (labeling was done under 30% of WHC and soil was slowly rewetted to 60% of WHC and further kept for 14d). Subsamples of soils and plants aboveground biomass were done 4 times: i) 1 day after ¹³C-labeling, ii) after soil reached drought (30% of WHC) or 60% of WHC after rewetting, iii) in the middle of a drought or rewetting stage and iv) at the end of the experiment. Constant moisture and unlabelled treatments were sampled at the same time points; total and ¹³C/¹⁵N were analyzed in plant biomass, dissolved organic matter (DOM), microbial biomass, and soil; NO₃^- and NH₄⁺ contents were measured in DOM extracts; additionally, activities of C, N, and phosphorus acquisition enzymes were measured. Thus, results from this experiment will shed light on the fate of rhizodeposits depending on the drought conditions and pulse water regime. The comparison of monoculture with intercropping will reveal how the presence of soybean can improve the nutrition of maize regarding N uptake, and whether microorganisms can be less affected by drought under intercropping.  

How to cite: Zhou, Q., Guo, Z., Hu, Y., Ludwig, B., Kuzyakov, Y., and Gunina, A.: Monoculture vs intercropping under drought: functioning of microorganisms and the fate of maize and soybean 13C-labeled rhizodepositions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12315, https://doi.org/10.5194/egusphere-egu22-12315, 2022.

The Paleoproterozoic Aravalli Supergroup has an unconformable relationship with the Mewar Gneissic Complex (MGC) of Archean age. The MGC in the Umra area, Rajasthan, India is represented by gneisses, granite, and amphibolite, and has development of a paleosol horizon below the unconformity surface separating it from the Aravalli Supergroup. The Aravalli metasediments have undergone metamorphism up to greenschist facies and have a scattered age of about 2.3-1.8 Ga. The lower part of the Paleoproterozoic succession comprises conglomerates, volcanics, carbonates with microbial mats, and carbonaceous phyllites. These metasediments provide geochemical proxies to the seawater conditions during the Great Oxidation Event (GOE) as these happen to lie on the age range where the Archean-Proterozoic transition occurred.  

Conglomerates present in the lower part of the Aravalli Supergroup are polymictic, clast to matrix-supported, and massive. Euhedral magnetite grains scattered in the matrix indicate a suboxic-anoxic condition of deposition. The overlying carbonates of the area show both negative as well as positive δ¹³C_V-PDB   excursions.  The positive δ¹³C excursion can be correlated with the Lomagundi event. This excursion indicates a warm water condition at the time of the GOE and is supported by the presence of microbial mats in the carbonate unit. On the other hand, the negative excursion may be representative of the Paleoproterozoic glaciation event. The PAAS normalized REE data of the carbonates have HREE>LREE, with a flat to positive Ce* and Eu* anomalies and no significant Y-anomaly, indicating a suboxic-anoxic condition of seawater during deposition in a shallow sea. The carbon phyllite present above the carbonate unit in the area hosts uranium mineralization which is associated with chalcopyrite and pyrite. The Δ³⁴S data of the pyrites show a reducing environmental condition of mineralization. The studies related to the metasediments of the Aravalli Supergroup provide a clue to the seawater conditions that prevailed during the Archean-Proterozoic transition and also provide deeper insights into the Great Oxidation Event.  

How to cite: Goswami, A., Mohanty, S. P., and Sarangi, S.: Sea water chemistry during Archean-Paleoproterozoic transition: Insight from the Aravalli Supergroup, India., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1575, https://doi.org/10.5194/egusphere-egu22-1575, 2022.

The terrestrial carbon cycle is a well researched topic, but for this we basically assume a perennial water flow, which transports the organic carbon along terrestrial flow pathways from the soils to the fluvial network. Climate change will accelerate hydrological processes within water basins and lead to more intermittent catchments, where internal water fluxes will be interrupted and subsurface water flow pathways disturbed due to dry periods. From this perspective, the question arises, what is the impact of an intermittent catchment in the low mountain range on water-soluble carbon transport in the headwaters? To meet this question soil samples were taken seasonally including snowmelt events both in an intermittent and a perennial catchment of a headwater stream over a period of one year. A total number about 700 soil samples were collected in five field campaigns. A transect-based system was used to sample the slopes along the upper slope, middle slope and lower slope. The sample points were chosen to cover all catchment slopes and also the exposures. In the laboratory, the following indicators were determined with the help of a TOC analyser (Shimadzu), C/N analyser (Elementar), photometer (Thermo Fischer) and fluorescence spectrometer (Shimadzu): DOC (WSOC), TOC, SUVA₂₅₄, spectral slopes, BIX, FI and freshness index. The analysis of the comprehensive dataset aimed to build a bridge between the clear changes in the individual events and the annual course and to show how changes, such as the dry fall in the intermittent catchment, 'first flush' effects during heavy rainfall events and new carbon input in autumn through leaf fall impact the spatial and temporal variability of WSOC, which indicates changing of subsurface transport pathways. Preliminary results indicate that indices and total dissolved carbon in the intermittent catchment show differences (factor 0.9) compared to the perennial catchment.

How to cite: Santowski, A. and Chifflard, P.: Spatio-temporal variation of water soluble organic carbon in an intermittent catchment (Hesse, Germany) , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5256, https://doi.org/10.5194/egusphere-egu22-5256, 2022.

Secondary minerals such as iron and aluminum (oxyhydr)oxides are a well-known key factor determining the accumulation and persistence of organic carbon (OC) in soil. Manganese (Mn) oxides, although being less abundant in soil than other oxide minerals, may also bind and stabilize organic matter. In addition, they exhibit a high redox activity that may promote oxidation of refractory organic compounds into substrates easily available to microorganisms. However, little is known about the adsorption and oxidation of dissolved OC (DOC) by Mn oxides. Therefore, we investigated the adsorption of dissolved organic matter (DOM) to vernadite, acid birnessite, and cryptomelane, by varying DOM type (beech and pine-derived), pH (4 and 7), and background electrolyte composition (no salt addition, 0.01 M NaCl or CaCl₂). Preliminary results show that the extent and kinetics of DOM adsorption as well as oxidative DOM transformation strongly differed with Mn oxides and sorption conditions. Overall, DOM adsorption was higher at pH 4 than at pH 7. Vernadite was most sorptive, retaining 68% to 85% of added DOC at pH 4. At pH 7, on average 30% less DOC was adsorbed by Mn oxides. After reaction, reduced specific ultraviolet absorbance at 280 nm of DOM indicates preferential adsorption of aromatic moieties. Contact of DOM with Mn oxides also resulted in high concentrations of dissolved low-molecular-weight (LMW) organic acids, consisting mainly of formic, acetic, oxalic, and citric acid. In addition, we will present results from liquid chromatography/Fourier transform ion cyclotron resonance mass spectrometry and X-ray diffraction on the molecular transformation of reacted DOM and reductive changes of reacted Mn oxides, respectively. Consequently, interactions of DOM and Mn oxides may promote selective sorptive stabilization of organic matter as well as support microbial growth due to oxidative production of easily available organic compounds.

How to cite: Brüggenwirth, L., Lechtenfeld, O., Behrens, R., Kaiser, K., Mikutta, R., and Mikutta, C.: Interactions of dissolved soil organic matter and manganese oxides, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-296, https://doi.org/10.5194/egusphere-egu22-296, 2022.

Dissolved Organic Matter (DOM) in soils has received much research attention over the years. This is not surprising given the important role this most mobile fraction of soil organic matter plays in processes such as pedogenesis, transport and bioavailability of natural and anthropogenic compounds, and the soil’s C cycle. With the increasing advancement of analytical chemical tools, our capabilities of studying the behaviour and interaction of DOM have developed dramatically over the years. Particularly interesting has been the development of advanced molecular characterization tools such as LC-QTOF-MS. However, while showing great promise, the interpretation and aggregation of the vast amounts of data produced by such advanced molecular approaches is a challenge

Here we show how non-target screening by LC coupled to high resolution QTOF-MS detection can be applied to obtain meaningful information about the molecular composition of DOM derived from coniferous and deciduous tree leaf litter material. We highlight both the chemical analysis and the subsequent data interpretation steps needed to arrive at identification of chemical compounds and formulas. For the latter we used a data processing workflow with the in-house developed open-source patRoon software package. As a specific example of the new possibilities opened by this type of detailed characterization methods, we present the results of its application to shed light on the role of DOM in the formation of Podzols

References                        

Brock, O., Helmus, R., Kalbitz, K., & Jansen, B. (2020). European Journal of Soil Science, 71(3), 420-432. https://doi.org/10.1111/ejss.12894

Helmus, R., Ter Laak, T., Van Wezel, A., De Voogt, P. & Schymanski, E. (2021). Journal of Cheminformatics, 13(1). https://doi.org/10.1186/s13321-020-00477-w

How to cite: Jansen, B., Brock, O., and Helmus, R.: LC-QTOF-MS analyses shed new light on dissolved organic matter composition and podzolization, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-909, https://doi.org/10.5194/egusphere-egu22-909, 2022.