Mechanisms of soil organic matter stabilization and carbon sequestration

Soil organic matter (SOM) is well known to exert a great influence on physical, chemical, and biological soil properties, thus playing a very important role in agronomic production and environmental quality. Globally SOM represents the largest terrestrial organic C stock, which can have significant impacts on atmospheric CO2 concentrations and thus on climate. The changes in soil organic C content are the result of the balance of inputs and losses, which strongly depends on the processes of organic C stabilization and protection from decomposition in the soil. This session will provide a forum for discussion of recent studies on the stabilization and sequestration mechanisms of organic C in soils, covering any physical, chemical, and biological aspects related to the selective preservation and formation of recalcitrant organic compounds, occlusion by macro and microaggregation, and chemical interaction with soil mineral particles and metal ions.

Co-organized by BG3/CL3.1
Convener: César Plaza | Co-conveners: Claire Chenu, Beatrice GiannettaECSECS, Claudio Zaccone
vPICO presentations
| Thu, 29 Apr, 14:15–17:00 (CEST)

vPICO presentations: Thu, 29 Apr

Chairpersons: Beatrice Giannetta, César Plaza, Claudio Zaccone
Leiyi Chen, Kai Fang, Bin Wei, Shuqi Qin, Xuehui Feng, Tianyu Hu, Chengjun Ji, and Yuanhe Yang

Elucidating the processes underlying the persistence of soil organic matter (SOM) is a prerequisite for accurately projecting soil carbon feedback to climate change. However, the potential role of plant carbon input in regulating the SOM preservation over broad geographic scales remains unclear. Based on large-scale soil radiocarbon(△14C) measurements from the Tibetan Plateau and International Soil Radiocarbon Database (ISRaD), we found that plant carbon input was the major contributor to topsoil carbon destabilization at the regional and global scales, despite the universal associations of topsoil ∆14C with climatic and mineral variables as well as SOM chemical composition. By contrast, mineral protection by iron-aluminum oxides and cations became more important in preserving SOM in deep soils. These findings illustrate divergent controls of SOM persistence across soil layers, which provide new insights for constraining models to better predict multi-layer soil carbon dynamics under changing environments.

How to cite: Chen, L., Fang, K., Wei, B., Qin, S., Feng, X., Hu, T., Ji, C., and Yang, Y.: Soil carbon persistence governed by plant input and mineral protection at the regional and global scales, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-762,, 2021.

Mario Reichenbach, Peter Fiener, Gina Garland, Marco Griepentrog, Johan Six, and Sebastian Doetterl

Stabilization of organic carbon in soils (SOC) depends on several soil properties, including the soil weathering stage and the mineralogy of parent material. As such, tropical SOC stabilization mechanisms likely differ from those in temperate soils due to contrasting soil development. To better understand these mechanisms, we investigated SOC dynamics at three soil depths under pristine tropical african mountain forest along a geochemical gradient from mafic to felsic and a topographic gradient covering plateau, slope and valley positions. We conducted a series of soil C fractionation experiments in combination with an analysis of the geochemical composition of soil and a sequential extraction of pedogenic oxides. Overall, we found that reactive secondary mineral phases drive SOC properties together with aggregation. These key mineral stabilization mechanisms for SOC were strongly related to soil geochemistry and independent of topography in the absence of detectable erosion processes. We also detected fossil organic carbon (FOC) at several sites, constituting up to 52.0 ± 13.2 % of total SOC stock in the C depleted subsoil. FOC decreased strongly towards more shallow soil depths, indicating decomposability of FOC by microbial communities under topsoil conditions. Regression analysis showed that variables affiliated with soil weathering, parent material geochemistry and soil fertility, together with soil depth, explained up to 75 % of the variability of SOC stocks and Δ14C. Furthermore, the same variables explained 44 % of the variability in the relative abundance of C associated with microaggregates versus free silt and clay associated C fractions. We conclude that despite long-lasting weathering, geochemical properties of soil parent material leave a footprint in tropical soils that affects SOC stocks and links two of the most important mineral related C stabilization mechanisms. While the identified stabilization mechanisms and controls are similar to less weathered soils in other climate zones, their relative importance is markedly different in the investigated tropical soils.

How to cite: Reichenbach, M., Fiener, P., Garland, G., Griepentrog, M., Six, J., and Doetterl, S.: Organic carbon stabilization controlled by geochemistry in tropical rainforest soils, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7323,, 2021.

Peng Yu, Xiaolu Tang, Yuehong Shi, Chunju Cai, Yuxuan Han, Zhou Li, Luo Deng, Ci Song, and Jingji Li

Soil organic carbon turnover time (τ, year) is an important indicator of soil carbon stability and sequestration capacity. However, our understanding of the spatial heterogeneity of subsoil τ still was poorly qualified over a large scale, even though subsoil organic carbon below 0.2 m accounts for the majority of total soil organic carbon. We compiled a dataset that consisted of 630 observations in subsoil (0.2 - 1 m) from published literatures to investigate the spatial heterogeneity of subsoil τ (defined as the ratio of soil carbon stock and net primary production) and explore its main environmental drivers using structural equation modelling (SEM) in forest ecosystems across China. Results indicated that mean (± standard deviation) subsoil τ was 72.4 ± 68.6 years with a large variability ranging from 2.3 to 896.2 years. Subsoil τ varied significantly with forest types that mean subsoil τ was the longest in deciduous broadleaf forest (82.9 ± 68.7 years), followed by evergreen needleleaf forest (77.6 ± 60.8 years), deciduous needleleaf forest (75.3 ± 78.6 years) and needleleaf and broadleaf mixed forest (71.3 ± 80.9 years), while the shortest τ in evergreen broadleaf forest (59.9 ± 40.7 years). SEM suggested that soil environment was the most important factor in predicting subsoil τ. However, the dominant driver differed with forest types, i.e. soil environment for evergreen broadleaf forest and climate for evergreen needleleaf forest. This study highlights the different dominant controlling factors in subsoil τ and improve our understanding of biogeographic variations of subsoil τ. These findings are essential to better understand (and reduce uncertainty) in biogeochemical models of subsoil carbon dynamics at regional scales.

How to cite: Yu, P., Tang, X., Shi, Y., Cai, C., Han, Y., Li, Z., Deng, L., Song, C., and Li, J.: Spatial heterogeneity of subsoil organic carbon turnover times in forest ecosystems across China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1902,, 2021.

Yuehong Shi, Xiaolu Tang, Peng Yu, Li Xu, Guo Chen, Longxi Cao, Ci Song, Chunju Cai, and Jingji Li

Soil carbon turnover time (τ, year) is an important indicator of soil carbon stability, and a major factor in determining soil carbon sequestration capacity. Many studies investigated τ in the topsoil or the first meter underground, however, little is known about subsoil τ (0.2 – 1.0 m) and its environmental drivers, while world subsoils below 0.2 m accounts for the majority of total soil organic carbon (SOC) stock and may be as sensitive as that of the topsoil to climate change. We used the observations from the published literatures to estimate subsoil τ (the ratio of SOC stock to net primary productivity) in grasslands across China and employed regression analysis to detect the environmental controls on subsoil τ. Finally, structural equation modelling (SEM) was applied to identify the dominant environmental driver (including climate, vegetation and soil). Results showed that subsoil τ varied greatly from 5.52 to 702.17 years, and the mean (± standard deviation) subsoil τ was 118.5 ± 97.8 years. Subsoil τ varied significantly among different grassland types that it was 164.0 ± 112.0 years for alpine meadow, 107.0 ± 47.9 years for alpine steppe, 177.0 ± 143.0 years for temperate desert steppe, 96.6 ± 88.7 years for temperate meadow steppe, 101.0 ± 75.9 years for temperate typical steppe. Subsoil τ significantly and negatively correlated (p < 0.05) with vegetation index, leaf area index and gross primary production, highlighting the importance of vegetation on τ. Mean annual temperature (MAT) and precipitation (MAP) had a negative impact on subsoil τ, indicating a faster turnover of soil carbon with the increasing of MAT or MAP under ongoing climate change. SEM showed that soil properties, such as soil bulk density, cation exchange capacity and soil silt, were the most important variables driving subsoil τ, challenging our current understanding of climatic drivers (MAT and MAP) controlling on topsoil τ, further providing new evidence that different mechanisms control topsoil and subsoil τ. These conclusions demonstrated that different environmental controls should be considered for reliable prediction of soil carbon dynamics in the top and subsoils in biogeochemical models or earth system models at regional or global scales.

How to cite: Shi, Y., Tang, X., Yu, P., Xu, L., Chen, G., Cao, L., Song, C., Cai, C., and Li, J.: Subsoil organic carbon turnover is dominantly controlled by soil properties in grasslands across China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2413,, 2021.

Guanghui Yu

Plant residues in soil create temporal and spatial hotspots of extremely high microbial activities leading to very intensive greenhouse gas (GHG) fluxes that challenge our mechanistic understanding and predictive power. Using a series of well-controlled soil microcosm experiments, we examine how abiotic processes (e.g., iron reduction-oxidation cycling) at residue/soil interfaces contribute to hotspot dynamics. We quantify for the first time the contributions of microbially-initiated Fenton reactions, which produce strongly oxidizing hydroxyl radicals (HO•), to organic matter solubilization and mineralization in hotspots 0–3 mm from the litter surface. The concentrations of ferrous iron (Fe2+), hydrogen peroxide (H2O2) and HO• were 2.1–3.0, 3.0–9.0 and 2.6–2.8 times higher, respectively, at the straw-soil interface than in the bulk soil. Thus, iron minerals, especially in concert with microorganisms, produce a burst of hydroxyl radicals that explain extremely high GHG fluxes from soil hotspots. Our findings highlight how Fe minerals and microorganisms synergistically influence global carbon cycling and stability. Our findings highlight the relevance of free radical-related mechanisms in soil to the cycling, stabilization, and storage of carbon and also extend our mechanistic understanding of processes occurring within hotspots.

How to cite: Yu, G.: Free radical-related mechanisms in soil and their relevance  to the cycling, stabilization, and storage of carbon, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3576,, 2021.

Alice Puppin, Marcella Roner, Alvise Finotello, Massimiliano Ghinassi, Laura Tommasini, Marco Marani, and Andrea D'Alpaos

Salt-marsh evolution importantly depends on complex feedbacks between hydrodynamic, morphological, and biological processes. These crucial ecogeomorphic structures support a diverse range of ecosystem services, including coastal protection and biodiversity increase. In addition, they are among the most carbon‐rich ecosystems on Earth, as their high primary production coupled with rapid surface accretion results into the ability to sequester atmospheric carbon at high rates. However, salt-marsh future is at risk today, due to the effects of climate changes and local anthropogenic disturbances, in particular sea-level rise and reduced fluvial sediment delivery to the coasts. The organic matter captured and stored by salt marshes results from the balance between inputs and outputs and may contribute to marsh surface accretion, which determines their ability to keep pace with sea-level rise. Therefore, a better understanding of the processes regulating organic matter dynamics on salt marshes is a critical step to elucidate their carbon sink potential and to address salt-marsh management and conservation issues. Toward this goal, we analysed organic matter decomposition processes within salt-marsh ecosystems by burying 712 commercially available tea bags within different marshes in the Venice Lagoon (Italy), following the Tea Bag Index protocol. The process provides the values of two key parameters: the decomposition rate (k) and litter stabilisation factor (S). Based on standardized litter bag experiments, the Tea Bag Index focuses on the effects of abiotic conditions, neglecting litter-quality influences. The mean values of the decomposition metrics from our analyses are in general consistent with previous results and indicate a quite fast decomposition of the organic matter with a remaining mass of about 34% of the initial labile mass after 90 days. We next explore the possible dependence of k and S on environmental drivers. Temperature showed the most significant relationship with decomposition processes, suggesting an organic-matter decay acceleration with warming temperature, in line with previous literature. Moreover, the statistical analysis indicated some significant trends of the decomposition rate also with surface elevation and distance from the marsh edge. This suggests that, at the marsh scale, higher and probably less frequently flooded sites are exposed to faster decomposition, likely due to greater oxygen availability enhancing microbial respiration. In conclusion, the organic matter decay we observed is rapid enough to consume all the labile material before it can be buried and stabilized, hence increased global temperatures may not have a significant effect in increasing organic matter decomposition in coastal marshes. Therefore, we argue that, at least in the short term, the remaining mass of the organic matter contributing to carbon sequestration and marsh accretion, strongly depends on the initial litter quality, recalcitrant or labile, which may differ considerably between different species and plant parts and may be affected by climate change effects.

How to cite: Puppin, A., Roner, M., Finotello, A., Ghinassi, M., Tommasini, L., Marani, M., and D'Alpaos, A.: Characterization of organic matter decomposition in the Venice Lagoon using the Tea Bag Index, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3355,, 2021.

Adriano Sofo, Luca Zurlo, Giuseppe Vitale, and Assunta Maria Palese

Olive is a widespread crop within Mediterranean area and Italy is one of the biggest producer of olives and oil in the world. From an environmental point of view, centered on carbon (C) sequestration, managing olive orchards sustainably is an urgent and actual issue.

This trial was done in a 2-ha olive orchard (Olea europaea L., cv. ‘Maiatica’; 70-year-old plants, with a distance of 8 × 8 m and NE orientation) located in Ferrandina (Southern Italy, Basilicata region; N 40°29’; E 16°28’). The soil is a sandy loam (Haplic Calcisol - WRB), with a mean bulk density of 1.30 g cm–3 and sediment as parental material. The major landform is plain, the slope form is classified as convex-straight and the gradient class as gently sloping (2-5%). Half of the orchard has been managed using sustainable agricultural practices (sustainable management, Sung) for 20 years (2000-2020). Trees were drip-irrigated from March to October with urban wastewater. A light pruning was carried out every year during winter. The soil was permanently covered by spontaneous self-seeding weeds, mowed twice a year. Cover crop residues and prunings were shredded and left along the row as mulch.

The other half of the orchard was kept as ‘control’ plot. It was rainfed and conducted with a locally conventional management (Cmng), according to the practices usually adopted by farmers. The Cmng was managed by tillage performed 2-3 times per year to control weeds. Intensive pruning was carried out every two years, but pruned residues were removed from the orchard. A mineral fertilization was carried out once per year, during the fruit set and pit hardening phase (early spring).

The average value (n = 5; 0-100 cm soil depth) of baseline soil organic carbon (SOC) stock (related to the Cmng) in the 20-year period was 4.79 t SOC ha–1, with an average additional SOC storage potential because of the adoption of the Smng of 0.15 t SOC ha–1 yr–1, and a SOC stock after 20 years of Smng of 7.75 t SOC ha–1 yr–1.

In the Smng system, soil acted as a significant sink for C, especially due to the supplies of the organic resources internal to the system. The Smng system, made up of mature olive trees, was also able to fix in its aboveground and belowground components, a > 2-times higher total amount of C than the Cmng. Spontaneous vegetation was the most important pool, sequestering about 35% of the total fixed C. Also pruning material had a substantial importance in C fixation. Emissions of CO2 eq per kg of olives, calculated according to the Life Cycle Assessment (LCA), were 0.08 kg in the Smng system and 0.11 kg in the Cmng system. Besides C sequestration, the application of the Smng markedly improved physical, chemical, and biological soil fertility, with benefits on plants and production.

The application of a sustainable soil and plant management makes olive growing a multifunctional rural activity, not only aimed at production, but including many other objectives, such as environmental, landscaping cultural, social and recreational.

How to cite: Sofo, A., Zurlo, L., Vitale, G., and Palese, A. M.: Carbon sequestration in a Mediterranean olive orchard managed sustainably over a 20-year period, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1488,, 2021.

Chairpersons: Claire Chenu, Claudio Zaccone, César Plaza
Moritz Mohrlok, Victoria Martin, Alberto Canarini, Wolfgang Wanek, Michael Bahn, Erich M. Pötsch, and Andreas Richter

Soil organic matter (SOM) is composed of many pools with different properties (e.g. turnover times) which are generally used in biogeochemical models to predict carbon (C) dynamics. Physical fractionation methods are applied to isolate soil fractions that correspond to these pools. This allows the characterisation of chemical composition and C content of these fractions. There is still a lack of knowledge on how these individual fractions are affected by different climate change drivers, and therefore the fate of SOM remains elusive. We sampled soils from a multifactorial climate change experiment in a managed grassland in Austria four years after starting the experiment to investigate the response of SOM in physical soil fractions to temperature (eT: ambient and elevated by +3°C), atmospheric CO2-concentration (eCO2: ambient and elevated by +300 ppm) and to a future climate treatment (eT x eCO2: +3°C and + 300 ppm). A combination of slaking and wet sieving was used to obtain three size classes: macro-aggregates (maA, > 250 µm), micro-aggregates (miA, 63 µm – 250 µm) and free silt & clay (sc, < 63 µm). In both maA and miA, four different physical OM fractions were then isolated by density fractionation (using sodium polytungstate of ρ = 1.6 g*cm-3, ultrasonication and sieving): Free POM (fPOM), intra-aggregate POM (iPOM), silt & clay associated OM (SCaOM) and sand-associated OM (SaOM). We measured C and N contents and isotopic composition by EA-IRMS in all fractions and size classes and used a Pyrolysis-GC/MS approach to assess their chemical composition. For eCO2 and eT x eCO2 plots, an isotope mixing-model was used to calculate the proportion of recent C derived from the elevated CO2 treatment. Total soil C and N did not significantly change with treatments.  eCO2 decreased the relative proportion of maA-mineral-associated C and increased C in fPOM and iPOM. About 20% of bulk soil C was represented by the recent C derived from the CO2 fumigation treatment. This significantly differed between size classes and density fractions (p < 0.001), which indicates inherent differences in OM age and turnover. Warming reduced the amount of new C incorporated into size classes. We found that each size class and fraction possessed a unique chemical fingerprint, but this was not significantly changed by the treatments. Overall, our results show that while climate change effects on total soil C were not significant after 4 years, soil fractions showed specific effects. Chemical composition differed significantly between size classes and fractions but was unaffected by simulated climate change. This highlights the importance to separate SOM into differing pools, while including changes to the molecular composition might not be necessary for improving model predictions.    

How to cite: Mohrlok, M., Martin, V., Canarini, A., Wanek, W., Bahn, M., Pötsch, E. M., and Richter, A.: Effects of climate change on soil organic matter chemical composition and carbon content in different physical fractions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9216,, 2021.

Sophie F. von Fromm, Alison M. Hoyt, Benjamin M. Butler, Asmeret Asefaw Berhe, Sebastian Doetterl, Stephan M. Haefele, Steve P. McGrath, Keith D. Shepherd, Johan Six, Erick K. Towett, Leigh A. Winowiecki, and Susan E. Trumbore

Recent compilations of global soil radiocarbon data suggest that current Earth System Models underestimate the mean age of soil carbon (C). The discrepancy between data-derived estimates and model calculations might be due to an inadequate representation of processes that control C persistence in soils – especially in understudied regions.

Here, we investigate the relationships between soil mineralogy, soil properties, climate and radiocarbon (Δ14C) in soils sampled as part of a comprehensive soil survey (AfSIS) for sub-Saharan Africa. A total of 510 samples were analyzed, comprised of soils collected from two depths (0–20 cm and 20–50 cm) at 30 sites in 14 countries. To determine soil mineralogy, we analyzed X-ray powder diffraction (XRPD) data, which provides a precise and detailed mineralogical signature of each soil sample. The studied soil profiles vary greatly in their mineralogy, reflecting a diverse range of parent materials and soil forming factors.

The median soil C age is 182 years in the topsoils and 563 years in the subsoils, corresponding to a total Δ14C value range of -432 to 95 ‰. In general, Δ14C values decrease (older mean C ages) with increasing clay particle size fractions. This corresponds to an increase in short range-order minerals expressed as oxalate-extractable aluminum and iron (Alox and Feox). Separately, mineralogically defined variables – derived from the XRPD data using principal component analysis – are found to correlate strongly with a range of soil properties (pH, weathering status, exchangeable calcium, Alox and Feox, and soil texture) and climatic variables (aridity index and mean annual temperature). This provides a holistic assessment of the processes that have formed each soil along with the properties that it currently exhibits. Our analyses with random forests show that these XRPD-derived mineralogical variables alone can explain up to 30% of the variation in Δ14C across sub-Saharan Africa. They also allow the identification of specific minerals that contribute to this variation and how they are linked to the C mean age of the soil. In conclusion, our results suggest that soil mineral data can help to better understand C persistence in subtropical and tropical soils.

How to cite: von Fromm, S. F., Hoyt, A. M., Butler, B. M., Berhe, A. A., Doetterl, S., Haefele, S. M., McGrath, S. P., Shepherd, K. D., Six, J., Towett, E. K., Winowiecki, L. A., and Trumbore, S. E.: Soil carbon persistence linked to mineralogy across sub-Saharan Africa, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10703,, 2021.

Vjacheslav Polyakov and Evgeny Abakumov

Black carbon is one of the short-lived climatically significant factors. This term refers to climate-forming substances that are located for a short amount of time in the atmosphere - from several days to several years. To identify the role of cryoconite in the conditions of a possible climatic crisis, the stabilization of organic matter isolated from cryoconite holes was assessed. Humic acids are part of the organic matter accumulating in soils and cryoconites and are heterogeneous systems of high-molecular condensed compounds formed as a result of the decomposition of organic remains of plants and animals in terrestrial and aquatic ecosystems. Climatic parameters, precursors of humification, and the local position in the landscape determine the diversity of the composition and properties of HAs. Stabilization of organic material is defined as the transformation of organic matter into a state inaccessible to soil microorganisms, and the very property of stabilization is a characteristic stage in the dynamics of carbon. Using 13C NMR spectroscopy, we determined the proportion of aromatic and aliphatic compounds in the composition of HAs in order to assess the stabilization of organic matter in cryoconites from Mount Elbrus (Caucasus Mountains, Russia), the Arctic (Severnaya Zemlya archipelago, Russia) and Antarctica (King George Island, West Antarctica).

Samples for qualitative analysis of carbon accumulated in cryoconites were carried out during fieldwork in 2020. The studied samples were analyzed at the Department of Applied Ecology, St. Petersburg State University. Humic acids (HAs) were extracted from each sample according to a published IHSS protocol. Solid-state CP/MAS 13C-NMR spectra of HAs were measured with a Bruker Avance 500 NMR spectrometer.

Thus, it follows from the obtained results that aliphatic fragments of humic acids predominate in all studied cryoconites. A similar composition of humic acids testifies to a single mechanism of accumulation and development of organic matter in glacier regions. Low biological activity and climatic features prevent condensation of high-molecular compounds in the organic matter of cryoconite holes. This is an essential prerequisite for high rates of carbon dioxide emissions into the atmosphere under the conditions of deglaciation of the studied regions. With the thawing of glaciers and the ingress of cryoconites into warmer conditions, an additional contribution of carbon dioxide to the atmosphere can occur and, therefore, increase the possible climate crisis on our planet.

This study was supported by Russian Foundation for Basic Research No. 19-05-50107.

How to cite: Polyakov, V. and Abakumov, E.: Stabilization of organic carbon isolated from cryoconite holes in polar and mountain systems by 13С NMR spectroscopy, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4156,, 2021.

Haichao Li, Jan Van den Bulcke, Orly Mendoza, Heleen Deroo, Geert Haesaert, Kevin Dewitte, Stefaan De Neve, and Steven Sleutel

Soil organic carbon (OC) levels generally increase with increasing clay and silt content under a similar climatic zone because of increased association of OC to clay minerals and stronger occlusion inside aggregates. Surprisingly though, in Western Europe many silt loam soils actually bear low topsoil OC levels compared to lighter textured soils. Soil texture obviously also strongly controls moisture availability with consequent indirect impact on heterotrophic activity. We hypothesized that with increasingly frequent summer drought: 1) soil microbial activity in sandy soils is more likely impeded due to their limited water holding capacity retention during droughts, while soil OC mineralization in silty soils remain be less drought-limited; 2) capillary rise from sufficiently shallow groundwater would, on the other hand, alleviate the water stress in lighter textures. To test these hypotheses, we established a one-year field trial with manipulation of soil texture, monitoring of soil moisture and maize-C decomposition via 13/12C-CO2 emissions. The upper 0.5 m soil layer was replaced by sand, sandy loam and silt loam soil with low soil OC. Another sandy soil treatment with a gravel layer was also included beneath the sand layer to exclude capillary rise. Soil texture did not affect maize-C mineralization (Cmaize-min) until April 2019 and thereafter Cmaize-min rates were higher in the silt loam than in the sandy soils (P=0.01). θv correlated positively with the Cmaize-min rate for the sand-textured soils only but not for the finer textures. These results clearly highlight that soil texture controlled Cmaize-min indirectly through regulating moisture under the field conditions starting from about May, when soils faced a period of drought. By the end of the experiment, more added Cmaize was mineralized in the silt loam soil (81%) (P<0.05) than in the sandy soil (56%). Capillary rise did not result in a significant increase in cumulative Cmaize-min in the sandy soil, seemingly because the capillary fringe did not reach the sandy topsoil layer. These results imply that, under future climate scenarios the frequency of drought is expected to increase, the largely unimpeded microbial activity in silty soils might lead to a further stronger difference in soil OC with coarser textured soils under similar management.

How to cite: Li, H., Van den Bulcke, J., Mendoza, O., Deroo, H., Haesaert, G., Dewitte, K., De Neve, S., and Sleutel, S.: Soil texture can predominantly control organic matter mineralization in temperate climates by regulating soil moisture rather than through direct stabilization, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4424,, 2021.

Simone Pesce, Enrico Balugani, Josè Miguel De Paz, Fernado Visconti, Carlotta Carlini, and Diego Marazza

In the context of sustainable development, agriculture holds a promising potential for CO2 sequestration and, accordingly, for the mitigation of climate change. This potential capacity can be developed through the adoption of less conventional farming techniques, such as the mulching of the topsoil with agricultural by-products where they are available, e.g., rice straw in the semiarid Valencia province (Eastern Spain). In general, the use of straw as mulching material has been found beneficial for soil quality as it reduces temperature excursions both daily and yearly, increases soil water content overall, and increases the activity of microbes. Moreover, it encourages the binding of organic matter and mineral particles into macro and micro aggregates, leading to: enhancement of the aggregate stability, restoration of stable C, and increase in the soil organic carbon (SOC) content and, thus, soil carbon sequestration. SOC dynamic models, like the widely used RothC, are useful to assess the soil carbon sequestration potential of different agricultural practices and to project their effects on the long term. However, there is a lack of studies focusing on the modelling of straw mulch effects on SOC dynamics.

Our work aimed at modelling the rice straw mulch degradation and its effects on the SOC dynamics in two citrus orchards, as observed during a short-term field experiment (2 years). In the orchards, the straw mulch was applied to the inter-rows once a year, and its effects on soil water content, temperature, respiration rate, and SOC contents (amidst other chemical and biological parameters) were compared with bare soil and natural grass formation

The RothC carbon dynamics model was modified by including the straw mulch effects on SOC dynamics as observed on the field and, additionally, by modelling the soil water dynamics with the HYDRUS1D model. The SOC pools for the RothC simulations were assessed following the fractionation of Zimmerman et al. (2007). The model parameters were calibrated with the soil respiration data.

The straw mulch model can be used for the estimation of the effects of the rice straw on the SOC in the short term. By changing the soil, climatic and agricultural practices inputs, the model can be applied to different fields in semiarid conditions, allowing the assessment of the soil carbon sequestration potential of different agricultural practices. However, the model still needs to be verified on long term field studies to deliver reliable long term sequestration projections.

How to cite: Pesce, S., Balugani, E., De Paz, J. M., Visconti, F., Carlini, C., and Marazza, D.: Modelling of soil carbon sequestration by use of rice-straw mulching in two citrus orchards in Valencia (Spain), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6323,, 2021.

Pengzhi Zhao, Daniel J. Fallu, Sara Cucchiaro, Paolo Tarolli, Clive Waddington, David Cockcroft, Lisa Snape, Andreas Lang, Sebastian Doetterl, Antony G. Brown, and Kristof Van Oost

Being the most common and widest spread man-made landform, terrace construction has resulted in an extensive perturbation of the land surface. Our mechanistic understanding of the underlying soil organic carbon (SOC) (de-)stabilization mechanisms and of the persistence of SOC stored in terraced soils, however, is far from complete. Here we explored the factors controlling SOC stability and temperature sensitivity (Q10) of heterotrophic soil respiration of abandoned prehistoric agricultural terrace soils in NE England. For this we combined soil fractionation and temperature sensitive incubation experiments under idealized, well-aerated topsoil conditions with measurements of terrace soil burial age. Results showed that a substantial part of the SOC stock in these terraced soils (43.5± 5.5%) was found in buried horizons. A significantly lower soil potential respiration was observed for buried terrace soils, relative to a control (non-terraced) profile. This suggests that the burial of soils is an important mechanism to slow down the decomposition of SOC in terraced soils. Furthermore, we observed a shift in the SOC pool composition from particulate organic C to mineral carbon mineral protected C with increasing burial age creating energetic barriers for microorganisms to overcome. This clear shift to more processed recalcitrant SOC with terrace soil burial age also contributes to SOC stability in terraced soils. Temperature sensitivity incubations revealed that as terraced and buried soil becomes older, lower C quality in buried horizons leads to an increase in temperature sensitivity of SOC. In conclusion, terracing in our study site has stabilized SOC as a result of soil burial during terrace construction with evolution to a more biologically processed SOC pool with increasing terrace soil burial age. These depth-age patterns of Q10 and SOC pool composition of terraced soils should be considered when assessing the effects of climate warming or terrace abandonment/removal on the terrestrial C cycle

How to cite: Zhao, P., J. Fallu, D., Cucchiaro, S., Tarolli, P., Waddington, C., Cockcroft, D., Snape, L., Lang, A., Doetterl, S., G. Brown, A., and Van Oost, K.: SOC stabilization mechanisms and temperature sensitivity in old terraced soils, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9099,, 2021.

Han Lyu, Tetsuhiro Watanabe, Ruohan Zhong, Method Kilasara, Arief Hartono, and Shinya Funakawa

Clarifying the controlling factors for soil organic carbon (SOC) stabilization is a primary issue in mitigating climate change. However, the mechanisms controlling soil carbon cycle are not well-understood, especially in tropical regions. Furthermore, the mechanisms are expected to differ between topsoil and subsoil. The objectives were to clarify the controlling factors for SOC pools partitioned by their stabilities, then to compare the differences in pools and controlling factors between topsoil and subsoil.

Both top (0–15 cm) and subsoil (20–40 cm) samples were collected at volcanic regions of Tanzania and Indonesia along an elevation gradient under mostly undisturbed vegetation (23 sites). A kinetic model, including labile, intermediate, and stable pools, was fitted to accumulative SOC mineralization curve obtained from 343-day incubation to determine the sizes of the labile and intermediate SOC pools (CL and CI) and their mean residence times, where the size of the stable SOC pool (CS) was measured as non-hydrolyzable carbon by fractionation. Correlation and path analyses were conducted to determine the controlling factors for each SOC pool, using the results of the model fitting and SOC fractionation and the data on climate, geochemistry, and biology (e.g., mean average temperature and precipitation, nanocrystalline mineral content (Alo+1/2Feo), and microbial biomass, respectively).

The intermediate pool (56.2 ± 10.4% of SOC) predominantly contributed to the storage and stability of total SOC (10 to 157 g kg−1) for both topsoil and subsoil with the mean residence time of years to decades (3400 to 31500 days). For both topsoil and subsoil, Alo+1/2Feo was strongly correlated with CI and CS, suggesting that organo-mineral complexation is a predominant factor that controls the intermediate and stable SOC pools, rather than soil pH or texture. Also, temperature negatively affected the sizes of all three pools, which indicates the low temperature retards the decomposition of all parts of SOC. The labile SOC pool was more controlled by biotic and climatic factors (i.e., microbial biomass and excess precipitation). Concerning differences between topsoil and subsoil, SOC was more in the intermediate than in the stable pool, and the effect of temperature on CS was more substantial in the subsoil. Moreover, Alo+1/2Feo controlled the mean residence time of the intermediate SOC pool, indicating the stability of subsoil SOC that had a labile nature would be more dependent on nanocrystalline minerals.

While temperature widely influences all SOC pools, geochemical factors control more stable pools and total SOC storage, whereas biotic factors and moisture mainly alter relatively labile SOC pools. The subsoil SOC would be more sensitive to climate change than topsoil SOC. The findings helped to understand SOC stabilization mechanisms for both top and subsoils in tropical volcanic regions.

How to cite: Lyu, H., Watanabe, T., Zhong, R., Kilasara, M., Hartono, A., and Funakawa, S.: Effect of Climate and Geochemistry on Organic Carbon Pools in Top and Subsoils of Tropical Volcanic Regions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9355,, 2021.

Hyeonji Song, Snowie Galgo, Ronley Canatoy, Hogyeong Chae, and Pil Joo Kim

Soil C sequestration is widely regarded as the most reasonable way to mitigate global warming. Traditionally, a high amount of organic carbon (OC) input is strongly recommended to increase soil organic carbon (SOC) stocks in croplands. However, according to the whole-soil saturation theory, stable SOC (mineral-associated SOC) accumulation can be limited at a certain point, relying on silt and clay contents. Most studies based on the theory were conducted in aerobic soil condition. This relationship is still uncertain in a rice paddy that makes up 10.8% of total arable land and has an anaerobic soil environment. In this study, we investigated high OC addition can enhance soil C sequestration in a rice paddy. We added different OC levels (0.5, 2.0, 2.9, and 4.6 Mg C ha-1 yr-1) in rice paddy by incorporating cover crop biomass for nine years. SOC stock and soil saturation degree were determined. Unprotected, sand-associated, silt-associated, and clay-associated SOC were separated via density and size fractionation. Respired C losses (CO2-C and CH4-C) were monitored using the static closed chamber method. SOC stock did not linearly increase with higher amount of OC input. The carbon sequestration efficiency (i.e. the increase of SOC per unit of OC input) decreases with the amount OC added. Higher OM input significantly increased unprotected labile SOC content. Unprotected SOC (<1.85 g cm-3) exponentially increased as the SOC saturation degree was higher. On the other hand, stable SOC content did not exhibit a linear relationship with the SOC saturation degree. The higher OC addition level exponentially increased respired C loss. In particular, C loss via CH4 was more sensitive to high OC addition. We conclude that higher OC addition in rice paddy without consideration in terms of SOC stock saturation point can accelerate global warming by increasing labile SOC accumulation and CH4 emission.

How to cite: Song, H., Galgo, S., Canatoy, R., Chae, H., and Kim, P. J.: High organic carbon input can accelerate global warming in rice paddy soil: increase unprotected soil organic carbon and CH4 emission, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9438,, 2021.

Marius Mayer, Andreas Fliessbach, Paul Mäder, and Markus Steffens

Soils contain more carbon (C) in the form of organic matter (soil organic matter = SOM) than the entire atmosphere and global vegetation combined. They are a central component of the global C cycle and its largest dynamic reservoir. Smart agricultural practices are discussed, on the one hand, as a way to mitigate climate change because they can increase the amount of SOM and thus actively remove C from the atmosphere. On the other hand, all intensively used soils lose C in the long term. The scientific key questions in this context revolve around the extent and dynamics of C storage, as well as the associated stabilization mechanisms involved and effects of agricultural use on the C budget.

The DOK experiment is a long-term agronomic field trial near Basel (Switzerland) that compares biodynamic, organic and conventional management systems since 40 years. Within the "DynaCarb" project, we investigate how the management systems affect SOM fractions during the 40-year experimental period. We compare the unfertilized control to a purely mineral, a purely organic, and a combined fertilized, mineral-organic variant (four field replicates each) during six crop rotation cycles. By using a combined density and particle size fractionation, the SOM is separated into particulate and mineral-associated fractions and their development is quantitatively investigated in archived samples from 1982, 1989, 1996, 2003, 2010, and 2017.

"DynaCarb" investigates the medium- and long-term effects of different agricultural systems on SOM. These results are of great importance for the evaluation of the C sequestration potential of agricultural soils and for the identification of suitable management and fertilization strategies.

How to cite: Mayer, M., Fliessbach, A., Mäder, P., and Steffens, M.: Carbon distribution between density and particle size classes of differently managed soils in a 40-year agronomic long-term trial, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9801,, 2021.

Carlos Romero, Xiying Hao, Paul Hazendonk, Timothy Schwinghamer, Martin Chantigny, Simon Fonteyne, and Nele Verhulst

Managing croplands for increased storage of soil organic matter (SOM) is a critical step towards developing resilient farming systems in a changing climate. We examined SOM dynamics in a wheat (Triticum durum L.) – maize (Zea mays L.) irrigated bed planting system established near Ciudad Obregón, Sonora, Mexico. Soil samples (0 – 15 cm) were collected from conventionally tilled raised beds (CTB) with all crop residues incorporated (CTB-I) and permanent raised beds (PB) with crop residues burned (PB-B), removed (PB-R), partly retained (PB-P) or fully retained (PB-K) receiving 0, 150 or 300 kg N ha-1, and analyzed for organic C (OC), total N (TN) and δ13C in whole-soil, light fraction (LF) and coarse- (sand) and fine- (silt and clay) mineral-associated organic matter (MAOM). Results indicated that PB-K and PB-B increased soil OC (P < 0.05) in whole-soil relative to CTB-I, mainly through increases in sand- and silt-size MAOM, respectively. Similarly, N-fertilization increased soil OC and TN contents in whole-soil, coarse-MAOM, and fine-MAOM, but not in the LF pool. Soil δ13C was higher (P < 0.05) in PB-K (-20.18‰) relative to PB-B (-20.67‰), possibly due to the stabilization of partly decomposed maize-C in silt- and clay-size MAOM. The composition of SOM surveyed by CPMAS 13C NMR was not affected by tillage-residue management and roughly consisted of 35% O-alkyl-C, 31% alkyl-C, 24% aromatic-C, and 10% carboxyl-C. Our results indicate that long-term PB-K and PB-B adoption increased surface soil OC contents relative to CTB-I, even though pathways of SOM stabilization differed between systems. Under PB-K, accumulation of fine-MAOM was mostly related to straw-C inputs, whereas in PB-B it was closely associated with black-C precursors. Fine-MAOM appeared responsive to crop residue management and should be therefore considered when analyzing mechanisms of SOM stabilization in irrigated croplands.

How to cite: Romero, C., Hao, X., Hazendonk, P., Schwinghamer, T., Chantigny, M., Fonteyne, S., and Verhulst, N.: Tillage-residue management affects the distribution, storage and turnover of mineral-associated organic matter – A case study from northern Mexico , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2986,, 2021.

Shane Stoner, Carlos Sierra, Marion Schrumpf, Sebastian Dötterl, and Susan Trumbore

Soil organic matter (SOM) is a complex collection of organic molecules of varying origin, structure, chemical activity, and mineral association. A wide array of laboratory methods exists to separate SOM based on qualitative, biological, chemical, and physical characteristics. However, all present conceptual and logistical limitations, including the requirement of a substantial amount soil material.

An newly applied alternative method of fractionation relies on a conceptual analogue between biochemical stability in soil and thermal stability, e.g. more persistent SOM will require higher temperatures (greater energy inputs) to decompose than less persistent SOM. This accounts for both chemical complexity and mineral association as main factors in determining SOM persistence.

In this method, carbon is released by heating SOM to 900°C at a constant rate. The peaks of carbon release are grouped into activation energy pools, CO2 is collected, and analyzed for 13C and 14C. We seek to describe in finer detail the distribution of soil radiocarbon by adding another fractionation step following a different paradigm of SOM stability, and explore mineralogical effects on SOM quality and stability using thermal analysis, radiocarbon, and gas chromatography.

Here, we analyzed bulk soil and soil fractions derived from density separation and chemical oxidation, as well as mineral horizons dominated by diverse mineralogies. Density fractions contained a wide range of radiocarbon activities and that young SOM is stabilized across multiple fractions, likely due to organomineral complexation. Initial results showed that soil minerals with limited stabilization potential released C at lower temperatures than those with diverse stabilization mechanisms. High-temperature sub-fractions contained the oldest carbon across fractions and minerals, thus supporting the assumption that thermal stability can be used as a limited analogue for stability in soil. We present a fine-scale distribution of radiocarbon in SOM and discuss the potential of this method for comparison with other fractionation techniques.

How to cite: Stoner, S., Sierra, C., Schrumpf, M., Dötterl, S., and Trumbore, S.: Thermal fractionation of soil organic matter produces fine-scale distributions of SOM age, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15968,, 2021.

Laura Scrano, Francesca Mottola, Cosimo Mario Stefanelli, Filomena Lelario, Giuliana Bianco, and Sabino Aurelio Bufo

Advanced techniques have been recently used to obtain information on Natural Organic Matter (NOM). However, the current knowledge of the chemical structure of humic substances (HS) is still incomplete. These substances appear to be too complex mixtures of charged organic molecules, and their characterization remains one of the most stimulating challenges in modern environmental science. Knowledge of the chemical composition of NOM is of great importance for the definition of soil and water properties because it has a significant impact on the understanding of numerous molecular and global-scale processes.
This study aims to apply two-dimensional graphical methods to resolve homologous series in mass spectra of humic extracts (Suwannee River, Nordic Aquatic and Soil) obtained using FT-ICR / MS (Thermo LTQ FT, 7 Tesla) in negative ionization mode. Electrospray ionization (ESI) coupled with ultra-high resolution mass spectrometry offered by Fourier transformed ion cyclotron resonance (FT-ICR / MS) has emerged with great promise as it can provide an overview of the NOM composition and details on a molecular scale. NOM's very high-resolution FT-ICR spectra can be extremely complicated. These spectra usually contain many peaks at each nominal mass and thousands of peaks across the entire spectrum. Each peak can represent a chemically distinct compound. This complexity poses an analytical challenge to the study of spectra for structural interpretation. Two-dimensional graphing methods, such as Kendrick and van Krevelen graphs, have been successfully applied to very high-resolution mass spectra, allowing peaks to be sorted into complicated spectra from their homologous relatives across the mass range.
In van Krevelen plots, ionic signals corresponding to structural similarities between homologous series of compounds involved in the loss or gain of functional groups are found on straight lines. We identified many interesting homologous regions and compared the three humic standards with each other. Finally, we recognized the structural relationships of the homologous series obtained through Kendrick graphs.
The results showed homologous series in the Suwannee River and Nordic Aquatic samples compared to the soil-extracted samples (soil-FA and soil-HA). In particular, homologous series signals related to methylation/demethylation, hydrogenation/dehydrogenation, hydration/dehydration, and oxidation/reduction processes were lower in the soil-FA van Krevelen diagrams. On the contrary, the differences were not so evident in all the homologous series for the soil-HA samples.

How to cite: Scrano, L., Mottola, F., Stefanelli, C. M., Lelario, F., Bianco, G., and Bufo, S. A.: Graphical resolution of  Humic structures, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11108,, 2021.

Jussi Heinonsalo, Anna-Reetta Salonen, Rashmi Shrestha, Subin Kalu, Outi-Maaria Sietiö, and Karoliina Huusko

Soil C sequestration through improved agricultural management practices has been suggested to be a cost-efficient tool to mitigate climate change as increased soil C storage removes CO2 from the atmosphere. In addition, improved soil organic carbon (SOC) content has positive impacts on farming though better soil structure and resilience against climate extremes through e.g. better water holding capacity. In some parts of the world, low SOC content is highly critical problem for overall cultivability of soils because under certain threshold levels of SOC, soil loses its ability to maintain essential ecosystem services for plant production. Soil organic amendments may increase soil C stocks, improve soil structure and boost soil microbial activities with potential benefits in plant growth and soil C sequestration. Additional organic substrates may stimulate microbial diversity that has been connected to higher SOC content and healthy soils.

We performed a two-year field experiment where the aim was to investigate whether different organic soil amendments have an impact on soil microbial parameters, soil structure and C sequestration.

The experiment was performed in Parainen in southern Finland on a clay field where oat (Avena sativa) was the cultivated crop. Four different organic soil amendments were used (two wood-based fiber products that were leftover side streams of pulp and paper industry; and two different wood-based biochars). Soil amendments were applied in 2016. Soil C/N analysis was performed in the autumns 2016-2018 and soil aggregate in the summer and autumn 2018, as well as measures to estimate soil microbial activity: microbial biomass, soil respiration, enzymatic assays, microbial community analysis with Biolog ®  EcoPlates and litter bag decomposition experiment. The relative share of bacteria and fungi was determined using qPCR from soil samples taken in the autumns 2016, 2017 and 2018.

Data on how the studied organic soil amendments influence soil structure and C content, as well as soil microbial parameters will be presented and discussed.

How to cite: Heinonsalo, J., Salonen, A.-R., Shrestha, R., Kalu, S., Sietiö, O.-M., and Huusko, K.: Organic soil amendments as a tool to increase biological activity and C sequestration in clay soil, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11965,, 2021.

Ji Chen

Soil extracellular enzymes (EEs) catalyze rate-limiting steps in soil carbon (C) decomposition, which may have important implications for soil C cycling. Hydrolytic and oxidative EEs are targeting the decomposition of different soil C pools with distinct microbial C use efficiency. Here, we analyzed the responses of hydrolytic and oxidative EEAs to experimental warming, enhanced N deposition and altered precipitation. Experimental warming profoundly increased oxidative EEAs by 21%, while having no effect on hydrolytic EEAs. Enhanced N addition significantly decreased oxidative EEAs by 21% but enhanced hydrolytic EEAs by 15%. Increased precipitation substantially stimulated oxidative EEAs by 21%, while having no effect on hydrolytic EEAs. On the contrary, decreased precipitation significantly suppressed oxidative EEAs by 11% but enhanced hydrolytic EEAs by 26%. Those results together showed that hydrolytic and oxidative EEAs generally responded asymmetrically to the experimental treatments, representing the trade-offs between microbial hydrolytic and oxidative EEs production. Moreover, experimental treatments were more likely to have positive effects on soil C stock when oxidative EEAs respond negatively, and vice versa. One explanation might be that degradation of soil C pools that targeted by oxidative EEs were typical with lower microbial C use efficiency, since additional energy was required for the deconstruction of those complex and recalcitrant soil C pools. Altogether, our results highlight that soil EEAs can potentially be harnessed towards soil C sequestration if we can better understand the underlying mechanisms associated with the trade-offs between hydrolytic and oxidative EEAs.

How to cite: Chen, J.: Trade-offs between hydrolytic and oxidative extracellular enzyme activity under climate change: Implications for soil carbon cycle, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13165,, 2021.

Steffen A. Schweizer, Emanuele Lugato, Carmen Höschen, and Ingrid Kögel-Knabner

Agricultural sandy soils with high organic matter (OM) contents are generally unexpected under the current paradigm of organic matter formation and stabilization. These so-called black sand soils occur in North-Western Europe and have been related to historical heathland vegetation. The properties and mechanisms of the high OM sequestration in these soils are not clear as they exceed common observations of OM stored in coarse-textured soils. In this study, we analyzed a subset of samples with ‘black sand’ properties from the European soil database “Land Use/Cover Area frame statistical Survey” (LUCAS). Through particle size fractionation, we isolated the fine fraction <20 µm which contained, on average, 55 % of the total soil organic carbon (OC), in only 8 % of the corresponding soil mass. The fine fraction <20 µm contained 301 mg OC g-1 with a C:N ratio of 17.4 on average and was positively correlated with the bulk soil OC. The characterization of OM composition in the fine fractions by solid-state 13C nuclear magnetic resonance (NMR) spectroscopy revealed that the share of alkyl C increased with OC concentrations whereas O/N-alkyl C decreased. To analyze the distribution of OM at the microscale, we analyzed five samples from the <20 µm fraction containing a gradient of 245-378 mg OC g‑1 with nanoscale secondary ion mass spectrometry (NanoSIMS) at a spatial resolution of 120 nm. These microscale measurements revealed fine mineral particle structures associated with heterogeneously distributed OM. Using image analysis, we found that the proportion of OM-dominated area (indicated by 12C2- and 26CN-) increased from 52 to 80 % on average with increasing OC concentration of the fine fractions. A majority of OM-dominated area was correlated with higher 42AlO- counts, which might suggest a preferential co-localization. In turn, the particle area which was dominated by minerals (indicated by 16O, 28Si, 42AlO and 72FeO) contained less Al and more Si. This shows that the more alkylated and OM-rich fine fractions are related with distinct patterns of organo-mineral structures at the microscale.

How to cite: Schweizer, S. A., Lugato, E., Höschen, C., and Kögel-Knabner, I.: Organic matter in black sand soils related to alkyl carbon and organo-mineral structures at the microscale, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13614,, 2021.

Beatrice Giannetta, Danilo Oliveira De Souza, Giuliana Aquilanti, and Daniel Said Pullicino

Paddy soils experience long-term redox alternations affecting the interactions between the biogeochemical cycling of iron (Fe) and carbon (C). Although the higher soil organic matter (SOM) accumulation rates in paddy topsoils with respect to non-paddy soils is generally assumed to be due to limited mineralization under anoxic soil conditions resulting from frequent field flooding, there is growing evidence questioning this assumption. Moreover, differences in particle aggregation and SOM turnover are likely to both affect and be affected by the trajectory of Fe mineral evolution/crystallinity with redox fluctuations. We hypothesized that redox cycling in paddy soils will affect the particle aggregation, the distribution and mineralogy Fe (hydr)oxides between aggregate size fractions, and consequently the mechanisms of SOM stabilization. In particular, we expect finer aggregate and particle size classes to have a higher proportion of short-range ordered (SRO) Fe oxides with respect to larger aggregates under paddy management, compared to non-paddy management, and that paddy management can result in lower amounts of Fe(hydr)oxides in the topsoil with respect to non-paddy soils.  

We tested these hypotheses by evaluating mineralogical changes, and the distribution of Fe species and organic C between different aggregate and particle-size fractions in topsoil (eluvial) and subsoil (illuvial) horizons of soils under long-term paddy (P) horizons (Arp1, Arp2, Arpd, Brd1, Brd2) and non-paddy (NP) horizons (Ap1, Ap2, Bgw) in NW Italy. Soil aggregates (microaggregates: <200 μm, free silt: (53-2 μm), free clay: <2 μm, and, after sonication, fine sand, silt and clay within microaggregates) have been obtained frombulk soils using an aggregate and particle size physical fractionation method. After fractionation, Fe phases were evaluated by selective extraction procedures, X-ray diffraction (XRD) and Fe K-edge extended X-ray fine structure (Fe EXAFS) spectroscopy (Elettra XAFS beamline).

Our results indicate: (1) a depletion in the contents of ferrihydrite in the P topsoil horizons with respect to NP, though redox cycling favoured an increase in ferrihydrite in the P subsoil, possibly due to Fe(II) translocation from topsoil to subsoil, with consequent ferrihydrite precipitation and aggregates formation; (2) more crystalline Fe mineral phases were associated with intra-aggregate clay fraction in the P topsoil. In the clay fraction in the Brd2 subsoil horizon magnetite was observed. In the NP soil the illuvial horizons were not characterized by a significant increase in ferrihydrite. Our hypothesis that finer aggregate and particle size classes have a higher proportion of SRO Fe oxides with respect to larger aggregates under P management, with respect to NP management, was confirmed; (3) more organic C was associated with the fine fraction in P with respect to NP suggesting that redox cycling enhances the chemical stabilization of mineral-associated SOM.

These findings focused on localized Fe dynamics and biogeochemical coupling with SOM, suggesting that redox-driven changes in aggregate-size classes distribution were also linked to the differences in organic C and Fe stocks in these two agro-ecosystems.

How to cite: Giannetta, B., Oliveira De Souza, D., Aquilanti, G., and Said Pullicino, D.: Redox-driven changes in the distribution of Fe minerals between aggregate-size classes in illuvial and elluvial horizons of a hydromorphic soil, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14354,, 2021.

Nicolas Jean Bernard Puche, Cornelia Rumpel, and Nicolas Bottinelli

Carbon sequestration in soils became a major issue that governments have to face under their sustainable development objectives and the international 4p1000 program. Although, earthworms are recognized to play a key role in the structure and dynamics of organic matter (OM) in soils, their contribution to soil OM cycling is not taken into account in biogeochemical models nor well understood. In particular, the fate of OM protected in earthworm casts is unknown. In this study, we investigated the effects of ageing under field conditions on the OM dynamics contained in casts produced by the anecic earthworm Amynthas adexilis in North Vietnam. To this end we investigated (1) the microscale organisation of particulate organic matter and pores during the exposure of casts and control aggregates during 12 months and (2) compared it to the potential OM mineralisation during a laboratory incubation.

Our results indicated that fresh casts contained significantly more particulate organic matter (POM) than control soil aggregates and field aged earthworm casts. Conversely, the porosity was higher in soil control aggregates than in casts and the porosity of casts tended to increase with their ageing. The analyses of micro-CT images also revealed that POM and Pores contents between casts samples presented strong variabilities even in the youngest casts category. We found, on average, higher mineralisation rates for casts than for controls and a reduction of the OM mineralisation with the ageing of casts. Our results also highlighted a strong positive correlation (r2 = 0.89) between POM contents determined by the segmentation of micro CT images and CO2 emissions from the incubation experiment. We conclude that earthworms impact the microscale organisation of POM and pores in their casts and thereby influence soil OM dynamics.

How to cite: Puche, N. J. B., Rumpel, C., and Bottinelli, N.: Effects of ageing under field conditions on soil organic matter in earthworm casts produced by the anecic earthworm Amynthas adexilis in northern Vietnam, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14982,, 2021.