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
Convener: César Plaza | Co-conveners: Claire Chenu, Claudio Zaccone
| Attendance Wed, 06 May, 14:00–15:45 (CEST)

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Chat time: Wednesday, 6 May 2020, 14:00–15:45

Chairperson: Cesar Plaza, Claire Chenu, Claudio Zaccone
D1977 |
Albert C. Brangarí, Stefano Manzoni, and Johannes Rousk

Soils are continuously exposed to recurrent cycles of drying and rewetting, for instance, when extended periods of drought are followed by rainfall events. For nearly a century it has been known that the balance of the soil C budget is affected by these moisture fluctuations, which is characterized by very large mineralization losses when dry soils are rewetted. In some ecosystems, the soil C losses resulting from this phenomenon (“the Birch effect”) even represent a dominant fraction of the annual C-transfer from soils to the atmosphere. However, to balance the soil C budget, the microbial control of C input to the soil during these events also needs to be known. It was recently discovered that the growth of microorganisms, driving C stabilization in soils, has a far slower response to rewetting than does respiration. This results in a pronounced and dynamic disconnection between the mechanisms controlling microbial respiration and growth. Despite the significance of this decoupling for the C budget and the long-term balance of soil C stocks, this feature has so far been entirely overlooked by biogeochemical models, potentially leading to a failure to capture the capacity of soils to mitigate the effects of climate change.

To close this knowledge gap, we developed a new process-based soil microbial model that includes a wide range of physical, chemical and biological mechanisms to explore the nature of soil C dynamics induced by moisture changes. The model was validated using respiration data from soils exposed to repeated cycles of drying and rewetting which has been frequently studied (Miller et al., 2005, Soil Biol Biochem) and compared to other models existing in the literature. The proposed model was able to capture, at once and for the first time, the respiration data and the decoupled behaviour of growth. Simulation results identified the drought-legacy effects on C use efficiency and microbial physiology as the main mechanisms controlling the soil responses to moisture fluctuations. This represents a critical step towards unravelling the C sequestration capacity of soils, its drivers and feedback on climate.

How to cite: Brangarí, A. C., Manzoni, S., and Rousk, J.: Uncovering the diverging factors that control microbial carbon sequestration and respiration in soils exposed to moisture fluctuations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-415, https://doi.org/10.5194/egusphere-egu2020-415, 2020.

D1978 |
| Highlight
Dedy Antony, Jo Clark, Chris Collins, and Tom Sizmur

Soils are the largest terrestrial pool of organic carbon and it is now known that as much as 50% of soil organic carbon (SOC) can be stored below 30 cm. Therefore, knowledge of the mechanisms by which soil organic carbon is stabilised at depth and how land use affects this is important.

This study aimed to characterise topsoil and subsoil SOC and other soil properties under different land uses to determine the SOC stabilisation mechanisms and the degree to which SOC is vulnerable to decomposition. Samples were collected under three different land uses: arable, grassland and deciduous woodland on a silty-clay loam soil and analysed for TOC, pH, C/N ratio and texture down the first one metre of the soil profile. Soil organic matter (SOM) physical fractionation and the extent of fresh mineral surfaces were also analysed to elucidate SOM stabilisation processes.

Results showed that soil texture was similar among land uses and tended to become more fine down the soil profile, but pH did not significantly change with soil depth. Total C, total N and C/N ratio decreased down the soil profile and were affected by land use in the order woodland > grassland > arable. SOM fractionation revealed that the free particulate organic matter (fPOM) fraction was significantly greater in both the topsoil and subsoil under woodland than under grassland or arable. The mineral associated OC (MinOC) fraction was proportionally greater in the subsoil compared to topsoil under all land uses: arable > grassland > woodland. Clay, Fe and Mn availability play a significant role (R2=0.87) in organic carbon storage in the top 1 m of the soil profile.

It is evidently clear from the findings that land use change has a significant effect on the dynamics of the SOC pool at depth, related to litter inputs to the system.

How to cite: Antony, D., Clark, J., Collins, C., and Sizmur, T.: The chemical and physical stability soil organic carbon in the top 1 m of the soil profile under different land uses in the UK, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-514, https://doi.org/10.5194/egusphere-egu2020-514, 2020.

D1979 |
| Highlight
Yiqi Luo, Feng Tao, and Xiaomeng Huang

It has long been known that processes that determine soil carbon dynamics are spatially heterogeneous. However, the spatially heterogeneous mechanisms have not been well characterized nor incorporated into Earth system models for predicting soil carbon sequestration in response to climate change. This presentation shows our recent results from an integrated approach that combines deep learning, data assimilation, big data with >100,000 vertical soil organic carbon (SOC) profiles worldwide, and the Community Land Model version 5 (CLM5) to optimize the model representation of SOC over the world. Our results indicate that CLM5 that is trained by >100,000 data via data assimilation alone is constrained with spatially homogeneous parameter values over the globe. However, CLM5 that is not only trained by data assimilation but also optimized by deep learning from the big data is constrained with spatially heterogeneous parameter values. Our further analysis suggests that those parameters representing microbial carbon use efficiency greatly vary across space. The spatial heterogeneity in carbon use efficiency is caused by interactions of edaphic, climate and vegetation factors. When the spatially heterogenous parameterization is applied to simulation over time with temporal variation, CLM5 predicts substantial carbon sequestration under climate change. In contrast, CLM5 with the spatially homogeneous parameters predicts carbon loss. Our study demonstrates the importance to uncover and represent spatially heterogeneous mechanisms underlying soil carbon sequestration in order to realistically predict SOC dynamics in the future.

How to cite: Luo, Y., Tao, F., and Huang, X.: Spatially heterogeneous mechanisms underlying soil carbon sequestration as revealed via big data-driven Earth system modelling and deep learning, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2473, https://doi.org/10.5194/egusphere-egu2020-2473, 2020.

D1980 |
Nicholas Jarvis, Elsa Coucheney, Claire Chenu, Anke Herrmann, Thomas Keller, Thomas Kätterer, and Katharina Meurer

The aggregated structure of soil is known to reduce rates of soil organic matter (SOM) decomposition and therefore influence the potential for long-term carbon sequestration. In turn, the storage and turnover of SOM strongly determines soil aggregation and thus the physical properties of soil. The two-way nature of these interactions has not yet been explicitly considered in soil organic matter models. In this study, we present and describe a new model of these dynamic feedbacks between SOM storage, soil pore structure and soil physical properties. We show the results of a test of the model against measurements made during 61 years in a field trial located near Uppsala (Sweden) in two treatments with different OM inputs (bare fallow, animal manure). The model was able to successfully reproduce long-term trends in soil bulk density and organic carbon content (SOC), as well as match limited data on soil pore size distribution and surface elevation. The results suggest that the model approach presented here could prove useful in analyses of the effects of soil and crop management practices and climate change on the long-term potential for soil organic carbon sequestration.

How to cite: Jarvis, N., Coucheney, E., Chenu, C., Herrmann, A., Keller, T., Kätterer, T., and Meurer, K.: Modelling dynamic interactions between soil structure and soil organic matter, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3613, https://doi.org/10.5194/egusphere-egu2020-3613, 2020.

D1981 |
Yakov Kuzyakov, Ezekiel Bore, and Michaela Dippold

Microbial transformation of organic substances is a key process of soil organic matter (SOM) formation. Carbon (C) entering the soil can be transformed in three main directions: i) stabilization over long period without relevant microbial utilization, ii) recycling by microorganisms for production of new and reparation of old cells, and iii) microbial utilization for energy production leading to C losses from soil as CO2. So, individual compounds within huge diversity of the organic substances entering the soil will follow predominantly one of these directions, depending on the substance chemistry, soil properties, microbial activities and environmental conditions. Therefore, organic substances can have two general trends: i) they converge from any initially distinct compounds (e.g. in litter or rhizodeposition) to completely mixed, so that it is impossible to trace back their origin; or ii) divergence: the substances maintain their differences despite microbial transformations by SOM formation.

We proved two opposite hypotheses that convergence and divergence of the fate of organic substances depends on microbial utilization at two levels: 1) intermolecular: high recycling intensity leads to convergence, whereas stabilization leads to divergence of the C originated from various organic compounds, and 2) incorporation of C from various molecule positions into microbial metabolic cycles define the C fate at intramolecular level. We tested the first hypothesis based on own and literature data to the fate of polymeric substances: sugars, proteins, lipids and lignin. The second hypothesis was tested by the C atoms from various positions of pentoses and hexoses by position-specific 13C and 14C labeling.

The polymeric substances as well as monomers from the same chemical group clearly converge to three groups stabilization, recycling and losses. Carboxylic acids will be nearly completely mineralized and are lost from soil. The fate and functions organic compounds depend mainly on microbial recycling. Proteins, amino acids and sugars - key components of microbial biomass - are intensively recycled and e.g. proteins remain relatively long in soil.

For the intramolecular differences, we traced the fate of position-specific 13C labeled glucose and ribose under field conditions for 800 days. Both sugars were simultaneously metabolized via glycolysis and pentose phosphate pathway. The similarity between position-specific 13C recovery in microbial biomass and soil reflected high contribution of microbial necromass to SOM. The mean residence time of uniformly labeled 13C ribose in the soil was 3 times longer than that of glucose. Consequently, ribose and glucose were incorporated into different cellular components, defining their long-term fate in soil. The convergence of glucose C positions in soil and microbial biomass revealed that recycling dominated glucose transformation. In contrast, divergence of ribose C positions in soil revealed that intact ribose-derived cell components are reused or preserved in SOM.

Thus, convergence vs. divergence distinguished the two general trends explaining the long persistence of C at inter- and intra-molecular levels: microbial recycling leads to convergence, whereas slow decomposition and preservation define the divergence of C pathways in soil.

How to cite: Kuzyakov, Y., Bore, E., and Dippold, M.: Soil organic matter formation: Convergence and divergence of three carbon pathways: Stabilization, recycling and losses, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6174, https://doi.org/10.5194/egusphere-egu2020-6174, 2020.

D1982 |
Yan Zhang

Improvements in management practices can prevent the decline of soil organic carbon (SOC) storage caused by conventional tillage practice in Northeast China. Density and size fractionation can track the transformation of plant residue into SOC and its location in soil matrix. We used a long-term field study in China to evaluate these changes as a result of improved management involving tillage and cropping systems. Experimental treatments included no-till (NT) and moldboard ploughing (MP) under monoculture maize (Zea mays L.) (MM) and maize-soybean (Glycine max Merr.) rotation (MS); these were compared to the traditional management involving conventional tillage (CT) under MM. An incubation study was conducted to evaluate mineralization and the biodegradability of SOC. The soils were also physically fractionated by density (light fraction, LF) and size (sand, silt, clay). With improved management, the SOC storage in the clay showed the largest increase across all fractions. This increase was greater for MS than MM. The NTMS treatment resulted in a decline in silt-OC storage compared to CTMM. The SOC mineralization (mg CO2-C g-1 soil) was affected by tillage and driven by LF-OC and was observed in the order: NTMM (2.06) > MPMM (1.72) ≈ NTMS (1.71) > CTMM (1.52) ≈ MPMS (1.41). Both cropping and depth affected the biodegradability of SOC. Considering the plough layer (0-20 cm), treatments under MM had larger proportion of biodegradable SOC than under MS. We conclude that the significant differences in SOC storage in physical fractions and SOC biodegradation were caused by differences in soil management.

How to cite: Zhang, Y.: Tillage and cropping effects on soil organic carbon: biodegradation and storage in density and size fractions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8186, https://doi.org/10.5194/egusphere-egu2020-8186, 2020.

D1983 |
Anna Gunina and Yakov Kuzyakov

Aggregation of mineral and organic particles is a key process of soil development, which promotes carbon (C) stabilization by hindering decomposition of plant and microbial residues. All microbial utilization and C stabilization processes lead to 13C fractionation and consequently to various δ13C values of organic matter in aggregate size classes, sand, silt, and clay-sized particles, as well as density fractions. Differences in δ13C within the aggregates and density fractions may have two reasons: 1) preferential stabilization of organic compounds with light or heavy δ13C and/or 2) stabilization of organic materials after passing one or more microbial utilization cycles, leading to respiring of 13C depleted CO2 and heavier δ13C in remaining C. Assuming these two reasons, the new approach based on the natural differences in stable C isotopic composition between SOM fractions was proposed and tested on soils developed solely under C3 vegetation (arable, coniferous and deciduous forests) in boreal climate (Gunina and Kuzyakov, 2014). This approach assumes that: 1) 13C enrichment between the SOM fractions corresponds to successive steps of SOM formation; 2) 13C fractionation (but not the δ13C signature) depends mainly on the transformation steps and not on the C precursors. Consequently, 13C enrichment of SOM fractions allows reconstructing the SOM formation pathways. To prove these initial results we reviewed  δ13C values of soils globally and focused on the i) estimation of the validity of this approach for soils developed under various climatic conditions and parent materials, and depending on fertilization, and ii) C flows not only between aggregate size classes and density fractions but also between various particle size classes of the soils (i.e. sand, silt, and clay) and iii) on revealing the intensities of natural 13C fractionation during the stabilization of litter C in aggregates, particle size classes, and density fractions. Results showed that density fractions were 13C enriched in the order: free particulate organic matter (POM) < light occluded POM < heavy occluded POM < mineral fraction, with the strongest increase between the light occluded and heavy occluded POM. The maximum 13C fractionation during stabilization of litter C in density fractions and aggregate size classes was < 2‰. Δ13C enrichment of the SOM fractions showed that the main direction of C flows within the aggregates and SOM fractions was from the macroaggregate-free POM to the mineral microaggregate fraction. Thus, despite some limitations, δ13C natural abundance approach based on 13C fractionation within individual steps of SOM formation is very useful and probably the sole approach to estimate C flows under steady-state without labeling.

How to cite: Gunina, A. and Kuzyakov, Y.: Carbon flows by soil organic matter formation: A review based on 13C natural abundance, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10730, https://doi.org/10.5194/egusphere-egu2020-10730, 2020.

D1984 |
Angelika Xaver, Taru Sandén, Heide Spiegel, Luca Zappa, Gerhard Rab, Drew Hemment, and Wouter Dorigo

Soil organic matter plays a key role within the nutrient cycle, serves as an agent to improve soil structure, and is also known to impact concentrations of greenhouse gases and stabilize soil pollutants. Thus, the soil organic matter content and its potential losses through decomposition are of high interest, especially in the light of a changing climate. As the decomposition process is significantly influenced by climatic conditions, it is important to understand the relationship between decomposition and environmental variables. Previous studies primarily focused on determining the influence of air temperature and precipitation on litter decomposition, but the impact of soil moisture has hardly been investigated.

In this study, we evaluate the relationship between plant litter decomposition, using commercial tea bags (Green and Rooibos tea) as standardized plant litter, and soil moisture, observed with low-cost sensors used within the European citizen science project GROW Observatory (GROW; https://growobservatory.org/). The low-cost soil moisture sensors were placed alongside the tea bags at eight different locations, covering four different land cover types, within the Hydrological Open Air Laboratory (HOAL), a small agricultural catchment in Petzenkirchen, Austria. Data has been collected for two years providing decomposition rates (k) and stabilization factors (S) for the four different seasons of both years. Apart from soil moisture, we investigate air and soil temperature, precipitation and soil parameters as drivers for litter decomposition.

We will show preliminary results on the relationship between decomposition and different environmental variables, in particular soil moisture, throughout all seasons and various land cover classes.


This study was funded by the GROW Observatory project of the European Union’s Horizon 2020 research and innovation programme (https://growobservatory.org/).

How to cite: Xaver, A., Sandén, T., Spiegel, H., Zappa, L., Rab, G., Hemment, D., and Dorigo, W.: Impact of soil wetness on plant litter decomposition using low-cost soil moisture sensors and off-the-shelf tea bags, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18700, https://doi.org/10.5194/egusphere-egu2020-18700, 2020.

D1985 |
Vibeke Ernstsen

Studies of the distribution of soil organic matter (SOM) below intensively cultivated fields on clayey sediments of Quaternary age in Denmark show three markedly different zones from the surface and down to about 10 meters below surface. Each zone reflects the balance between input and losses of since the last glaciation.

The upper zone makes up the uppermost few meters below the soil surface. Here, the inherited bioavailable pool of SOM has mineralized and only small amounts of not-bioavailable SOM are present. The largest pool of SOM is renewable and derives from crops grown in the field.

In the underlying middle zone, the content of inherited SOM is very low and seems well protected against biological decomposition (not-bioavailable). No renewable source of SOM from the crops seems to reach down to this middle zone.

In the third and deepest zone, only inherited SOM is present. The SOM origins from the sediments deposited during the last glaciation, about 12,000 years ago.

Typically, the lowest contents of SOM is in the middle zone.

Zonation by content, composition, and bioavailability of the organic matter in soils and deeper sediments is important for the fate of many environmentally substances and for the quality of soil water as well of the quality of other parts of the aquatic environment. In addition, the SOM- pools of different composition in the three zones will most likely behave different to future changes in atmospheric CO2 and climate change adoption.

How to cite: Ernstsen, V.: Postglacial zonation of soil organic matter in clayey till sediments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20704, https://doi.org/10.5194/egusphere-egu2020-20704, 2020.

D1986 |
Eito Nonomura, Soh Sugihara, Mayuko Seki, Hidetoshi Miyazaki, Muniandi Jegadeesan, Pandian Kannan, and Haruo Tanaka

An understanding of the mechanisms of soil organic carbon (SOC) stabilization is essential to develop the appropriate management for C sequestration and soil health. In southern India, where neutral-alkaline soils are mainly distributed, soil C stocks are inherently low in cropland, despite relatively high clay contents (Clay>ca. 30%, OC<ca. 5 g C kg-1 soil). To consider this reason of low SOC in this area, we evaluated the fractionated C contents and its controlling factors, by measuring the particulate organic matter (POM). The objective of this study was to evaluate the effect of land management on the amount and composition of each fraction of soil in southern India. We collected the surface soils (0-10 cm) from two representative sites of southern India; Vertisols with alkaline soil pH (8.4-8.8) and Alfisols with neutral soil pH (6.0-7.0). At each site, two different land management were selected; forest and cropland of Vertisols, and cropland with no organic matter application (no-OM) and with manure application (with-OM) of Alfisols. Soils were separated into the four fractions; (1) Light Fraction; LF (<1.7 g cm-3) , (2) Coarse POM; cPOM (>1.7 g cm-3, 250-2000 µm), (3) Fine POM; fPOM(>1.7 g cm-3, 53-250 µm), and (4) Silt+Clay; S+C (>1.7 g cm-3, <53 µm). Each fraction was analyzed by elemental analysis (C, N) and CPMAS 13C NMR spectroscopy. In Vertisols, C contents of cPOM, fPOM, S+C were significantly higher in forest (0.65, 0.91, 4.8 g kg-1 soil, respectively) than those of cropland (0.17, 0.22, 4.1 g kg-1 soil, respectively), causing the higher total SOC in forest (7.8 g kg-1 soil) than in cropland (4.5 g kg-1 soil). C concentration of cPOM, fPOM, and S+C fractions were also significantly higher in forest (3.7, 7.6, 6.7 g kg-1 fraction, respectively) than those of cropland (1.0, 2.7, 5.4 g kg-1 fraction, respectively). In particular, increasing rates in cPOM and fPOM (180-280 %) were greater than S+C (24 %), possibly suggesting that forest management should increase the relatively active and intermediate SOC pools through the C accumulation in cPOM and fPOM fractions of Vertisols. In Alfisols, C contents in LF and S+C were significantly higher in with-OM (1.1 and 5.2 g kg-1 soil, respectively) than in no-OM (0.76 and 4.7 g kg-1 soil, respectively). C concentration of S+C fraction was significantly higher in with-OM (14 g kg-1 fraction) than in no-OM (11 g kg-1 fraction), but not of cPOM and fPOM fractions. It suggests that the OM application to cropland should increase the slow SOC pool through the C accumulation in S+C fractions of Alfisols. These results indicate that different fraction may contribute to SOC stabilization between Vertisols and Alfisols in southern India.

How to cite: Nonomura, E., Sugihara, S., Seki, M., Miyazaki, H., Jegadeesan, M., Kannan, P., and Tanaka, H.: Fractionation of soil organic carbon under different land management in dry tropics, south India, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21025, https://doi.org/10.5194/egusphere-egu2020-21025, 2020.

D1987 |
Luciano Beneduce, Cesar Plaza, Stefano Trotta, and Claudio Zaccone

In a previous study (Zaccone et al., 2018. Appl. Soil Ecol. 130, 134-142), we evaluated the potential ecological partition of microbial and plant DNA across soil organic matter (SOM) fractions linked to conceptual stabilization mechanisms. We found that different microbial taxa (bacterial and fungal) seem to be specifically associated to SOM fractions. In the present work we investigated the short-term distribution of exogenous microbial population in SOM fractions following inoculation, in order to track the fate of bacterial DNA (in the form of spores) artificially spiked in bulk soils. The main hypothesis was that the colonization of external organisms proceeds from the unprotected fraction (FR) towards those protected physically and/or chemically by soil minerals from decomposition (i.e., into macro and micro-aggregates [MA, MI] or interacting with mineral surfaces [MIN]).

Three different soils with different pH, SOM content and texture were used in the experiment. One aliquot of soil was spiked with approx. 8 Log cfu of spore of Bacillus clausii from a commercial preparation of 4 strains. DNA was extracted from soil and recovered from SOM pools isolated using a physical fractionation method [Plaza et al., 2012. CLEAN-Soil Air Water 40, 134-139] and quantified by fluorescence (Qubit).

DNA recovered from spiked vs. non-spiked samples followed two different patterns of distribution, according to the SOM fractions. Total DNA in the bulk soils varied according to the soil types and the effect of spiking 8 cfu was negligible. In the SOM fractions, while MI and MIN showed different concentration according to the soil type (no apparent influence of spiking), total DNA in FR was clearly higher for spiked samples, while MA had a putative interaction between soil type and spiking. Even if very preliminary, our results point out a possible mechanism of short-term distribution of exogenous DNA (through spores and potential vegetative forms of B. clausii germinated during the incubation) from the free SOM to the macroaggregates, with no apparent influence on MI and MIN yet.

Further analyses (e.g., PCR-ARISA and qPCR) will allow to disclose whether indigenous vs. exogenous bacterial DNA are differentially distributed in SOM, possibly enhancing the description of the mechanisms underlying the distribution of microbial communities in soil, according to the different organization of the SOM in soil aggregates.

How to cite: Beneduce, L., Plaza, C., Trotta, S., and Zaccone, C.: Interaction of exogenous microbial inoculum with soil organic matter fractions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6731, https://doi.org/10.5194/egusphere-egu2020-6731, 2020.

D1988 |
Gergely Jakab, Tibor Filep, Csilla Király, Balázs Madarász, Dóra Zacháry, Marianna Ringer, Anna Vancsik, Malihe Masoudi, and Zoltán Szalai

Soil organic matter (SOM) is in the focus of research as it plays crucial role in soil fertility, carbon sequestration, and all adsorption related processes in the soil. Nevertheless, its compound and the methods to investigate it are rather diverse. Some approach prefers to define different theoretical carbon pools in the soil based on input and mineralization dynamics using mean residence times. Other studies apply physical and/or chemical fractionations of the soil to separate the various eg. mineral phase associated or aggregate occluded carbon pools to gain less heterogeneous material. However, in practice, these two approaches are hardly met each other. As a considerable part of SOM is strongly associated with the mineral colloid fraction or even cations its investigation reveals the question of extractions. Traditional methods aimed to extract pure SOM fractions such as fulvic and humic acids (FA; HA)  and characterized the whole SOM based on them, even though these pure fractions represented only a small part of the total SOM and were not present under natural conditions. Recent methods try to characterize the SOM using in situ samples where the role of organic mineral complexes is still not fully understood. As a result, findings based on several approaches are hardly comparable with each other. The present study aims to characterize SOM based on parallel in situ solid-phase investigation FA separation, and water dissolved organic matter extraction. The study site is a haplic Luvisol under plowing and conservation tillage. Fourier transform infrared spectroscopy on the solid phase fractions resulted in an inverse proportion between organic carbon content and aromaticity independently from tillage. The aggregate occluded SOM was characterized by the lack of aliphatic components, whereas the fine fraction, and the bulk soil associated SOM seemed to be rich in them. The water-soluble SOM revealed molecular size increase in both the fine fraction related and the aggregate occluded organic matter owing to plowing, nonetheless, aggregates occluded the same sized OM molecules as those attached to the fine fraction. In general, FA fractions provided more humified organic matter, whereas water dissolved SOM showed a more intensive microbiome origin. The photometric properties of the FA fractions did not differ between the tillage systems, except for the SUVA254, which provided higher aromaticity under conservation tillage due to the lack of plowing. Also, the water-soluble part of SOM showed more humified composition and increased aromaticity under conservation tillage compared to plowing tillage. As a consequence, beneath the fingerprint of recent microbial activity, DOM reflects soil organic matter composition as well, therefore it seems to be suitable as a direct SOM proxy. The present research was supported by the Hungarian National Research and Innovation Office (NKFIH) K-123953, which is kindly acknowledged.

How to cite: Jakab, G., Filep, T., Király, C., Madarász, B., Zacháry, D., Ringer, M., Vancsik, A., Masoudi, M., and Szalai, Z.: Characterizing soil organic matter differences among extracts and the solid phase – the role of conservation tillage, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6997, https://doi.org/10.5194/egusphere-egu2020-6997, 2020.

D1989 |
Amicie Delahaie, Pierre Barré, Lauric Cécillon, François Baudin, Camille Resseguier, Thierry Morvan, Denis Montenach, Florence Savignac, Aurélia Michaud, Florent Levavasseur, and Sabine Houot

The term Organic Waste Products (OWPs) encompasses a wide range of byproducts such as manure, sewage sludge or green waste compost. The use of OWPs impacts soil quality and functioning, agricultural yields, carbon (C) sequestration, biogeochemical cycles of nutrients like nitrogen (N) or phosphorus, and organic matter (OM) dynamics. These impacts likely depend on the considered OWP.

Taking advantage of 3 mid to long-term experimental trials (6 to 20 years) located in the Northern part of France (Paris region; Brittany; Alsace), we investigated the impact of 16 different OWPs on C content and stability. To do so, surface soil samples from replicated plots amended with the different OWPs used either alone or in addition with mineral N fertilization and appropriated control treatments were analyzed using Rock-Eval 6® thermal analyses. Samples taken up at the onset of the experiment and after 6, 18 and 20 years for the 3 sites respectively were analyzed. It resulted in the analyses of 248 different samples whose Rock-Eval 6® (RE6) signature can be used as a proxy for soil organic carbon (SOC) biogeochemical stability. In particular, we determined 2 RE6 parameters that were related to SOC biogeochemical stability in previous studies (e.g. Barré et al., 2016): HI (the amount of hydrogen-rich effluents formed during the pyrolysis phase of RE6; mgCH.g-1 SOC), and T50 CO2 oxidation (the temperature at which 50% of the residual organic C was oxidized to CO2 during the RE6 oxidation phase; °C). We also computed the amount of centennially stable SOC from RE6 parameters using the model developed in Cécillon et al. (2018).  


Our results showed that no clear effect of OWPs addition can be established for the youngest site (6 years). On the contrary, OWPs amendments had a clear effect on SOC quantity and quality at the sites having experienced 18 and 20 years of OWPs addition. For these sites, OWPs amendments increased SOC content, decreased SOC thermal stability (T50 CO2 oxidation) and increased the Rock-Eval 6® Hydrogen Index (HI) compared to control plots. OWPs amendments tended to increase slightly the amount of centennially stable SOC at the sites having experienced 20 years of repeated OWPs application. Our results suggest that if OWPs addition does increase SOC content, at least in the long run, the majority of this additional SOC is labile and may be quickly lost if OWPs additions are stopped.



Barré P., Plante A.F., Cécillon L., Lutfalla S., Baudin F., Bernard S., Christensen B.T., Eglin T., Fernandez J.M., Houot S., Kätterer T., Le Guillou C., Macdonald A., van Oort F. & Chenu C. (2016) The energetic and chemical signatures of persistent soil organic matter. Biogeochemistry, 130: 1-12.

Cécillon L., Baudin F., Chenu C., Houot S., Jolivet R., Kätterer T., Lutfalla S., Macdonald A.J., van Oort F., Plante A.F., Savignac F., Soucémarianadin L.N. & Barré P. (2018) A model based on Rock-Eval thermal analysis to quantify the size of the centennially persistent organic carbon pool in temperate soils. Biogeosciences, 15, 2835-2849.

How to cite: Delahaie, A., Barré, P., Cécillon, L., Baudin, F., Resseguier, C., Morvan, T., Montenach, D., Savignac, F., Michaud, A., Levavasseur, F., and Houot, S.: Influences of repeated application of organic waste products on soil organic carbon content and stability assessed using Rock-Eval 6® thermal analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9794, https://doi.org/10.5194/egusphere-egu2020-9794, 2020.

D1990 |
Ivan F. Souza, Aline A. Vasconcelos, Robert L. Johnson, Luis Fernando J. Almeida, Emanuelle M. B. Soares, Klaus Schmidt-Rohr, and Ivo R. Silva

Several processes are involved in soil organic matter (SOM) formation and turnover across the soil profile. Interestingly, while deep soil C appears to turn over very slowly, it remains unclear whether this trend is related to the molecular chemistry of SOM retained therein or its interaction with the mineral matrix. Besides, the extent to which the molecular chemistry of SOM is related to the chemistry, amount, and frequency of C inputs in topsoil and subsoil horizons remains unclear. We addressed these questions by collecting soil samples from three deep Ferralsols (a Gibbsic, a Ferritic, and a Haplic Ferralsol) across the Brazilian Cerrado to include samples with distinct texture classes and mineralogical combinations. Interestingly, the vegetation of the Brazilian Cerrado is characterized by different proportions of trees and grasses, implying different depth of rooting. Moreover, the Cerrado biome is subjected to frequent fire events, which could affect the input rate and the chemistry of C added to the soils. At each site, samples were taken from topsoil (0–10 cm) and subsoil horizons (60–100 cm) and incubated with a double-labeled (13C and 15N) eucalypt litter for 12 months under laboratory conditions. After the incubation, the samples were submitted to physical fractionationation to isolate SOM within the particle-size fractions (PSF) greater and smaller than 53 µm. Subsequently, we quantified the total C and N remaining in these PSF. Subsamples of the clay+silt fraction (<53 µm) were treated with a 10% HF solution to concentrate SOM. The molecular composition of SOM within the HF-insoluble fraction was assessed by 13C/15N Nuclear Magnetic Resonance (NMR) spectroscopy by applying a multi/cross-polarization (multi/CP) pulse sequence, yielding a quantitative solid-state magic-angle spinning (MAS) 13C/15N NMR. After the incubation, litter-C was retained at approximate proportions in both PSF evaluated, while a larger fraction of the litter-N was concentrated within the clay+silt fraction. Based on the multi-CP MAS NMR results, carbohydrates (65–110 ppm) accounted for most of the total C forms identified in the HF-insoluble fraction, regardless of soil type, soil depth, and plant litter addition. In topsoil, differences in the molecular chemistry of SOM between samples treated with plant litter and the controls were small. Otherwise, plant litter inputs to subsoil led to major changes in the chemistry of SOM, with a substantial reduction in the proportion of non-protonated aromatics and the aromaticity degree of SOM. Although observed in the topsoil, this effect was much less pronounced for the three Ferralsols evaluated. In addition, following eucalypt litter addition the molecular composition of SOM in topsoil and subsoil tended to converge, becoming enriched in alkyl-C (0–46 ppm), carboxylic and/or amide groups (160–190 ppm for 13C and 120 ppm for 15N-NMR). Our results suggest that in topsoil, SOM molecular chemistry is consistent with a continuous supply of fresh plant litter. Otherwise, the deep burial of plant litter appears to be less relevant for SOM formation in subsoil horizons, where the accumulation of charred/pyrogenic materials are significant.

How to cite: F. Souza, I., A. Vasconcelos, A., L. Johnson, R., J. Almeida, L. F., M. B. Soares, E., Schmidt-Rohr, K., and R. Silva, I.: Larger contribution of non-protonated aromatics for organic matter in subsoil than topsoil horizons in Brazilian Ferralsols, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11972, https://doi.org/10.5194/egusphere-egu2020-11972, 2020.

D1991 |
Axel Don, Christina Hagen, Erik Grüneberg, and Cora Vos

Soil disturbance and disruption is assumed to enhance mineralisation and cause losses of soil organic carbon. Therefore, no tillage is promoted as soil carbon sequestration measure. However, the experimental evidence of enhanced carbon turnover due to soil disturbance is rare.  We investigated soil disturbance in forest ecosystems with simulated bioturbation of wild boar. Wild boar are effective at mixing and grubbing in the soil and wild boar populations are increasing dramatically in many parts of the world. In a six-year field study, we investigated the effect of wild boar bioturbation on the stocks and stability of soil organic carbon in two forest areas at 23 plots. The organic layer and mineral soil down to 15 cm depth were sampled in the disturbed plots and adjacent undisturbed reference plots.

No significant changes in soil organic carbon stocks were detected in the bioturbation plots compared with non-disturbed reference plots. However, around 50% of forest floor carbon was transferred with bioturbation to mineral soil carbon and the stock of stabilised mineral-associated carbon increased by 28%. Thus, a large proportion of the labile carbon in the forest floor was transformed into more stable carbon. Carbon saturation of mineral surfaces was not detected, but carbon loading per unit mineral surface increased by on average 66% due to bioturbation. This indicates that mineral forest soils have non-used capacity to stabilise and store more carbon.

Our results indicate that soil disturbance and bioturbation alone does not affect soil carbon turnover and stocks, but only change the distribution of carbon in the soil profile. This is in line with results from no-tillage experiments. The prevailing effect is a redistribution of carbon in the soil profile with no changes in total soil carbon stocks. We discuss these findings in the light of soils as potential sinks for carbon.


How to cite: Don, A., Hagen, C., Grüneberg, E., and Vos, C.: Does soil disturbance result in soil carbon losses? – A case study on bioturbation effects of wild boar , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18999, https://doi.org/10.5194/egusphere-egu2020-18999, 2020.

D1992 |
Anna Yudina, Dmitriy Fomin, Yulian Farkhodov, Polina Tregubova, Konstantin Abrosimov, and Vladimir Cheptsov

Soil organic matter (SOM) as one of the main aggregation factors supports the hierarchy structure organization of soils. Structure organization at the micro-level (µm-mm) is based on soil primary particles, composite building units and microaggregates characterized by different SOM composition, stabilization mechanisms and dynamics (Yudina et al., 2018; Yudina & Kuzyakov, 2019). This presentation aims to show the specifics in composition and sensitivity of soil microstructure to ecosystem type and land use changes. The studied objects are Haplic Chernozems (Kursk region, Russia) under 6 land use types differing in vegetation: natural steppe, natural forest, conventional arable field, long-term bare fallow, and afforestation. Separation of C pools associated with particle size distribution (PSD) were obtained with high resolution by laser diffraction technique. Mathematical computations with PSD's allow to find localization of particles sensitive to the chosen factor. Two parameters (mean volume diameter MVD, µm and content of particles, %) for each of the three particle types (organo-mineral OMp, particulate organic matter POM, microaggregates µA) can be calculated. POM is the smallest (3 or less %) but the most labile solid phase C pool and very sensitive indicator to changes in land use and C accumulation with the soil depth. OMp is sensitive to long-term factors and were the lowest in bare fallow soil. Since Chernozems are well-structured soils, the content of µA is less sensitive than MVD, which vary from 50 µm under bare fallow to 170 µm in forest soil. Presented indicators in combination with C storage characterize role of SOM in soil microstructure organization. We have supposed that the differences in dynamics between OMp, POp and µA is attributable to internal particle structure and microbial availability of SOM. The marking particle types were separated for physical and biological justification of suggested indicators. Their thermal stability, specific surface area, microporosity, microbial activity and composition were characterized. Following hypotheses were tested: 1) content of the thermostable organic C fraction will increase from POM to µA and OMp; 2) value of specific surface area and porosity will be higher in µA compare to OMp. The proposed approach to describe C dynamics based on combination of high-resolution PSD data is a simple, sensitive and effective tool for monitoring of SOM pools dynamics.

Yudina, A. V., Fomin, D. S., Kotelnikova, A. D., & Milanovskii, E. Y. (2018). From the Notion of Elementary Soil Particle to the Particle-Size and Microaggregate-Size Distribution Analyses: A Review. Eurasian soil science, 51(11), 1326-1347. DOI: 10.1134/S1064229318110091

Yudina, A., & Kuzyakov, Y. (2019). Saving the face of soil aggregates. Global change biology, 25(11), 3574-3577. DOI: 10.1111/gcb.14779

The reported study was funded by Russian Foundation for Basic Research according to the research project No 18-34-00825.

How to cite: Yudina, A., Fomin, D., Farkhodov, Y., Tregubova, P., Abrosimov, K., and Cheptsov, V.: Soil microstructure is sensible to ecosystem and land use changes: simple approach to monitoring C pools, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-983, https://doi.org/10.5194/egusphere-egu2020-983, 2020.

D1993 |
Ivan Alekseev and Evgeny Abakumov

Polar soils play a key role in global carbon circulation and stabilization as they contain maximum stocks of soil organic matter (SOM) within the whole pedosphere. Cold climate and active layer dynamics result in the stabilization of essential amounts of organic matter in soils, biosediments, and grounds of the polar biome. Chemical composition of soil organic carbon (SOC) determines its decomposability and may affect soil organic matter stabilization (SOM) rate (Beyer, 1995). This is quite important for understanding variability in SOC pools and stabilization rate in context of changes in plant cover or climate (Rossi et al. 2016). 13C nuclear magnetic resonance spectroscopy, which provides detailed information on diversity of structural composition of humic acids and SOM, may also be used to study the SOM dynamics under decomposition and humification proceses (Kogel-Knabner, 1997; Zech et al., 1997). This study aims to characterize molecular organization of the humic acids, isolated from various permafrost-affected soils of Yamal region and to assess the potential vulnerability of soils organic matter in context of possible mineralization processes. Organic carbon stocks for studied area were 7.85 ± 2.24 kg m-2 (for 0-10 cm layer), 14.97 ± 5.53 kg m-2 (for 0-30 cm), 23.99 ± 8.00 kg m-2 (for 0-100 cm). Results of solid-state 13C-NMR spectrometry showed low amounts of aromatic components in studied soils. All studied humic powders are characterized by predominance of aliphatic structures, and also carbohydrates, polysaccharides, ethers and amino acids. High content of aliphatic fragments in studied humic acids shows their similarity fulvic acids. Low level of aromaticity reflects the accumulation in soil of lowly decomposed organic matter due to cold temperatures. Our results provide further evidence of high vulnerability and sensitivity of permafrost-affected soils organic matter to Arctic warming. Consequently, these soils may play a crucial role in global carbon balance under effects of climate warming.

How to cite: Alekseev, I. and Abakumov, E.: Soil organic matter in soils of the Russian Arctic: insights from 13C-NMR spectroscopy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1058, https://doi.org/10.5194/egusphere-egu2020-1058, 2020.

D1994 |
Mark E. Hodson

As the climate emergency gathers pace there is a growing imperative to reduce carbon emissions and move to a low carbon economy. Such shifts in global economics and politics will inevitably take time and therefore there is also a pressing need to identify immediate actions that can help limit or reduce carbon emissions.


Soils store large quantities of carbon. However, chemical, physical and biological properties limit the amount of carbon that any particular soil can store. It may be possible to alter soil properties such that the carbon sequestration potential of a soil is increased without causing a reduction in (or even increasing) other important ecosystem services delivered by the soil.


Soil aggregates are widely acknowledged to play an important role in the storage of carbon in soil. One limiting factor for aggregate formation in some soils is the amount of iron oxide present; the iron oxide is an important binding agent, holding aggregates together.


Ochres comprise a variety of poorly crystalline iron (III) oxides and form in a number of environments such as mine drainage when water moves to increasingly oxygenated environments and dissolved iron (II) is oxidised and precipitates from solution. In many countries these ochres are treated as wastes and are landfilled.


In batch experiments in which soil was amended with 0, 0.5 or 5% by mass ochre and shaken with water in a ratio of c. 1:5 (g:mL) ochre amendments reduced the concentration of dissolved organic carbon released into solution by almost a factor of 2. In experiments that are more realistic of field deployment of ochre amendments to increase soil carbon sequestration in which soils were amended with 0, 0.5 or 5% by mass ochre and kept moist for c. 9 weeks with treatments comprising presence/absence wheat and presence/absence earthworms, ochre amendments reduced the concentration of cold water extractable carbon by a factor of 2 and hot water extractable carbon by a factor of 1.3.

In this presentation the above results together with additional results relating to impacts on other soil properties will be presented. The data confirm the potential of waste iron ochres as a soil amendment to increase soil carbon sequestration though further work with more soil types and a variety of ochres is needed and the carbon footpring of applying the amendments needs to be calculated. Whilst methods such as these may provide vital time to transition to low carbon lifestyles, it is the move to such lifestyles that must be the ultimate solution to the climate emergency.

How to cite: Hodson, M. E.: Ochre amendments as a means of increasing carbon sequestration in soil, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1653, https://doi.org/10.5194/egusphere-egu2020-1653, 2020.

D1995 |
Noelia Garcia-Franco, Martin Wiesmeier, Roswitha Walter, Luis Carlos Colocho-Hurtarte, Vincent Buness, Bernd Josef Berauer, Marcus Zistl-schlingmann, Ralf Kiese, Michael Dannenmann, and Ingrid Kögel-Knabner

Alpine and pre-alpine grassland soils in Bavaria provide important ecosystem services and are hotspots for soil organic carbon (SOC) storage.  However, information on the underlying factors that control SOC stabilization via soil aggregation is limited. In three grassland soils with the same parent material but at different elevation (Fendt: 600 m.a.s.l, Graswang: 860 m a.s.l and Esterberg: 1,260 m a.s.l), we studied the soil aggregate distribution and associated SOC according to aggregate size classes (large-macroaggregates > 2,000 µm, small-macroaggregates 250-2000 µm, microaggregates 63-250 µm, silt plus clay particles <63 µm). Furthermore, the biomass and abundance of different ecological groups of earthworms were determined. Our results showed an increase in SOC contents and aggregate stability with elevation. SOC and N stocks of bulk soils showed the same trend as OC contents in aggregates.  Principal component analysis revealed that carbonates, SOC, aboveground plant biomass and the earthworm biomass are the main facilitating agents of aggregation and SOC and N storage in grassland soils of the Northern Limestone Alps of Germany

How to cite: Garcia-Franco, N., Wiesmeier, M., Walter, R., Colocho-Hurtarte, L. C., Buness, V., Berauer, B. J., Zistl-schlingmann, M., Kiese, R., Dannenmann, M., and Kögel-Knabner, I.: Biotic and abiotic controls on carbon storage in aggregates from grassland soils in the Northern Limestone Alps of Germany, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3020, https://doi.org/10.5194/egusphere-egu2020-3020, 2020.

D1996 |
Oscar Pascal Malou, David Sebag, Patricia Moulin, Tiphaine Chevallier, Yacine Badiane Ndour, Abou Thiam, and Lydie Chapuis-Lardy

Soil organic carbon (SOC) is a key element in the functioning of agrosystems. It ensures soil quality and productivity of cultivated systems in the Sahelian region. This study uses Rock-Eval pyrolysis to examine how cultural practices impact SOC quantity and quality of cultivated sandy soils in the Senegal groundnut basin. Such thermal analysis method provides cost-effective information on SOC thermal stability that has been shown to be qualitatively related to SOC biogeochemical stability. Soils were sampled within 2 villages agricultural plots representative of local agricultural systems and for local preserved areas. Total SOC concentrations ranged from 1.8 to 18.5 g.kg-1 soil (mean ± standard deviation: 5.6 ± 0.4 g.kg-1 soil) in the surface layer (0-10 cm) and from 1.5 to 11.3 g.kg-1 soil (mean ± standard deviation: 3.3 ± 0.2 g.kg-1 soil) in 10-30 cm deep layer. SOC of cultivated soils significantly (p-value < 0.0001) decreased according to treatments in the following order: +organic wastes > +manure > +millet residues > no input. Our results show that the quantity and the quality of SOC are linked to each other and both depend on land-use and agricultural practices, especially the nature of organic inputs. This correlation is very strong in the tree plantation (R² = 0.98) and in the protected shrubby savanna (R² = 0.97). It remains important for cultivated soils receiving organic wastes (R² = 0.82), manure (R² > 0.75), or millet residues (R2 = 0.91) but it’s no more significant in no-input situations. The Rock-Eval based indexes were depicted in a I/R diagram that illustrate the level of SOC stabilization and plotted against comparable results from literature. The Senegalese sandy soils have thermal signatures showing an inversion of the I and the R indexes compared to data from the literature and highlighting SOC stabilization as a function of soil depth. Indeed, the studied soils were characterized by a more abundant refractory pool (A5 which ranged from 7.7 to 21.3 % in 0-10 cm layer and from 12.5 to 24.3 % in 10-30 cm, respectively) compared to other tropical soils. The SOC in these sandy soils while positively affected by organic inputs is dominated by labile forms that mineralize quickly which is excellent for the needs of productivity of these agrosystems but not for mitigation of climate change.

Keywords: Soil organic carbon; Organic inputs; Thermal analysis; Agrosystems; West Africa

How to cite: Malou, O. P., Sebag, D., Moulin, P., Chevallier, T., Badiane Ndour, Y., Thiam, A., and Chapuis-Lardy, L.: Does agricultural practices impact the quantity and the forms of organic carbon stored in cultivated soils of the Senegal groundnut basin? A Rock-Eval approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11229, https://doi.org/10.5194/egusphere-egu2020-11229, 2020.

D1997 |
The potential for full inversion tillage to increase soil carbon storage following pasture renewal in New Zealand
Mike Beare, Erin Lawrence-Smith, Denis Curtin, Sam McNally, Frank Kelliher, Roberto Calvelo-Pereira, and Mike Hedley
D1998 |
Benjamin Bukombe, Laurent Kidinda, Alison Hoyt, Cordula Vogel, Marijn Bauters, Florian Wilken, Karsten Kalbitz, Peter Fiener, and Sebastian Doetterl

Tropical ecosystems and the soils therein have been reported as one of the most important and largest terrestrial carbon (C) pools and are considered important climate regulator. Carbon stabilization mechanisms in these ecosystems are often complex, as these mechanisms crucially rely on the interplay of geology, topography, climate, and biology. Future predictions of the perturbation of the soil carbon pool ultimately depend on our mechanistic understanding of these complex interactions.

Using laboratory incubation experiments, we investigated if carbon release from soils through heterotrophic respiration in the African highland forests of the Eastern Congo Basin follows predictable patterns related to topography, soil depth or geochemical soil properties that can be described at the landscape scale and ultimately be used to improve the spatial accuracy of soil C respiration in mechanistic models. In general, soils developed on basalt and granite parent material (mafic and felsic geochemistry of parent material) showed significantly (p <0.05) higher specific respiration than soils developed on sedimentary rocks (mixed geochemistry) with highest rates measured for soils developed on granite. For soils developed on basalt, specific respiration decreased two-fold with soil depth, but not for soils developed on granite or sedimentary rocks. No significant differences in respiration under tropical forest were found in relation to topography for any soil and geochemical background.

Using a non-linear,  stochastic gradient boosting machine learning approach we show that soil biological, physical and chemical properties can predict the pattern of specific soil respiration (R2=0.41, p<0.05). An assessment of the relative importance of the included predictors for soil respiration resulted in 43 % of the model being driven by geochemistry (pedogenic oxides, nutrient availability), 12 % driven by soil texture and clay mineralogy, 34 % by microbial biomass, C:N, and C:P ratios and 11 % by topographic indices. 

We conclude that, in order to explain soil C respiration patterns in tropical forests, a complex set of variables need to be considered that differs depending on the local bedrock chemistry. Its effect is likely related to the varying strength of C stabilization with minerals as well as nutrient availability that might drive C input patterns and microbial turnover.

How to cite: Bukombe, B., Kidinda, L., Hoyt, A., Vogel, C., Bauters, M., Wilken, F., Kalbitz, K., Fiener, P., and Doetterl, S.: Soil carbon respiration in tropical forest soils along geomorphic and geochemical gradients, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13609, https://doi.org/10.5194/egusphere-egu2020-13609, 2020.

D1999 |
Florian Schneider and Axel Don

Agricultural soils in Germany store about 2.5 Pg (1 Pg = 1015 g) of organic carbon in 0-100 cm depth. If this carbon was all powdered charcoal, it would fill a train with 61 million carriages, extending 2.5 times the distance to the moon. This study aimed at better understanding the origin of the organic carbon contained in mineral soils under agricultural use. For this, total organic carbon (TOC), C:N ratios and particulate organic carbon (POC) of 2,939 crop- and grassland sites scattered in a 8x8 km grid across Germany were evaluated. RandomForest algorithms were trained to predict TOC, C:N, POC and their respective depth gradients down to 100 cm based on pedology, geology, climate, land-use and management data. The data originated from the first German Agricultural Soil Inventory, which was completed in 2018, comprising 14,420 mineral soil samples and 36,163 years of reported management.

In 0-10 cm, land-use and/or texture were the major drivers for TOC, C:N and POC. At larger depths, the effect of current land-use vanished while soil texture remained important. Additionally, with increasing depth, soil parent materials and/or pedogenic processes gained in importance for explaining TOC, C:N and POC. Colluvial material, buried topsoil, fluvio-marine deposits and loess showed significantly higher TOC and POC contents and a higher C:N ratios than soil that developed from other parent material. Also, Podzols and Chernozems showed significantly higher TOC and POC contents and a higher C:N ratio in the subsoil than other soil types at similar depths because of illuvial organic matter deposits and bioturbation, respectively. In 30-70 cm depth, many sandy sites in north-western Germany showed TOC, POC and C:N values above average, which was a legacy of historic peat- and heathland cover. The depth gradients of TOC, POC and C:N showed only little dependence on soil texture suggesting that they were robust towards differences in carbon stabilization due to organo-mineral associations. Instead, these depth gradients were largely driven by land-use (redistribution of carbon in cropland by ploughing) and variables describing historic carbon inputs (e.g. information on topsoil burial). Hardpans with packing densities > 1.75 g cm-3 intensified the depth gradients of TOC, POC and C:N significantly, suggesting that such densely packed layers restricted the elongation of deep roots and therefore reduced organic carbon inputs into the subsoil.

Today’s soil organic carbon stocks reflect past organic carbon inputs. Considering that in 0-10 cm, current land-use superseded the effect of past land-cover on TOC while land-use showed no effect on POC and C:N, we conclude that topsoil carbon stocks derived from relatively recent carbon inputs (< 100 years) with high turnover. In the subsoil, however, most carbon originated from the soil parent material or was translocated from the topsoil during soil formation. High C:N ratios and POC content of buried topsoils confirm low turnover rates of subsoil carbon. The contribution of recent, root-derived carbon inputs to subsoils was small but significant. Loosening of wide-spread hardpans could facilitate deeper rooting and increase carbon stocks along with crop yield.

How to cite: Schneider, F. and Don, A.: What do carbon fractions and C:N ratios tell us about the origin of carbon in German agricultural soils?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18540, https://doi.org/10.5194/egusphere-egu2020-18540, 2020.