Organic and inorganic carbon dynamics in calcareous soils – SIC-SOC-DYN

Analytical difficulties in measuring SOC and SIC contents separately have often discouraged studies of SOC dynamics in carbonate soils. The C stocks of these soils are therefore poorly characterised and little taken into the accounts of the global C balance. Therefore, SIC-SOC-DYN aims (i) to document the dynamics of SOC in soils containing SIC, and (ii) to understand and formalize the interactions between these two forms of C in soils.

The main activities of the project are (i) to establish protocols on C measurements and especially test the potential of Rock-Eval® analysis to characterize the forms of C in calcareous soils, (ii) to characterize SIC and SOC forms by soil fractionation and (iii) to study the origin of the CO2 emissions from calcareous soils. A large range of contrasted calcareous soil in terms of SIC and SOC contents will be studied.

A collection of limestone soils from France, Italy, Germany, Hungary, Tunisia and Canada has been assembled.

Adjustments to the standard RockEval® analysis cycle have been proposed to measure SOC and SIC in a same run and without pre-treatment. However, statistical corrections are still necessary to obtain data close to the elemental analysis.

Adjustments to the analysis cycle are still on going to avoid statistical corrections.

Soil fractionation and soil incubations are underway on the proposed soil collection in order to characterise the mechanisms by which organic matter is stabilised in calcareous soils.

Hazera, J., Sebag, D., Kowalewski, I., Verrecchia, E., Ravelojaona, H., Chevallier, T., 2023. Adjustments of the Rock-Eval® thermal analysis for soil organic and inorganic carbon quantification. Biogeosciences 20, 5229-5242. doi.org/10.5194/bg-20-5229-2023

Soil is the largest continental carbon (C) sink and contributes to the global C cycle. Two thirds of soil C is organic (Soil organic carbon, SOC). SOC results from the balance between captured atmospheric CO2 (via photosynthesis) and the incorporation of litter decomposition products, and the CO2 emissions, via the respiration of roots and heterotrophic microorganisms. SOC plays a key role in the physical, chemical, and biological properties of soils. Understanding its dynamics is a major challenge for maintaining soil fertility while participating in the storage of C. However, one third of soil C is inorganic (Soil Inorganic Carbon SIC). SIC consists of lithogenic, or petrogenic, (primary) carbonate inherited from the bedrock and pedogenic (secondary) carbonate precipitated in the soil. Because SIC pools are generally considered more stable and less impacted by human activities than SOC pools, the SIC dynamics are of less interest in the short-term. Moreover, analytical di?culties in studying SOC and SIC separately have impeded knowledge on the dynamics of SOC in carbonate soils. The C stocks of these soils, even though they cover one third of the Earth's surface, are given little consideration in the global C balance.
Although interactions between SIC and SOC pools have been described in the short-term, they are poorly understood. Isotopic analyses have shown that carbonate soils emit CO2 from both C pools. There is also an inherited or neoformed origin of SIC, as organisms (bacteria, roots, fungi) have the ability to precipitate biominerals from metabolic pathways of organic matter transformation.
Our project proposes the study of both SOC and SIC contents, quality and dynamics in various contexts. The main objectives are to propose innovative analytical tools and to acquire knowledge on the C balance in carbonate soils according to their use and management.
Knowledge of these processes and the development of analytical methodologies speci?c to these soils will facilitate the acquisition of data distinctive of these soils. Knowledge sharing will focus on training of young scientists (MS, PhD, post-doc), exchanges between scienti?c communities (geochemists, soil scientists, agronomists) and raise awareness in soil analysis.
The project is based on 3 scienti?c work packages and a work package for coordination:
1- Integrated methodology to study SOC and SIC forms
We propose to develop soil analysis on C pools in carbonate soils. A diversity of carbonate soils in terms of SIC features will be characterized. Protocols to analyse SOC and SIC pools will be compared and develop. We propose to couple thermal and isotopic analyses to characterise the forms of SIC and SOC in carbonate soils in a rapid and partially automated way.
2- Processes of SOC stabilisation in carbonate soils
Thermal, physical (size), chemical and morphological analysis of SOC in di?erent calcareous contexts will help to explore the relationships between soil properties and SOC. The quanti?cation of SOC in its di?erent forms would allow understanding the processes of SOC stabilisation in these speci?c soils.
3- Contributions of SIC and SOC to C ?uxes between soil and atmosphere
Relationships between soil properties, SIC and SOC forms and dynamics will be studied through soil incubations and modelled.
Research will mainly focus on the solid phases of SOC and SIC in a collection of soils with varying SOC and SIC contents. Speci?cally, our objectives are (i) to establish protocols to measure SIC and SOC contents and their 13C natural abundance (ii) to identify stabilized SOC pools in soils with di?erent SIC features and (iii) to approach C balance in various carbonate environments. Finally, our ambition is to develop a scienti?c community studying C cycle in carbonate soils.