Agronomist Francisco Ruiz working in the field (photo: Beatriz Marchese Silva/ESALQ-USP)

Published on 01/28/2025

By Luciana Constantino  |  Agência FAPESP – The combination of materials rich in calcium, magnesium and kerogen (a chemical compound that is a precursor of petroleum) makes it possible to produce a type of modified soil (Technosol) capable of restoring areas degraded by mining with high stability, i.e. with more carbon capture and with less possibility of emitting CO₂ into the atmosphere over the years. The assessment of this stability was only possible thanks to the creation of an integrated methodology combining thermal analysis, chemical extractions and incubations.

This new method is described in research published in the scientific journal Soil Biology and Biochemistry.

Among the novelties of the work is the use of the Rock-Eval technique, which is rarely used in soil analysis. It is widely used in the oil and gas industry to assess the quantity and quality of organic matter present in sedimentary rocks. It involves a chemical process called pyrolysis, which preserves the original composition of the organic matter. This is done by heating the material to high temperatures, usually above 500 °C, in an oxygen-free environment, resulting in the breaking of chemical bonds and the formation of gases that are chemically characterized.

Equipment used in the Rock-Eval technique (photo: Francisco Ruiz/ESALQ-USP)

It differs from other methods in that it provides two important pieces of information: the chemical characterization and the thermal stability of the organic material. This allows researchers to determine the proportion of elements in the organic matter – mainly carbon, hydrogen and oxygen – and how resistant it is to heat.

When organic matter is stable, the carbon remains in the soil, contributing to CO₂ sequestration. In addition, organic matter contributes to soil fertility by slowly providing essential nutrients to vegetation and improving the physical structure of the soil, making it more resistant to compaction and erosion. Understanding this stability process is therefore critical to the development of sustainable land management, whether for agriculture, mining or forest restoration.

“The motivation for the study came from Technosols with high carbon concentrations. We wanted to know if the stabilization of the organic matter was only due to the geological carbon or if some other process was involved. By using Rock-Eval, we were able to separate the material. We had two different sources of organic matter: the one that came from plant growth and grazing, and the geological one, in the form of kerogen,” explains agronomist Francisco Ruiz, a researcher at the Department of Soil Science of the Luiz de Queiroz College of Agriculture of the University of São Paulo (ESALQ-USP) in Brazil and first author of the study.

In the work, the scientists analyzed an area with a Technosol rich in calcium, magnesium and kerogen (insoluble fraction of organic matter present in sedimentary rocks) that is under pasture cultivation and compared it with sites with carbon-rich shales and natural soils under long-term pasture cultivation.

“We saw that there was a synergy between the kerogen and the plant material in the Technosol, which gave greater stability than the two separately,” adds Ruiz, who was supported by FAPESP through two scholarships, including a Research Internship Abroad.

During this period, he carried out part of his work at the Institute of Ecology and Environmental Sciences at the Sorbonne University in France, under the guidance of researcher Cornelia Rumpel, who also signed the article and whose work is considered one of the most important on organic matter in terrestrial ecosystems, including soil carbon sequestration mechanisms.

Samples

The study area is located within a limestone mine in the town of Saltinho, in the interior of the state of São Paulo (Brazil). About 20 years ago, reclamation of part of the site began, with wells being filled in with mine tailings, including fragments of sedimentary rock. The region’s climate is humid subtropical and the native vegetation is Atlantic Forest.

“We used Technosol, but the method could work to understand the stability of organic matter in other types of soil with this mixed composition, such as biochar for agricultural areas and Amazonian dark earth,” Ruiz told Agência FAPESP.

Biochar is a type of agricultural biocarbon produced by pyrolysis, the controlled carbonization of plant biomass (which can be anything from straw to wood) in the absence of oxygen. The name comes from the combination of the English words biomass and charcoal. Studies have shown that the product increases agricultural productivity and root and shoot growth (read more at: revistapesquisa.fapesp.br/en/biochar-can-have-benefits-in-agriculture/).

Using the new method, the researchers showed that the way minerals bind to organic matter can affect its stability. For example, they discovered that the stability of organic materials during oxidation is related to the amount and type of interactions between minerals and compounds, particularly calcium (Ca²⁺) and magnesium (Mg²⁺).

“This work has a methodological innovation, but it also makes an important contribution in the sense that it proves, in a more powerful way, some processes that Francisco himself had already demonstrated in previous studies. Once science knows the process, it’s possible to create the conditions for it to happen, opening up a path of possibilities for thinking ahead,” says ESALQ-USP professor Tiago Osório Ferreira, corresponding author of the article and Ruiz’s advisor during his doctorate.

Research published last year and also signed by Ruiz, Rumpel and Ferreira showed that Technosol built from mine waste is capable of sequestering carbon, with the potential to capture up to 1 gigaton of CO₂ equivalent (read more at: agencia.fapesp.br/44801).

Soil is one of the planet’s four major carbon reservoirs, along with the atmosphere, oceans and vegetation. However, in a degraded state, it can release CO₂, as happens with forests.

A study published in 2023 concluded that Brazil stores in its soil the equivalent of 70 years of the country’s CO₂ emissions. The data come from MapBiomas, a collaborative network made up of NGOs, universities and technology startups that maps land cover and land use in Brazil. Of the total 37 gigatons of soil organic carbon (SOC) in Brazil in 2021, almost two-thirds (63%) were in areas with stable native cover (23.4 Gt SOC), mostly in the Amazon.

According to Ferreira, the new methodological process developed is now being replicated in other objects of study, thus opening up new lines of research.

The article “Combining thermal analyses and wet-chemical extractions to assess the stability of mixed-nature soil organic matter” can be read at: www.sciencedirect.com/science/article/abs/pii/S003807172300278X?via%3Dihub.  

 

Source: https://agencia.fapesp.br/53788