Methodology allows evaluating soil stability in an area degraded by mining

Luciana Constantino | FAPESP Agency – The combination of materials rich in calcium, magnesium and kerogen (a chemical precursor to petroleum) makes it possible to produce a type of modified soil (Technosol) capable of recovering areas degraded by mining with high stability, that is, capturing more carbon and with less possibility emission of CO2 into the atmosphere over the years. The assessment of this stability was only possible thanks to the creation of an integrated methodology, which combined 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, the use of the Rock-Eval technique, little adopted in soil analysis, stands out. It is widely used in the oil and gas industry to evaluate the quantity and quality of organic matter present in sedimentary rocks. It involves a chemical process called pyrolysis, which preserves the original composition of organic matter. To do this, the substance is heated to high temperatures, generally above 500° C, in an environment without oxygen, leading to 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 by providing two important pieces of information: the chemical characterization and thermal stability of organic matter. With this, researchers are able to know the proportion of elements that organic matter contains – mainly carbon, hydrogen and oxygen – and how resistant it is to heat.

When organic matter is stable, carbon remains in the soil, contributing to CO2 sequestration. Furthermore, organic matter helps with soil fertility, slowly providing essential nutrients to vegetation and improving the physical structure, making it more resistant to compaction and erosion. Therefore, understanding this stability process is crucial for developing sustainable land management, whether for agriculture, mining or forest restoration.

“The motivation for the study comes from Technosols with a high concentration of carbon. We wanted to know whether the stabilization of organic matter occurred solely because of geological carbon or whether there was some other process 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 pasture and the geological one, in the form of kerogen”, explains the agricultural engineer Francisco Ruizresearcher at the Department of Soil Science at the Luiz de Queiroz College of Agriculture, at the University of São Paulo (Esalq-USP), and first author of the study.

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

“We saw that there was a synergy between the kerogen and the plant material in the Technosol that gave greater stability than the two separately”, adds Ruiz, who received support from FAPESP through two bagsincluding an Internship Abroad (BEPE).

During this period, he developed part of the work at the Institute of Ecology and Environmental Sciences at Sorbonne Université (France), under the guidance of researcher Cornelia Rumpel, who also wrote the article and whose work has been considered one of the most important linked to organic matter in ecosystems. terrestrial resources, including soil carbon sequestration mechanisms.

Samples

The study area is located inside a limestone mine in the city of Saltinho, in the interior of the State of São Paulo. Around 20 years ago, part of the site began to be recovered, with the filling of wells using mine waste, which includes fragments of sedimentary rocks. The region’s climate is humid subtropical, and the native vegetation is the Atlantic Forest.

“We use Technosol, but the method can work to understand the stability of organic matter in other types of soil with this mixed composition, such as biochar for agricultural areas and terra preta from the Amazon,” Ruiz tells FAPESP Agency.

Biochar is a type of biochar aimed at agricultural use, produced from pyrolysis, originating from the controlled carbonization process of plant biomass (which can range from straw to wood) without the presence of oxygen. The name comes from the union of words in English biomass (biomass or organic matter) and charcoal (coal). Studies have shown that the product increases agricultural productivity, root and shoot growth (Read more at: lojapesquisa.fapesp.br/biocarvao-pode-trazer-ganhos-a-agricultura/).

Using the new methodology, the researchers pointed out that the way minerals bind to organic matter can influence its stability. They discovered, for example, that, during oxidation, the stability of organic materials is related to the amount and type of interactions between minerals and compounds, especially calcium (Ca2+) and magnesium (Mg2+).

“This work has a methodological innovation, but it also makes an important contribution in the sense of proving, in a more conclusive way, some processes that Francisco himself had already demonstrated in previous studies. Once science knows the process, it is possible to create conditions for it to happen, opening up an avenue of possibilities for thinking from now on”, assesses the Esalq professor Tiago Osório Ferreiracorresponding author of the article and who was Ruiz’s advisor during his doctorate.

Research published last year and also signed by Ruiz, Rumpel and Ferreira demonstrated that Tecnossol built from mine waste was capable of sequestering carbon, with the potential to capture up to 1 gigaton of CO2 equivalent (Read more at: agencia.fapesp.br/41887).

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

Released last year, lifting from MapBiomas concluded that Brazil stores the equivalent of 70 years of the country’s CO2 emissions in the soil. Of the total 37 gigatons of soil organic carbon (SOC) existing in Brazil in 2021, almost two thirds (63%) were in areas under stable native coverage (23.4 Gt COS), mainly in the Amazon.

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

The article Combining thermal analyzes 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.