Rare Amazon dark soils could help forest restoration, study shows

Beneath the luxurious foliage and the majestic trees of the Amazon Rainforest lies a counterintuitive reality: a mostly infertile soil that becomes swiftly depleted once its plant coverage is stripped away. Cattle ranchers and farmers searching for arable land continually encroach upon the forest, only to find the soil exhausted within a few years. This drives the intrusion into new, pristine swaths of forest, perpetuating deforestation cycles.

This grim picture could change in the next few years. Researchers from the University of São Paulo, in partnership with the Brazilian Agricultural Research Corporation (Embrapa) and the National Institute of Amazonian Research (INPA) have found that Amazonian Dark Earths (ADEs), a unique and somewhat rare type of soil, could help in the recovery of pastures and of the forest itself due to the soil’s high nutrient and microbiological contents.

Their study, published in Frontiers in Soil Science, showed that ADEs could promote the development of trees used in ecological restoration projects.

The dark soil used in the experiment can only be found in a few spots in the Amazon, usually covering 2-5 hectares (4.94-12.35 acres) and located close to rivers and to places that have previously seen some human activity. Some theories regarding the origin of ADEs, also called terra preta, include the existence of large sedentary pre-Columbian societies that improved, intentionally or not, the fertility of the soil with ashes, ceramics, bones of animals and other waste, increasing the levels of organic matter.

The decomposition process that could have lasted at least 2,000 years (and up to 8,500 years) made these dark soils highly fertile, with some native populations using them for subsistence farming. Researchers are now studying what makes this soil more fertile in order to create a protocol for improving planting in deforested areas. This could become a lifeline to the Amazon Forest.

Mimicking reforestation

The experiment, which proved the dark soil’s “exceptional” fertility and pointed to its possible use to enhance the growth of pasture and trees, entailed simulating the changes that happen in the soil when pasture in deforested areas is actively restored to forest.

The researchers filled 36 4-liter (1-gallon) pots with 3 kilograms (6.6 pounds) of soil each in a greenhouse with a mean temperature of 34 degrees Celsius, about 93 degrees Fahrenheit (anticipating global warming beyond current temperatures in the Amazon Forest, currently 22-28 degrees Celsius, or 71.6-82.4 degrees Fahrenheit). A total of 12 pots received regular soil, another third of the pots had a mixture of regular soil (80%) and ADE (20%), and the rest were filled with 100% ADE.

Mimicking the effects of pasture upon the soil, the researchers planted seeds of palisade grass (Brachiaria brizantha), extensively used in Brazil for livestock. After two months of growth, the grass was cut, but its roots stayed in the soil. The scientists then planted three tree seeds: Ambay pumpwood (Cecropia pachystachya, a colonizing species), Peltophorum dubium (typical of secondary forests) and cedro blanco (Cedrela fissilis, typical of mature forests).

After 90 days of growth, they measured how seedlings’ roots evolved; quantified changes in soil pH, texture, concentration of organic matter and of many chemical elements, including potassium, calcium and magnesium; and assessed the soil’s microbial diversity.

They found that the presence of nutrient-rich ADEs boosted plant growth up to eight times even in samples with mixed soils and lowered soil depletion after the experiment. ADEs also contained microorganisms that are better for the transformation of soils and also more beneficial families of bacteria.

These optimistic results are considered just a first step in researching dark soils. Since ADEs are rare and using them directly to improve plant growth would be unfeasible, researchers plan to better understand their composition and microbiology in order to create a product that could be used in agricultural processes or reforestation projects.

“The idea is to extract those factors and to create a formula, having the dark soil as a basis, but using materials that don’t need to come directly from the dark soil to amplify [forest] restoration projects,” Anderson Freitas, one of study’s lead authors, told Mongabay in a video interview.

Freitas said he believes that creating a biofertilizer based on dark soils might be ambitious, but the aim is to create a better protocol showing which microorganisms could be used to improve reforestation. “Today, there are almost no reforestation projects in the Amazon. … But when you offer a technology for this, you manage to make the interest grow a little more too,” he said.

This new protocol could also help mitigate the current scarcity of certain fertilizers, which was worsened by the Russia-Ukraine war, two of the largest world suppliers before the conflict. ADEs are rich, for instance, in phosphorus, a nutrient much needed by Brazilian soils, but which has a very limited supply chain. Its total global reserves could last a maximum of 50-100 years, scientists estimate. “The dark soil is a fertility oasis in the Amazonian soil,” said Aleksander Muniz, researcher of Embrapa, who advised on the experiment and is responsible for monitoring dark soils research in Brazil.

Muniz said he hopes dark soils research will help recover pasture areas, which would in turn have a greater impact on reducing deforestation. “Around 80% of deforestation in the Amazon comes from extensive cattle raising. By studying plant-soil feedbacks, the intention is to recover degraded areas. Pasture suffers from chemical degradation, and then from biological degradation, until complete depletion of the area,” Muniz told Mongabay in a video interview.

Recovering deforested areas

In 2021 alone, almost 1 million ha (2.47 million acres) of forest were cleared in the Amazon, according to MapBiomas.

Between 1985 and 2022, the Amazon Rainforest lost 12% of the total area it occupied, or about 44 million ha (108.7 million acres) of native vegetation. On the other hand, according to data from Brazilian conservation nonprofit Imazon, in 2019, there were 7.2 million hectares (17.8 million acres) that had been in a natural process of regeneration for at least the six previous years, with 73% located in areas classified as “low agricultural propensity.” Imazon advocates for protection of these areas and also for projects of assisted natural regeneration.

ADEs research has been gaining more interest recently. Christoph Müller, professor of experimental plant ecology at the Justus-Liebig Giessen University in Germany, plans to investigate ADEs later in 2023. According to Müller, dark soils have a double function. “First of all, [they can] increase carbon sequestration, but we also can hopefully minimize or at least reduce the amount of fertilizer. … The only drawback to all of this is that it’s a long-term process,” Müller told Mongabay in a phone interview.

By speeding up forest and pasture recovery, ADEs research aims to also halt further deforestation. “If you wait for the environment to recover, it takes much longer. With the technology that is being developed, I’m going to put a little black soil, which has fantastic microbiota, to recover this soil, together with the plants. … We are running against the clock to recover these pastures. Every hectare of pasture recovered, it’s a new area that won’t need to be deforested in the future,” Muniz says.

Citation:

• Freitas AS, Zagatto LFG, Rocha GS, Muchalak F, Silva SS, Muniz AW, Hanada RE and Tsai SM (2023) Amazonian dark earths enhance the establishment of tree species in forest ecological restoration. Soil Sci. 3:1161627. doi: 10.3389/fsoil.2023.1161627
• Barbosa, J. Z., Motta, A. C. V., Corrêa, R. S., Melo, V. F., Muniz, A. W., Martins, G. C., Silva, L. C. R., Teixeira, W. G., Young, S. D., & Broadley, M. R. (2019). Elemental signatures of an Amazonian Dark Earth as result of its formation process. Geoderma, vol. 361

Banner image: Image © Christian Braga/Greenpeace.

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