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Want to grow supertrees? Grab a handful of dark earth from the Amazon forest.

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A type of soil called terra preta da Amazônia, or Amazon dark earth (ADE), promotes faster growth of trees and enhances their development in qualitative terms, according to an article published in the journal Frontiers in Soil Science.

“ADE is rich in nutrients and supports communities of microorganisms that help plants grow, among other things. Native people of the Amazon have used ADE to grow food for centuries and don’t need fertilizer for plants,” said Luís Felipe Guandalin Zagatto, a master’s student at the University of São Paulo’s Center for Nuclear Energy in Agriculture (CENA-USP) in Piracicaba, Brazil, and one of the authors of the article.

The researchers discovered that the microbiota (bacteria, archaea, fungi, and other microorganisms) found in ADE greatly benefit the growth of plants. When ADE was added to the soil, the development of the three tree species they examined was boosted. Seedlings of Brazilian cedarwood (Cedrela fissilis) and Yellow poinciana (Peltophorum dubium) grew two to five times taller than normal in soil with 20% ADE and three to six times taller with 100% ADE, compared to growth in control soil. Ambay pumpwood (Cecropia pachystachya) did not grow at all in the control soil (soil without ADE) but thrived in 100% ADE. The dry mass of Brachiaria forage grass increased over three times in soil with 20% ADE compared to control soil, and over eight times in 100% ADE.

“The bacteria in ADE convert certain molecules in the soil into substances that can be absorbed by plants. Using a very rudimentary analogy, you could say the bacteria act as miniature ‘chefs’ by transforming substances that can’t be ‘digested’ by plants into substances they can profitably metabolize,” said Anderson Santos de Freitas, first author of the article. He is a Ph.D. candidate at CENA-USP and co-author of the podcast Biotec em Pauta.

The ADE soil had more nutrients than the control soil. For example, it had 30 times more phosphorus and three to five times more of the other measured nutrients, except for manganese. It also had a higher pH.

Zagatto and colleagues gathered samples of the ADE at the Caldeirão Experimental Field in Amazonas state. The control soil was taken from experimental croplands maintained by Luiz de Queiroz College of Agriculture (ESALQ-USP) in Piracicaba, São Paulo state.

In a greenhouse with an average temperature of 34°C, resembling the potential impact of global warming, 36 four-liter pots were filled with 3kg of soil each. A third of the pots were filled with control soil, a third with a mixture of control soil and ADE, and a third with 100% ADE. Seeds of Brachiaria forage grass (Urochloa brizantha) were planted in every pot to mimic pasture. After 60 days, the grass was cut, but the roots were left to simulate the restoration of degraded pasture by sowing seeds of the three tree species.

Biotech applications

The group does not propose use of ADE as such, Zagatto explained, since it is a finite resource and well protected. The point of their research is to analyze ADE’s chemical properties (nutrients, organic matter and pH) as well as the enzyme activity and other biological and biochemical aspects that benefit plants.

“We need to understand exactly which microorganisms are responsible for these effects, and how we can use them without requiring ADE as such. We can then try, for example, to replicate these characteristics by means of biotech developments. This study was a first step in that direction,” he said.

Deforestation is a significant problem in Brazil, not only in the Amazon. There are various reasons for this, such as the replacement of forests with pastures or croplands. It is crucial to find ways to restore these areas quickly, so that the forest can regrow, and ecosystem services can resume. These services provide many benefits to the environment and human populations, including regulation of climate and air quality, as well as carbon storage in the soil.

“In the study, we set out to evaluate a possible driver of improvement for tropical forest ecological restoration projects, more specifically in the Amazon, so that in future these areas can return as near as possible to their original state,” Zagarro said. “We believe these results are promising and show that using the characteristics of ADE in seedling production or even directly in the field can be a way to accelerate tropical forest ecological restoration.”

The findings reported in the article resulted from studies supported by FAPESP (projects 20/08927-018/19000-4 and 14/50320-4) under the aegis of its Biodiversity, Characterization, Conservation, Restoration and Sustainable Use Program (BIOTA).

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