The mystery of Amazonian ‘dark earth’ could aid in global forest regeneration.

Between 450 BCE and 950 CE, millions of Amerindians living in the present-day Amazonia changed the poor soil through a variety of processes. Charcoal from their low-intensity fires was used to cook and burn garbage, animal bones, broken pottery, compost, and manure over many generations, enriching the soil. The resulting substance is called Amazonian dark earth (ADE) or terra preta. It is extremely fertile due to its abundance of nutrients and stable organic matter derived from charcoal, which is what gives it its black color.

Now, Brazilian scientists demonstrate that ADE may be a “secret weapon” for increasing reforestation—not just in the Amazon, where 18 percent, or approximately 780,000 km2, of land has been lost since the 1970s, but globally. The findings are presented in the journal Frontiers in Soil Science.

“We show that the use of ADEs can enhance the growth of pasture and trees due to their high levels of nutrients as well as the presence of beneficial bacteria and archaea in the soil microbial community,” said Lus Felipe Zagatto, co-lead author and graduate student at the Center for Nuclear Energy in Agriculture of So Paulo University, Brazil.

“Here, we demonstrate how the usage of ADEs can promote pasture and tree growth because of their high nutritional content and the presence of advantageous bacteria and archaea in the soil microbial community.”

lead author Luís Felipe Zagatto, a graduate student at the Center for Nuclear Energy in Agriculture .

As a result, understanding the “ingredients” that make ADEs so fertile could be used to accelerate ecological restoration projects.

Imitating miniature reforestation

The purpose of the controlled experiments was to imitate the ecological succession and soil changes that occur when pasture in deforested areas is actively converted to forest. They wanted to find out how ADEs, or soils whose microbiome has been made up to look like ADEs, can speed up this process.

Zagatto and colleagues sampled agricultural soil from the Luiz de Queiróz Superior School of Agriculture in the state of So Paulo as a control, and ADE from the Caldeiro Experimental Research Station in the Brazilian state of Amazonas. They filled every one of 36 four-liter pots with 3 kg of soil inside a nursery with a mean temperature of 34 oC to expect an unnatural weather change past current temperatures in Amazonia somewhere in the range of 22 and 28 oC.

33% of the pots got just control soil, one more third got a 4:1 combination of control soil and ADE, and another third got 100 percent ADE. They gave each pot 60 days to grow seedlings of palisade grass (Urochloa brizantha), a common forage for livestock in Brazil, to mimic pasture. After that, they cut the grass back, leaving only its roots in the soil—a miniature piece of virgin land for reforestation. The researchers then replanted tree seeds in each of the three soils: either with the colonizing species Ambay pumpwood (Cecropia pachystachya), with Peltophorum dubium, common in auxiliary woodlands, or with Cedro blanco (Cedrela fissilis), normal in peak backwoods.

After allowing the seeds to germinate and the seedlings to grow for ninety days, the height, dry mass, and root length were measured. Throughout the course of the experiment, the researchers measured changes in the pH, texture, and concentration of organic matter, potassium, calcium, magnesium, aluminum, sulfur, boron, copper, iron, and zinc in the soil. They also measured changes in the soil’s microbial diversity using molecular techniques.

Rich in nutrients and beneficial

microbes At first, ADEs had more nutrients than the control soil: for instance, three to five times more of each of the other measured nutrients, with the exception of manganese, and thirty times more of phosphorus. ADE likewise had a higher pH and contained more sand and sediment, but less dirt. Although 100% ADE soils remained richer in nutrients than control soils, nutrient levels were intermediate in 20% ADE soils, suggesting that the plants had taken up some of the nutrients.

The final vases for Cecropia pachystachya show growth differences based on the soil. From right to left: control soil, 20% ADE, and 100% ADE. Credit: Luis Felipe Guandalin Zagatto

All through the trial, 20% or 100 percent ADE soils upheld a more noteworthy biodiversity of microbes and archaea than control soils.

“Microbes convert chemical soil particles into plant-acceptable nutrients.” According to Anderson Santos de Freitas, ADE’s co-lead author, “our data showed that ADE contains microorganisms that are better at this transformation of soils, thereby providing more resources for plant development.”

“For instance, the beneficial taxa of the bacterial families Paenibacillaceae, Planococcaceae, Micromonosporaceae, and Hyphomicroblaceae were found to be more abundant in ADE soils.”

In addition, the findings demonstrated that adding ADE to the soil enhanced plant growth and development. When compared to the soil that served as the control, palisade grass’s dry mass increased by 3.4 times in 20% ADE and 8.1 times in 100% ADE, respectively. The three species of trees also grew faster when ADE was added. Compared to seedlings in control soils, seedlings of Cedro blanco and P. dubium were 2.1 and 5.2 times taller in 20% ADE and 3.2 and 6.3 times taller in 100% ADE, respectively. Ambay pumpwood thrived in 100% ADE, whereas it did not even grow in control soils or 20% ADE.

The growth of Peltophorum dubium in the final vases varies depending on the soil. From left to right: control soil, 20% ADE, and 100% ADE. Credit: Lus Felipe Guandalin Zagatto

The researchers came to the conclusion that ADE can encourage plant growth. They wrote, “Our data point to a mixture of adapted microorganisms and soil nutrients [in ADE] to improve the establishment of plant trees in restoration.”

“ADE has taken thousands of years to accumulate and would take an equal amount of time to regenerate in nature if used,” said senior author Dr. Siu Mui Tsai, a professor at the same institute. Our proposals aren’t to use ADE itself, but rather to duplicate its attributes, especially its microorganisms, for use in future natural rebuilding projects.”

More information: Luís Felipe Zagatto et al, Amazonian dark earths enhance the establishment of tree species in forest ecological restoration, Frontiers in Soil Science (2023). DOI: 10.3389/fsoil.2023.1161627

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