How ancient soils can boost forest restoration across the globe

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Amazonian Dark Earth (ADE) soil profile. Note that there is a black and yellow layer. ADE only corresponds to the black class. Credit: Luis Felipe Guandaline Zagato

A study shows that adding dark Amazonian soil to soil promotes plant growth.

Adding Amazonian dark soil (ADE) to soil can greatly enhance plant growth, according to a study by Brazilian scientists published in the journal Frontiers in Soil Science. ADE, also known as terra preta, is highly fertile due to its nutrient richness and stable organic matter. Researchers conducted experiments with ADE and found that adding it to soil increased the growth and development of plants, as well as supporting greater biodiversity of bacteria and archaea. The study suggests that ADE can be used to accelerate environmental restoration projects around the world.

Between about 450 BC and 950 AD, the millions of Amerindians living in the Amazon today transformed the originally poor soil through various processes. Over many human generations, the soil has been enriched with charcoal from its low-intensity cooking fires, waste burning, animal bones, broken pottery, compost, and manure. The result is Amazonian Dark Earth (ADE) or terra preta, which is exceptionally fertile because it is rich in nutrients and the stable organic matter derived from charcoal, which gives it its black colour.

Now, scientists from Brazil have shown that ADE could be a ‘secret weapon’ to boost reforestation – not just in the Amazon, where 18%, or nearly 780,000 square kilometers, has been lost since the 1970s – but all over the world. The results are published in the journal Frontiers in Soil Science.

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

“This means that knowledge of the ‘components’ that make these areas so fertile can be applied to help speed up environmental restoration projects.”

The final vases of Cedrela fissilis show differences in growth depending on the soil. From left to right: 100% ADE, 20% ADE, soil control. Credit: Luis Felipe Guandaline Zagato

Simulation of reforestation in miniature

The researchers conducted controlled experiments to mimic the ecological succession and changes in soils that occur when grasslands are restored in areas that have been effectively deforested. Their goal was to study how ADEs, or soils that the microbiome is artificially made to mimic, might enhance this process.

Zagatto and colleagues took samples of ADE from the Caldeirão Research Experimental Station in the Brazilian state of Amazonas and, as control, agricultural soils from the Luiz de Queiróz Superior School of Agriculture in the state of São Paulo. They filled each of 36 four-liter pots with 3 kg of soil, inside a greenhouse with an average temperature of 34 degrees Celsius to predict global warming beyond current temperatures in the Amazon between 22 and 28 degrees Celsius.

The final vases of Cecropia pachystachya show differences in growth depending on the soil. From left to right: 100% ADE, 20% ADE, soil control. Credit: Luis Felipe Guandaline Zagato

One third of the pots received control soil only, another third a 4:1 mixture of control soil and ADE, and another third 100% ADE. To mimic pasture, they sow seeds of fennel grass (Urochloa brizantha), a popular forage for livestock in Brazil, in each pot and let its seedlings grow for 60 days. Then they cut down the grass and let only its roots remain in the soil – virgin land for reforestation in miniature. The researchers then replanted each of the three soils with tree seeds: either with the colonial species Ambay Pumpwood (Cecropia pachystachya), with Peltophorum dubium typical of secondary forests, or with cedro blanco (Cedrela fissilis), typical of peak forest.

The seeds were allowed to germinate, and the seedlings to grow for 90 days, after which the length, dry mass, and extension of the roots were measured. The scientists determined changes in soil pH, texture, organic matter concentration, potassium, calcium, magnesium, aluminum, sulfur, boron, copper, iron, and zinc over the course of the experiment. Using molecular methods, they also measured changes in soil microbial diversity.

The final vases of Peltophorum dubium show differences in growth depending on the soil. From left to right: 100% ADE, 20% ADE, soil control. Credit: Luis Felipe Guandaline Zagato

Rich in nutrients and beneficial microbes

Initially, the ADEs showed greater amounts of nutrients than the control soil: for example, 30 times more phosphorus and three to five times more than each of the other nutrients measured, with the exception of manganese. ADE also has a higher pH and contains more sand and silt, but less clay. After the experiment, the soil contained less nutrients than it did at the beginning, reflecting uptake by plants, but the 100% ADE soil remained richer in it than the control soil, while nutrient levels were average in the 20% ADE soil.

Throughout the experiment, 20% or 100% ADE soils supported greater biodiversity of bacteria and archaea than control soils.

Microbes convert soil chemical particles into nutrients that plants can absorb. “Our data showed that ADE contains microorganisms that are better at this soil transformation, thus providing more resources for plant development,” said joint lead author Anderson Santos de Freitas.

“For example, ADE soils contained more beneficial taxa from the bacterial families Paenibacillaceae, Planococcaceae, Micromonosporaceae, and Hyphomicroblaceae.”

Promote growth

The results also showed that the addition of ADE to the soil improved the growth and development of plants. For example, the dry mass of fennel grass was increased 3.4 times in 20% ADE, and 8.1 times in 100% ADE, compared to control soils. Addition of ADE also enhanced the growth of the three tree species: cedro blanco and P. dubium seedlings were 2.1 and 5.2 times taller in 20% ADE, and 3.2 and 6.3 times taller in 100% ADE, compared to control soils. Ambay firewood did not grow in control soil or 20% ADE, but thrived in 100% ADE.

The researchers concluded that ADE can promote plant growth. Our data indicate a mixture of soil nutrients and adaptive microorganisms [in ADE] To improve residence planting trees in the restoration process,” they wrote.

Senior author Dr. Siu Mui Tsai, Professor at the same institute, warned: “ADE has taken thousands of years to accumulate and would take equal time to regenerate in nature if used. Our recommendations are not to use ADE itself, but rather to copy its properties, in particular microorganisms, to use them.” in future environmental restoration projects.”

Reference: “Amazonian Dark Earth Promotes Tree Species Establishment in Forest Ecological Restoration” By Anderson Santos de Freitas, Luis-Philippe Guandalin Zagato, Gabriel Silvestre Rocha, Francil Muchalac, Solange dos Santos Silva, Alexandre Westphal-Muniz, Rogerio Eiji Hanada and Siu Mui Tsai, 5 May 2023, Frontiers in Soil Science. DOI: 10.3389/fsoil.2023.1161627

Funding: São Paulo Research Foundation, Brazilian National Council for Scientific and Technological Development, Coordination for the Improvement of Higher Education Personnel – Brazil

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