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Amazonian dark earth could be used to boost tree reforestation

Between 450 BCE and 950 CE, Amerindian people living in today’s Amazonia fundamentally transformed their habitat’s originally poor soils, enriching them with charcoal from their low-intensity fires used for cooking and burning refuse, as well as from manure, animal bones, broken pottery, and compost. The result is the exceptionally fertile Amazonian dark earth (ADE) – “tera preta” – which is rich in nutrients and stable organic matter derived from charcoal (an element that also gives it its dark color). 

In a new study published in the journal Frontiers in Soil Science, a team of Brazilian scientists has argued that Amazonian dark earth could be a “secret weapon” to boost reforestation efforts not only in the Amazon – where 18 percent of forests have been lost since the 1970s – but also worldwide.

“Here 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 to the presence of beneficial bacteria and archaea in the soil microbial community,” said co-lead author Luís Felipe Zagatto, a graduate student at the Center for Nuclear Energy in Agriculture at the University of São Paulo (USP) in Brazil. “This means that knowledge of the ‘ingredients’ that make ADEs so very fertile could be applied to help speed up ecological restoration projects.”

To investigate the ecological succession and soil changes occurring when pastures in deforested areas are actively restored to forests and assess the role of ADEs in boosting this process, the experts conducted controlled experiments involving 36 four-liter pots with three kilograms of soil kept inside a greenhouse with an average temperature of 34C (in order to anticipate global warming beyond the current 22-28 C average Amazonian temperatures). One third of these pots were filled with control soil, another third with a 4:1 mixture of control soil and ADE, and a last one fully with ADE.

After planting palisade grass (Urochloa brizantha) – a common forage for livestock in Brazil – in each pot and allowing it to grow for 60 days, they cut it and left only its roots in the soil, in order to mimic a “virgin territory” for reforestation. Then, they replanted each of the three types of soil with different types of tree seeds – the colonizing species Ambay pumpwood (Cecropia pachystachya), the Peltophorum dubium typical of secondary forests, and the cedro blanco (Cedrela fissilis) typical of climax forest. 

After the seedling were left to grow for three months, the researchers measured their height, extension of roots, and dry mass, as well as changes in the soil’s pH, texture, concentrations of potassium, calcium, magnesium, aluminum, boron, sulfur, copper, iron, and zinc. Finally, they quantified each type of soil’s microbial diversity.

The analyses revealed that ADEs had greater amounts of nutrients and supported a larger diversity of bacteria and archaea than control soils. 

“Microbes transform chemical soil particles into nutrients that can be taken up by plants. Our data showed that ADE contains microorganisms that are better at this transformation of soils, thus providing more resources for plant development,” explained co-lead author Anderson Santos de Freitas, a doctoral student at USP.

Moreover, adding Amazonian dark earth to the soils significantly improved the growth and development of plants (with the dry mass of palisade grass increasing 3.4 times in 20 percent ADE and 8.1 times in 100 percent ADE compared to the control soil), as well as the growth of the three tree species (for instance, seedlings of cedro blanco and P. dubium were 2.1 and 5.2 times taller in 20 percent ADE, and 3.2 and 6.3 times taller in 100 percent ADE than in control soils).

Although these findings provide clear evidence of the benefits of Amazonian dark earth in tree restoration, this type of soil has taken thousands of years to accumulate and would most likely take an equal amount of time to regenerate in nature if used. 

“Our recommendations aren’t to utilize ADE itself, but rather to copy its characteristics, particularly its microorganisms, for use in future ecological restoration projects,” concluded senior author Siu Mui Tsai, a professor of Cell and Molecular Biology at USP.

More about tree restoration 

Tree restoration refers to the process of repairing and restoring damaged, degraded, or destroyed forests and tree ecosystems. It aims to bring back the ecological, economic, and social benefits that healthy forests provide. This can be achieved through various methods, including natural regeneration, direct seeding, tree planting, and assisted natural regeneration.

Tree restoration is crucial for several reasons:

Biodiversity conservation

Forests are home to a vast range of plant and animal species. Restoring forests helps to conserve and protect this biodiversity, which is essential for maintaining the balance of ecosystems and supporting human livelihoods.

Climate change mitigation

Trees absorb and store carbon dioxide, which is a major greenhouse gas contributing to climate change. By restoring forests, we can help mitigate the impacts of climate change by increasing the carbon sequestration capacity of these ecosystems.

Erosion control

Tree roots stabilize soil and prevent erosion, which can lead to loss of fertile land and sedimentation in water bodies. Restoring forests can help protect and maintain soil quality and prevent erosion.

Water cycle regulation

Trees play a critical role in regulating the water cycle by absorbing and releasing water through transpiration. Restoring forests can help maintain water resources and reduce the risk of flooding and drought.

Livelihoods and economic benefits

Forests provide resources such as timber, non-timber forest products, and ecotourism opportunities. Restoring forests can create jobs and support local economies while providing valuable ecosystem services.

To achieve successful tree restoration, it is essential to consider factors such as site selection, appropriate tree species, timing of planting, and long-term maintenance.

Engaging local communities in restoration efforts and developing sustainable forest management practices can further enhance the success and sustainability of tree restoration projects.


By Andrei Ionescu, Staff Writer

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