Sustainable Footprint

Terra preta, catching carbon

Terra preta (“dark earth” in Portuguese) is the name of very dark, fertile soils found in the Amazon Basin. It owes its name to its very high charcoal content. It is also known as “Amazonian dark earth” or “Indian black earth. For a long time, the origins of the Amazonian dark earths were not clear and several theories were considered.

A typical riverside landscape where terra preta was found

One idea was that they resulted from ashfall from volcanoes in the Andes. Another theory considered its formation as a result of sedimentation in Tertiary lakes or in recent ponds.

Ancient civilisation
However, because of their elevated charcoal content and the common presence of pottery remains, it is now widely accepted that these soils are a product of indigenous soil management involving a labor intensive technique termed slash-and-char. This type of soil appeared between 450 BC and AD 950 at sites throughout the Amazon Basin. The Spanish explorer Francisco de Orellana, the 16th C explorer who was the first European to transverse the Amazon River, reported densely populated regions running hundreds of kilometers along the river, suggesting population levels exceeding even those of today. These populations left no lasting monuments because they used local wood as their construction material, which unfortunately rotted in the humid climate (stone was unavailable).
The semi-nomadic descendants make still a distinction among tribal indigenous societies of a hereditary, yet landless, aristocracy, which is strange for a society without a sedentary, agrarian culture. This suggests they once were more settled and agrarian. In the 16th and 17th century diseases were brought by Europeans, which caused a collapse of the societies. Moreover, many indigenous people were forced to adapt to a more mobile lifestyle in order to protect themselves against colonialism.
Recent excavations show that there has been a highly developed culture with cities, irrigation systems etc.

There is ample evidence for complex large-scale, pre-Columbian social formations, including chiefdoms, in many areas of Amazonia and even large towns and cities. For instance the pre-Columbian culture on the island of Marajo may have developed social stratification and supported a population of 100,000 people. The Native Americans of the Amazon rain forest may have used Terra preta to make the land suitable for the large scale agriculture .

The places in Amazonia where the terra preta has been found

Terra preta
Terra preta is characterized by the presence of charcoal in high concentrations; of high quantities of pottery sherds; of organic matter such as plant residues, animal faeces, fish and animal bones and other material; and of nutrients such as nitrogen (N), phosphorus (P), calcium (Ca), zinc (Zn), manganese (Mn). It also shows high levels of microorganic activities and other specific characteristics within its particular ecosystem.

Left the poor soil typical of Amazonia, right the fertile soil 'terra preta' rich in carbon and nutrients

It is less prone to leaching than surrounding soils. Terra preta zones are generally surrounded by terra comum, or "common soil"; these are infertile (yellow) soils.
Terra preta soils are of pre-Columbian nature and were created by man between ca. 5000 BC–1450 AD. The soil's depth can reach 2 metres (6 feet). Thousands of years after its creation it is reputedly known as self-regenerating at the rate of 1 centimetre per year by the local farmers in Brazil's Amazonian basin, and they seek it out for use and for sale as valuable compost.
Terra preta sites are also known in other South American areas (Ecuador, Peru, Guyana) , in West Africa (Benin, Liberia), and on the South African savannas. Similar soil was found in late Roman Britain.

Biochar

Seeing the good effects of charcoal mixed in the soil, people have started making charcoal to be mixed with soil. If organic materials like wood and waste are heated without air (pyrolysis) you get charcoal, or ‘biochar’. If this is mixed in the soil it creates a fertile soil that is ‘carbon-negative’.

The pyrolysis process

It works as a net withdrawal of atmospheric carbon dioxide stored in soil carbon that can stay ‘forever’.
Biochar-amended soil also keeps the nutrition better, therefore reduces the total fertilizer requirements. It is simply good for the climate and environmental impact of croplands. Char-amended soils have shown 50 – 80% reductions in nitrous oxide emissions and reduced runoff of phosphorus into surface waters and leaching of nitrogen into groundwater. Biochar significantly increases the efficiency of and reduces the need for traditional chemical fertilizers, while greatly enhancing crop yields. The soil is also is less easily eroded.

Moreover in the pyrolysis process renewable oils and gases are co-produced which can be used as fuel. Biochar thus offers promise for its soil productivity and climate benefits.
In an article Nature, Lehmann wrote that "Our calculations suggest that emissions reductions can be 12 to 84% greater if biochar is put back into the soil instead of being burned to offset fossil-fuel use."

Biochar used in a plantation

Making charcoal has been known for thousands of years, now it is again interesting because it could help to solve the problem of global warming. Mixed with topsoil in farming to remove the carbon from the crop's lifecycle, when magnified over several countries, could result in a sharp reduction of atmospheric carbon.
The technology is already available and the pyrolysis process, which also produces biofuel in the form of bio-oil, could make biochar use an economically attractive proposition. "The benefits are now clear and far enough advanced that economists can help to find opportunities to make it work," Lehmann said in a recent interview.

Using 'biofuel' is carbon neutral, using biochar is carbon negatieve: carbon is removed from the athmosphere.

What are the benefits of using biochar in the garden or in the fields?

The following benefits occur with additions of biochar
• Enhanced plant growth
• Suppressed methane emission
• Reduced nitrous oxide emission (estimate 50%)
• Reduced fertilizer requirement (estimate 10%)
• Reduced leaching of nutrients
• Stored carbon in a long term stable sink
• Reduces soil acidity: raises soil pH
• Reduces aluminum toxicity
• Increased soil aggregation due to increased fungal hyphae
• Improved soil water handling characteristics
• Increased soil levels of available Ca, Mg, P, and K
• Increased soil microbial respiration
• Increased soil microbial biomass
• Stimulated symbiotic nitrogen fixation in legumes
• Increased arbuscular mycorrhyzal fungi
• Increased cation exchange capacity

As modern agriculture, heavily relying on chemical fertilisers, has made the topsoil poor in organic matter, general use of biochar could not only reduce global warming, but also increase food production.

Adapted from Wikipedia and other sources

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