New light on dark earth

Amazonian dark earth (ADE), is a collective term for the dark-coloured soils in the Amazon delta which came about through human activity. The soils’ name reflects the black colour which results from the presence of little bits of charcoal and other organic matter. ADE contains high concentrations of the nutrients phosphorus, nitrogen and calcium.

The bits of charcoal fix the nutrients in the soil, turning ADE into a kind of oasis in the Amazon. The highest tracts of land in the region consist of severely weathered, acid soils which fix very few nutrients. Over the course of centuries, the Indians of the pre-Columbian period formed ADE – whether consciously or not – by enriching their fields with charcoal fish-bone, other bones and household waste.

Amazonian dark earth is interesting soil. As a cultural phenomenon, as agricultural soil, and potentially even as a carbon sink that could help solve the climate problem. Because you can more or less create ADE yourself by burying carbon (biochar) in the ground. This is good for the climate, as any carbon you bury underground does not get into the air in the form of carbon dioxide. It is also good for the soil, which becomes more fertile.

Wageningen is very much involved in the research going on through the Terra Preta Program, an INREF-funded project in which universities from Bolivia, Colombia and Brazil are also participating. The first three (of six) PhD candidates will be defending their theses on 1 July. This occasion will be preceded by a symposium the day before.

The Wageningen research on ADE covers several different aspects of the Amazonian soil, from its chemical composition to its influence on the composition of the rain forest (see: Richer soil, poorer biodiversity). And from current use of ADE by the local population (See: Marginal role in the food supply) to the potential and limitations of biochar as a weapon in the battle against climate change (see: Biochar is no panacea).

Richer soil, less biodiversity

Does Amazonian dark earth make its mark above the ground? Is the presence of a richer soil reflected in the composition and structure of the Amazon forest? Colombian researcher Estela Quintero Vellajo did research on this in the forest at La Chonta in Bolivia, on the southern edge of the Amazon basin. Her thesis shows that the effect of the dark earth in this part of the Amazon is remarkably modest. Quintero Vallejo compared places with and without ADE, both below and above ground.Her study revealed a striking difference between these soils in terms of chemical composition. ADE contains more calcium, potassium and phosphorus, and has a higher, more neutral pH. ‘But the effects of that greater wealth of nutrients are not necessarily positive,’ is her message. Forest undergrowth certainly flourishes on ADE, but with a smaller variety of species. This effect is particularly visible among ferns, by far the biggest category of plants present. The reason for this appears to be the higher pH. According to Quintero Vallejo, lower acidity may lead to fewer nutrients being available. Another explanation is that some species simply do not grow as well at a lower pH.

The fertile ADE has very little effect on the structure of the trees in the forest in La Chonta. Seedlings on Terra Preta grow more and larger leaves. But oddly enough this does not cause them to grow any faster. Quintero Vallejo think this has to do with the way seedlings respond to the higher calcium level in ADE. In some species this causes calcium poisoning. In other species it may be due to an imbalance in the available nutrients in the leaves. ‘Calcium can fi x phosphorus, so that it is no longer available. The higher pH may also cause nutrients to be less available here.’ All in all, the effects of ADE in La Chonta are not spectacular. According to Quintero Vallejo, the main reason for this is that the soil here is a lot less impoverished than the soils elsewhere in the Amazon. In other words, the soil’s ‘background fertility’ is relatively high. A useful finding, notes Quintero Vallejo. Enriching soil does not always improve it. So ADE does not have the same effect everywhere.

Biochar is no panacea

Adding biochar to Brazilian savannah soils is an unreliable way of storing carbon. Brazilian researcher Tatiana Rittl came to this striking conclusion in her study of the use of biochar for mitigating the effects of climate change. In theory, this sounds like a good idea. By digging biochar, carbonized biomass, into the ground, you can prevent carbon entering the atmosphere. With the added advantage that you improve the fertility of the resulting soil, creating in fact a new kind of ADE. But it is not as simple as that, as Rittl’s study shows. Under the conditions in the savannah, biochar from traditional charcoal stoves decomposes much faster than was thought. It does not remain in the ground for hundreds or even thousands of years, but disappears within a few centuries.

‘The projections about the stability and applicability of carbon storage were too optimistic,’ says Rittl. That optimism was based on the properties of biochar from modern installations in which biomass is burned using pyrolysis (in the absence of oxygen) under controlled conditions. But the composition of this biochar is different, Rittl discovered, and it breaks down much faster. Most of the biochar in countries such as Brazil is manufactured traditionally. According to Rittl, the potential of this biochar to help reduce the effects of climate change is uncertain.

The projections about the stability and applicability of carbon storage were too optimistic

That uncertainty is further increased by the fact that biochar also affects the breakdown of charcoal already present in the soil – either speeding it up or slowing it down. Biochar from the oil-producing sees of Jatropha, Rittl’s lab tests showed, delays the breakdown of organic matter in the soil by 82 percent. The soil itself and the climate can also influence the speed at which biochar breaks down. Sandy soils accumulate less biochar than clay soils. Hot and dry conditions increase the speed at which biochar breaks down. All in all, enough reason, says Rittl, to reconsider the idea that biochar could be a miracle cure for climate change.

Marginal role in the food supply

Historically, Amazonian dark earth played a key role in the food supply for the indigenous inhabitants of the Amazon region. Today, however, the soil only plays a limited role in the food supply. At least, this is the case in the central Caquetá region of the Colombian Amazon, where Clara Peña Venegas did her research. The farmers here do not use these soils any more often or more intensively than the soils around them to grow their cassava, the local community’s staple food. In the course of her research, Peña Venegas came across an astonishing 173 morphologically different cassava varieties. None of these was typically associated with ADE. Nor, says Peña Venegas, is the local ADE ‘clearly richer’ than the surrounding soils.