Archeologists frequently debate the real meaning of the black soil stains found in prehistoric sites in the central Amazon region, which stand out visually from the yellowish brown monotony typical of the region’s solid ground. Some of them believe that these dark soils indicate that pre-Columbian indigenous peoples lived for hundreds or even a few thousand years in complex and structured societies, based on sedentary agriculture and environmental husbandry in the midst of the rainforest. Others feel that the existence of this darker terrain, which often harbors many ceramic pieces, offers no definitive evidence that there was ancient and long-lasting human occupation in these areas prior to the arrival of European conquerors. However, there is consensus on one point concerning agricultural sciences rather than humanities: the black soil forms almost permanent oases of fertility in an area that has mainly poor quality soil incapable of holding nutrients for long. A recent study confirms that a key component of this soil variant is an unequivocal vestige of the establishment of human settlements: the feces of the Indians.
Concentrations of a biomarker linked to the depositing of human excrement in the environment, coprostanol (5ß-stanol), were found in black soil samples taken from five prehistoric sites in the Amazon region, according to a scientific article soon to be published by a team of researchers from Brazil and from Germany in the June issue of the Journal of Archaeological Science. Four such sites are in the state of Amazonas to the southwest of Manaus, on a strip of solid ground on the confluence of the Negro and Solimões rivers, and the other one is in the state of Pará southwest of the city of Santarém, in the lower Tapajós area. “Strictly speaking, the biomarker might also indicate the presence of the feces of domesticated swine,” states the agronomical engineer Wenceslau Geraldes Teixeira, from the company Embrapa Solos, in Rio de Janeiro, and one of the authors of the study. “But as these animals were only introduced into South America after the arrival of the Europeans, we discarded this possibility.” All the black soil samples analyzed were formed between 500 and 2,500 years ago, prior to the official discovery of the continent by Cristopher Columbus.
Rich in minerals associated with soil fertility, the black soil owes its dark color to its high content of so-called pyrogenic carbon, a stable form of aromatic carbon produced by the incomplete combustion of biomass. The way the ancient indigenous people of the Amazon region lived (they burned the remains of the animals they consumed, buried their dead and deposited their garbage and excrement in the areas around their communities) presumably accounts for the formation of this type of soil. “We’re trying to understand the chemical composition of the black soil and to discover what organic material was added to it that has kept it fertile to this day,” states the archeologist Eduardo Góes Neves, from the University of São Paulo (USP), another of the study’s authors and the coordinator of a FAPESP thematic grant project on the pre-colonial history of the Amazon region. “If we achieve this objective successfully, we might learn how to improve the fertility of poor soil and contribute to more sustainable tropical agriculture.” There are attempts under way to artificially reproduce the properties of the black soil, but they are still at an initial stage.
Some experts believe that the compounds in human feces play an important role in maintaining the long-term fertility of this variant of the Amazon region soil. Contrary to the impoverished latosol that is typical of the Amazon region, the black soil undergoes little lixiviation, a process that consists of losing nutrients due to the infiltration of rainwater, which “washes” the soil and draws away its chemical components. “The excrement contributes significantly to the nutrient content found in the black soil, such as nitrogen and phosphorus, besides helping to recycle its nutrients,” states Bruno Glaser, from the Martin Luther University in Halle-Wittenberg, Germany, who studies soil biochemistry and is yet another co-author of the study. “In modern societies this no longer happens, these nutrients being lost as a result of the depositing of sewage sludge in reservoirs.” In the black soil, the excrement is probably mixed into the soil thanks to worms, termites, ants and other organisms.
Though generally it is not directly considered an element capable of making soil more fertile, the pyrogenic carbon seems to contain a unique set of fungi and bacteria whose synergy may be connected with the black soil’s fertility. Work conducted by the team of the agronomical engineer Siu Mui Tsai, from the Center for Nuclear Energy in Agriculture, at USP in Piracicaba, showed that the form of coal found in this type of soil includes the DNA of as many as three thousand species of microorganisms. “This biodiversity is far greater than that found in the Amazon region in the ground around the black soil,” states Siu. “The Indians didn’t use toxic products and their system was balanced.” Nobody knows, however, whether the pre-Columbian peoples intentionally created black soil to enrich their agricultural land or whether it is the fortuitous result of the waste and garbage that their lifestyle produced.
Regional chronologies, gaps and discontinuities in the pre-colonial history of the Amazon region (nº 2005/60603-4); Type Thematic Research Grant; Coordinator Eduardo Góes Neves – MAE/USP; Investment R$ 577,619.78 (FAPESP)
BIRK, J. J. et al. Faeces deposition on Amazonian Anthrosols as assessed from 5ß -stanols. Journal of Archaeological Science. v. 38 (6). p-1209-20, June 2011.