In recent years, the terra preta (black earth) phenomenon has aroused increasing interest. These are man-made soils discovered in the middle of tropical rainforests that are up to 3000 years old, with high nutrient and organic matter in the soil. Terra preta has since been seen as a model for promoting sustainable agricultural practices in the humid tropics and as an example of long-term CO2 sequestration in terrestrial ecosystems with additional positive benefits for ecosystem services. These potentials of terra preta have inspired a large number of studies, but also stimulated the imagination as to how they were created in the first place. The article therefore aims to summarize the scientific knowledge about the terra preta properties and discuss its origin.
Terra preta is the product of inorganic (e.g. ash, bones (especially fish)) and organic (e.g. biomass waste, manure, excrements, urine and biochar) changes in the intensively weathered tropical soil. These inorganic and organic ingredients have been microbially decomposed. Fungi play a greater role than bacteria in surrounding ecosystems. Biochar plays a key role in this process due to its stability and its accumulation in the terra preta soils. However, it is not yet known whether terra preta is the result of an intentional or involuntary process. In addition, it is unclear how much time was needed to develop a terra preta after the above materials had been disposed of.
The role of biochar
Since biochar only contains small amounts of nutrients, it does not contribute significantly to the nutrient status. Nevertheless, biochar plays an important role in the development of terra preta as a pyrogenic material and the remainder of incomplete combustion of biomass (charcoal, soot). It is also considered probable that the biochar was applied to terra preta by human activities. However, the authors describe it as unlikely that the use of biochar was a key process that led to the formation of terra preta, since no terra preta is produced by slash-and-burn agriculture alone, although a lot of ash and biochar is applied at these sites. In addition, ash contains considerable amounts of Ca, K, Mg and P, while terra preta is highly enriched in P, for example.
The composition of terra preta
The soils naturally occurring in the Amazon region contain very small amounts of most nutrients. In contrast, terra preta is characterized by high concentrations of P, N, Ca and basic nutrients. The nutrient reserves in terra preta are many times greater than those of the surrounding soils. The researchers therefore assume that terra preta was produced from the following nutrient sources: Plant biomass, mammal and fish bones, ash, biochar and human excrement.