Back­ground

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.

Cre­ation

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.

Orig­i­nal article: State of the sci­en­tif­ic knowl­edge on prop­er­ties and genesis of Anthro­pogenic Dark Earths in Central Ama­zo­nia (terra preta de Índio)
Author: Bruno Glaser, Jago Jonathan Birk
Pub­lished in: Geo­chem­i­ca et Cos­mochim­i­ca Acta, Volume 82, p 39–51, Else­vi­er 2012