Terra preta
[4][5] Terra preta is characterized by the presence of low-temperature charcoal residues in high concentrations;[2] of high quantities of tiny pottery shards; of organic matter such as plant residues, animal feces, fish and animal bones, and other material; and of nutrients such as nitrogen, phosphorus, calcium, zinc and manganese.
[6] Fertile soils such as terra preta show high levels of microorganic activities and other specific characteristics within particular ecosystems.
[7] Deforested arable soils in the Amazon are productive for a short period of time before their nutrients are consumed or leached away by rain or flooding.
[8][9] Terra preta is less prone to nutrient leaching because of its high concentration of charcoal, microbial life and organic matter.
Many terra preta soil structures are now thought to have formed under kitchen middens, as well as being manufactured intentionally on larger scales.
[16] Amazonians formed complex, large-scale social formations, including chiefdoms (particularly in the inter-fluvial regions) and even large towns and cities.
[19][20] Beyond the geoglyphs, these populations left no lasting monuments, possibly because they built with wood, which would have rotted in the humid climate, as stone was unavailable.
[citation needed] Whatever its extent, this civilization vanished after the demographic collapse of the 16th and 17th century, due to European-introduced diseases such as smallpox[20] and bandeirante slave-raiding.
This might have made the benefits of terra preta, such as its self-renewing capacity, less attractive: farmers would not have been able to cultivate the renewed soil as they migrated.
[31] The variations in Amazonian dark earths prevent clearly determining whether all of them were intentionally created for soil improvement or whether the lightest variants are a by-product of habitation.
[citation needed] The processes responsible for the formation of terra preta soils are:[7] The transformation of biomass into charcoal produces a series of charcoal derivatives known as pyrogenic or black carbon, the composition of which varies from lightly charred organic matter, to soot particles rich in graphite formed by recomposition of free radicals.
By convention, charcoal is considered to be any natural organic matter transformed thermally or by a dehydration reaction with an oxygen/carbon (O/C) ratio less than 60;[32] smaller values have been suggested.
[42][43] The nucleus of black carbon particles produced by the biomass remains aromatic even after thousands of years and presents the spectral characteristics of fresh charcoal.
[47] Charcoal's porosity brings better retention of organic matter, of water and of dissolved nutrients,[42][48] as well as of pollutants such as pesticides and aromatic poly-cyclic hydrocarbons.
[30] Bechtold proposes to use terra preta for soils that show, at 50 centimeters (20 in) depth, a minimum proportion of organic matter over 2.0–2.5%.
The clay matter that exists in those soils is capable of holding only a small fraction of the nutrients made available from decomposition.
[51][52] The peregrine earthworm Pontoscolex corethrurus (Oligochaeta: Glossoscolecidae) ingests charcoal and mixes it into a finely ground form with the mineral soil.
P. corethrurus is widespread in Amazonia and notably in clearings after burning processes thanks to its tolerance of a low content of organic matter in the soil.
[citation needed] Some ants are repelled from fresh terra preta; their density is found to be low about 10 days after production compared to that in control soils.
[58] Perhaps the most important and unique part of the improvement of soil fertility is carbon, thought to have been gradually incorporated 4 to 10 thousand years ago.
Average poor tropical soils are easily enrichable to terra preta nova by the addition of charcoal and condensed smoke.
[60] Terra preta may be an important avenue of future carbon sequestration while reversing the current worldwide decline in soil fertility and associated desertification.
This area has many terra preta soil zones, demonstrating that this anthrosol was created not only in the Amazon basin, but also at higher elevations.
[62] A synthetic terra preta process was developed by Alfons-Eduard Krieger to produce a high humus, nutrient-rich, water-adsorbing soil.
