PRETIC - The new WRB diagnostic horizon for Amazonian Dark Earths

A new diagnostic horizon in WRB for anthropic topsoils in Amazonian Dark Earths (SouthAmerica)

Anjos, Lúcia Helena C. dos, UFRRJ;

Teixeira, Wenceslau .G., Embrapa Soils;

Schad, Peter, Technische Universitaet Muenchen;

Fontana, Ademir, Embrapa Soils

Introduction Characteristics ADEs Rationale Pretic Horizon Anthrosols

Why a new horizon for Amazonian Dark Earths?

The Amazonian Dark Earths (ADE), also called “Terra Preta de

Índio”, are classified according to WRB as Anthrosols (IUSS Working

Group WRB, 2006)

Different studies attribute to them the hortic, plaggic or terric horizon

according to base saturation, extractable P content, amount of artefacts,

animal activity, and other soil diagnostic criteria.

However they do not fit the central concept of any of them:

- deep cultivation, intensive fertilization and/or long-continued

application of human and animal wastes and other organic residues

(hortic horizon);

- addition of sods (topsoil layers together with vegetation) and animal

manure (plaggic horizon);

- addition of earthy manures, compost, beach sands or mud (terric

horizon).

Introduction Characteristics ADEs Rationale Pretic Horizon Anthrosols

Characteristics of the ADEs?

The ADEs are associated to pre-Columbian

Amerindian settlements and have been reported

mainly in the Amazon region.

They are variable in terms of depth of the

diagnostic horizon, physical and chemical properties,

but mostly have dark colors, high stocks of organic

carbon, extractable and total phosphorus, calcium,

magnesium, micronutrients (mainly zinc and

manganese) and other metals as strontium and

barium.

Contrasting with natural soils in the surrounding

area, the surface horizons influenced by human

activities have extraordinarily high CEC and nutrient

availability.

Artefacts TPI Rio Preto da Eva

TPI Rosa de Fogo Rio Preto da Eva - AM

Introduction Characteristics ADEs Rationale Pretic Horizon Anthrosols

Characteristics of the ADEs?

Artefacts of ceramic, stones and

bones and pieces of charcoal are

frequently found mixed into the soil.

Clear evidences of past human

occupation in the landscape

(constructions, gardens), shell mounds

(“sambaquis”), earthworks (geoglyphs)

are identifiable.

Sambaqui

fluvial Miracatu

Sambaqui –

Camacho 05_2007 154

Caldeirão, 2010 - P1050695

Introduction Characteristics ADEs Rationale Pretic Horizon Anthrosols

Rationale for a new Horizon!

Amazon Dark Earths Distribution (Teixeira, Wenceslau G.)

Introduction Characteristics ADEs Rationale Pretic Horizon Anthrosols

Rationale for a new Horizon!

The ADEs may not show the evidences of animal activity required for the

hortic horizons, and they were not influenced by manuring, as required for the

hortic horizons, since this activity was not part of the pre-Columbian Amerindian

culture.

Additionally, the surface anthropic horizons typical for the ADE show a high

base saturation (BS); the base cations were added from bones (Ca), ashes, and

many other organic materials (i.e. kitchen middens, palm leaves, human feces).

Horizons,

depth (cm)Color Corg

g/kg

Clay

g/kgCa+Mg

Base cations

cmolc/kgCEC

BS

%

P

mg/kg

A1 0-31 2.5Y 2.5/1 56.1 189 13.7 13.8 17.6 78 216

A2 31-43 2.5Y 3/1 22.5 203 5.5 5.5 9.2 60 66

AB 43-57 2.5Y 4/1 10.0 203 2.7 2.7 5.8 50 32

B2 130-190 2.5Y 7/6 1.9 320 1.2 1.2 2.3 52 15

Ferralsol, P18 (TB11-00), Humaita – Apuí, Amazon; 7º 51’ 29,6’’ S and 61º 17’ 52,8’’ WGr (Cunha, 2005)

Introduction Characteristics ADEs Rationale Pretic Horizon Anthrosols

Pretic Horizon (2014, update 2015)Pretic (from Portuguese preto, black)

General description

A pretic is a mineral surface horizon that results from human activities including the addition of charcoal. It

is characterized by its dark colour, the presence of artefacts (ceramic fragments, lithic instruments, bone

or shell tools etc.) and high contents of organic carbon, phosphorus, calcium, magnesium and

micronutrients (mainly zinc and manganese), usually contrasting with natural soils in the surrounding

area. It typically contains visible remnants of charcoal.

Pretic horizons are for example widespread in the Amazon Basin, where they are the result of pre-

Columbian activities and have persisted over many centuries despite the prevailing humid tropical

conditions and high organic matter mineralization rates.These soils with a pretic horizon are known as

‘Terra Preta de Indio’ or ‘Amazonian Dark Earths’. They generally have high organic carbon stocks. Many

of them are dominated by low-activity clays.

Diagnostic criteria:

A pretic horizon is a surface horizon consisting of mineral material and has:

1. a Munsell colour value of ≤ 4 and a chroma of ≤ 3, both moist; and

2. ≥ 1% organic carbon; and

3. a base saturation (by 1 M NH4OAc, pH 7) of ≥ 50%; and

4. exchangeable Ca plus Mg (by 1 M NH4OAc, pH 7) of ≥ 2 cmolc kg-1 fine earth; and

5. ≥ 30 mg kg-1of extractable P (Mehlich-1); and

Introduction Characteristics ADEs Rationale Pretic Horizon Anthrosols

Pretic Horizon (2014, update 2015)

Pretic (from Portuguese preto, black)

6. one or more of the following:

a. ≥ 1% artefacts (by volume, by weighted average); or

b. ≥ 1% charcoal (by volume, by weighted average); or

c. evidences of past human occupation in the surrounding landscape, e.g. constructions,

gardens, shell mounds (‘sambaquis’), or earthworks (geoglyphs); and

7. < 25% (by volume) of animal pores, coprolites or other traces of soil animal activity; and

8. one or more layers with a combined thickness of ≥ 20 cm.

Additional characteristics

Charcoal is an artefact only if it is intentionally manufactured by humans.

Relationships with some other diagnostics

Pretic horizons do not show the animal activity required for hortic and irragric horizons. The

diagnostic criteria for P concentrations in pretic and hortic horizons are based on different analytical

methods with a lower requirement for the pretic horizon.

The high base saturation sets the pretic horizon apart from plaggic horizons. It cannot be excluded

that a pretic horizon fulfils also the criteria of the plaggic horizon. In this case, the soil scientist

should use historical knowledge for making the final decision, whether the horizon is called plaggic

or pretic. Some pretic horizons may qualify as mollic or umbric horizons.

Introduction Characteristics ADEs Rationale Pretic Horizon Anthrosols

Anthrosols

Perfil Terra Preta Indio,

Lago do Limão, Ago 2004

Introduction Characteristics ADEs Rationale Pretic Horizon Anthrosols

Introduction Characteristics ADEs Rationale Pretic Horizon Anthrosols

Argissolo Amarelo Distrófico A antrópico,

Caldeirão, Iranduba, AM

Hor cm %C P mg kg-1 Ca1 Mg1 Zn2 Mn2

Au1 0-40 2.4 56 6.44 0.58 12 50

Au2 40-71 2.1 138 3.41 0.39 12 38

Au3 71-90 1.9 124 2.27 0.32 14 31

Bt2 165-200 0.2 84 1.95 0.29 6 8

1cmolc kg-1, 2 mg kg-1

Introduction Characteristics ADEs Rationale Pretic Horizon Anthrosols

Other classes

Perfil XI RCC Roraima, Rorainópolis

Lixic Pretic Xanthic Ferralsol

(Hypereutric, Amphiloamic, Vetic)

ARGISSOLO AMARELO Distrófico

antrópico plintossólico

Hor cm %C P mg kg-1 Ca* Mg*

Aup 0-14 1.07 47 3.8 0.6

Au1 14-41 0.79 36 3.6 0.5

Au2 41-63 0.53 25 1.8 0.5

*cmolc kg-1

Introduction Characteristics ADEs Rationale Pretic Horizon Anthrosols

What is the next step?

Database of soils identified as Amazonian Dark Earths (ADE), and their

classification;

To test properties and limits of diagnostic criteria (WRB, updated 2015);

Studies on different environments, beyond Amazon region; are this soils

limited to this area of Brazil?

How about other regions of the world? How widespread are the Dark

Earths?

IntroductionCharacteristics ADEs Rationale Pretic Horizon Anthrosols

Acknowledgement

Photos, courtesy from Wenceslau Geraldes

Teixeira <wenceslau.teixeira@embrapa.br> and

colleagues from different Institutions

Analytical data from: Cunha, T.J.F., 2005. Ácidos húmicos de solos

escuros da Amazônia (TPI). Thesis UFRRJ – CPGA-CS. 138p.