Geoscience Reference
In-Depth Information
potassium, and calcium. This means that they can be cultivated for longer periods—perhaps
indefinitely, and thus have the potential to contribute to more sustainable agriculture in the
Amazon, one of the world's richest and most threatened biodiversity hotspots (Glaser 2007).
Until recently, little was known of how
terra preta
were formed, why they are so stable, and
why they are so patchily distributed. Recent work has shown that they are of anthropogenic
origin and are associated with intensive human management, particularly with cultivated
areas and middens (Glaser 2007, Glaser and Birk 2012). Their formation is thought to be asso-
ciated with inputs of vast amounts of organic materials, including mammal bones, fish bones,
turtle shells, food waste, and human excrement, incorporated into the soil via incomplete
burns that result in a range of combustion residues, known as biochar, which have different
molecular ratios of oxygen to carbon (O/C) and hydrogen to carbon (H/C). Black carbon
(charcoal) is an essential ingredient of biochar; it comprises aromatic carbon rings that are
thought to be responsible the stability of
terra preta
and its ability to absorb organic mole-
cules, preventing nutrients from being leached from the soil and maintaining fertility (Fig-
ure 6.5b). Carbon in biochar is extremely resistant to decomposition, with a mean residence
time of about 2000 years (Glaser and Birk, 2012). Radiocarbon dating indicates that
terra
preta
soils are of pre-Columbian origin and were formed between 7,000 and 500 years bp
(Heckenberger et al. 2003, Glaser 2007).
These enriched soils allowed thriving agriculture and the establishment of extensive per-
manent settlements; in pre-Columbian times, the population of Amzonia may have been as
high as 8-10 million. Formation of
terra preta
most likely declined dramatically when popula-
tions, societies and their technologies in the Amazon were devastated by the introduction of
European diseases in the sixteenth and seventeenth centuries. However, as in pre-Columbian
times,
terra preta
are the preferred soils for agriculture today, and they are still used by Indios,
and other groups of farmers (Bozarth et al. 2009, Schmidt and Heckenberger 2009, Glaser
and Birk 2012). Midden areas are still important as home gardens and it seems likely that
terra
preta
will continue to form on these sites due to the continued addition of partially com-
busted carbon (biochar), potentially enabling sustainable settlements and at the same time
sequestering large amounts of carbon (Glaser et al. 2001, Glaser 2007, Schmidt and Hecken-
berger 2009, Glaser and Birk 2012).
As interest in the
terra preta
phenomenon has grown, attempts to recreate dark earths have
multiplied around the world. Using 'slash and char' rather than 'slash and burn' produces
incompletely combusted plant materials and a range of carbon products that can be incorpo-
rated into soils. Experiments have shown that vegetation can be charred rather than burned
by piling vegetation into dense mounds in order to restrict oxygen supply, thereby creating
the long-chain polymers that are the backbone of
terra preta
's stability. Charring rice and
corn husks can be easily done by villagers and can contribute to the stabilization of soils and
longer crop cultivation periods (Glaser 2007, Glaser and Birk 2012).
Pioneered by Wim Soembroek, the
terra preta
Nova group has assimilated archaeological,
anthropological, and soil science evidence that has vastly increased our understanding of
dark earths in Amazonia. Their work also suggested the potential for anthropogenic dark earth
formation in Africa (Woods et al. 2009). Revisiting the sites of their classic anthropological
Search WWH ::
Custom Search