|Authors: (Christoph Steiner, Institute of Soil Science and Soil Geography, Univ. of Bayreuth, Bayreuth, Germany)
Sustaining soil fertility is a major agricultural constraint in the Amazon Basin (Tiessen, et
al., 1994), thus shifting cultivation accompanied with slash and burn agriculture is the
prevailing agricultural practice in the humid tropics.
In addition to the predominant and unproductive Ferralsols and Acrilols, an exceptional
dark soil is well known by the indigenous people and colonists for its sustained soil fertility.
According to its dark color and origin ,the soil was termed Terra Preta de Indio. Smith
(1879) and Katzer (1903) were among the first who described the Terra Preta’s properties
and presumed its cultural origin. Later their assumptions were strengthened by the soil
scientists Sombroek (1966) and Zech, et al. (1990). Terra Preta contains significantly more
carbon (C), nitrogen (N), calcium (Ca), and phosphorus (P), and the cation exchange capacity
(CEC), pH value, and base saturation are significantly higher in Terra Preta soils than in the
surrounding Ferralsols and Acrisols (Zech, et al., 1990; Glaser, et al., 2000). Terra Preta soils
contain up to 70 times more black C than the adjacent soils. Due to its polycyclic aromatic
structure, black C is chemically and microbially stable and persists in the environment over
centuries (Glaser, et al., 2001b). Today, the anthropic origin of Terra Preta de Indio is
generally accepted and its fertility is most likely linked to an accumulation of P and Ca,
associated with bone apatite (Zech, et al., 1990; Lima, et al., 2002) and black C as charcoal
(Glaser, et al., 2001a). The evidence that Terra Preta was manmade, and thus the proven
feasibility to transform one of the most infertile soils into one of the most productive soil
inspired charcoal research.
Slash and Char was described as an alternative to slash and burn (Steiner, 2007) and
(Steiner, et al., 2004b) observed that charcoal is currently used by Amazonian settlers to
improve soil fertility. If a forest is burned, only around 2-3% of the above-ground C is
converted into charcoal (Fearnside, et al., 2001), but charcoal production can capture 50% of
the above-ground C. If the charcoal is not used as fuel (e. g., soil amendment) it has a very
high recalcitrance against biological or chemical decay and stores the carbon over centuries or
millennia. Slash and char is an alternative agricultural method producing charcoal out of the
aboveground biomass instead of converting it to CO2 through burning. If re-growing
resources are used, slash and char could establish as a significant carbon sink and could be an
important step towards sustainability and SOM conservation.
On a global scale, the total carbon release flux due to fire is of the order of 4-7 Pg of
carbon per year. This flux is almost as large as the rate of fossil fuel consumption (about 6 Pg
per year in 1990) (Goudriaan, 1995). Fearnside (2000) calculated a total net emission of
carbon from tropical land uses, equivalent to approximately 29% of the total anthropogenic
emission from fossil fuels and land-use change. These numbers emphasize the potential for C
management if only biomass is utilized being ablaze each year.
As a result of intensive research done in the 1980s, carbonized materials are formally
authorized for use as soil amendment material in Japan. Thus Japan used 27% of the total
charcoal consumption (50,835 Mg in 1999) for purposes other than fuel. By far the biggest
proportion was used for agriculture (30.6%) followed by livestock industries (22.3%),
gardening and golf courses (7.6 %) (Okimori, et al., 2003). A Japanese company established
charcoal production at an Indonesian tree plantation for pulp production. Their feasibility
study with conventional charcoal-making methods showed that 77,000 Mg charcoal could be
produced per year, and the carbon emission reductions by the project reaches 62,000 Mg C yr-
1 (= 230,000 Mg CO2 yr-1) at an annual wood harvest of 10,000 ha (Okimori, et al., 2003).