Managing Soil Carbon for Multiple Ecosystem Benefits – Positive Exemplars: Latin America (Brazil and Argentina) - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 280

the atmosphere, especially CO 2 (Lal, 1998;
Paustian et al ., 2000), compared to CT. There
is considerable evidence that the main effect
on SOC is in the topsoil layers (Six et al .,
2002; Abril et al ., 2005; Noellemeyer et al .,
2008), but significant increments in SOC
have also been reported for layers below
30 cm in depth in NT soils with high-input
cropping systems (Sisti et al ., 2004; Dieckow
et al ., 2005a,b).
Worldwide, approximately 63 Mha are
currently being managed under NT farming,
with the USA having the largest area (Lal,
2006), followed by Brazil and Argentina. In
Brazil, NT farming began in the southern
states in the 1970s as an alternative to the
misuse of land that was leading to unaccept-
able levels of soil losses by water erosion
(Denardin and Kochhann, 1993). Similarly,
in Argentina, NT began to be used in the
central rolling Pampas, where water erosion
also had become a major problem when
soybean-wheat double cropping was intro-
duced (Alvarez and Steinbach, 2009), and
NT was shown to reduce runoff velocity
and sediment load effectively (Castigilioni
et al ., 2006). The underlying land manage-
ment principles that led to the development
of NT systems were prevention of surface
sealing, caused by rainfall impact on the
soil surface, achievement and maintenance
of an open soil structure and reduction of
the volume and velocity of surface runoff.
Consequently, NT was based on two essen-
tial farm practices: (i) not tilling and
(ii) (ii) keeping soil covered all the time. This
alternative strategy expanded quickly and
the cropped area under NT has since then
increased exponentially.
In the early 1990s, the NT area in Brazil
was about 1 Mha, increased ten times by
1997, and currently is approximately 24 Mha.
This expansion includes the conversion
from CT in the southern region (72%) and
expansion of the agricultural frontier, clear-
ing the natural savannah in the central-
western area (28%). Recently, due to the
high profits, ranchers in the Amazon region
are converting old pastures to soybean/millet
under NT.
Changes in soil C stocks under NT have
been estimated in earlier studies for temperate

Soil Organic Carbon Stocks in
Conventional Versus No-tillage
Systems in Latin America
(Brazil and Argentina)

No-tillage (NT) is presumed to be the oldest
soil management system in agriculture and,
in some parts of the tropics, NT is still prac-
tised in slash-and-burn agriculture where,
after forest clearing by controlled burning,
seeds are placed directly into the soil without
any tillage operation. As mankind developed
more systematic agricultural systems, culti-
vation of the soil became an accepted prac-
tice as a means of preparing a more suitable
seedbed and environment for plant growth.
Indeed, tillage as symbolized by the mould-
board plough became almost synonymous
with agriculture (Dick and Durkalski, 1997).
No-tillage can be defined as a crop produc-
tion system where soil is left continuously
undisturbed, except in a narrow strip where
seed and fertilizer are placed.
Conversion of native vegetation to cul-
tivated cropland under a conventional till-
age (CT) system has resulted in a significant
decline in SOM content (Paustian et al .,
2000; Lal, 2002; Zach et al ., 2006). Farming
methods that use mechanical tillage, such
as the mouldboard plough for seedbed prep-
aration or disking for weed control, can
promote soil C loss by several mechanisms:
(i) they disrupt soil aggregates that protect
SOM from decomposition (Six et al ., 2002;
Soares et al ., 2005); (ii) they stimulate short-
term microbial activity through enhanced
aeration, resulting in increased levels of
CO 2 and other gases released to the atmos-
phere (Bayer et al ., 2000a,b; Kladivko,
2001); and (iii) they mix fresh residues into
the soil where conditions for decompos-
ition are often more favourable than on the
surface (Plataforma Plantio Direto, 2009).
Furthermore, tillage can leave soils more
prone to erosion, resulting in further loss of
soil C (Bertol et al ., 2005; Lal, 2006).
However, no-tillage farming, due to less
soil disturbance, often results in significant
accumulation of SOC (Bayer et al ., 2000b;
Sá et al ., 2001; Schuman et al ., 2002) and in
consequent reductions of GHG emissions to