Agriculture Reference
In-Depth Information
and tropical regions. Reicosky
et al
. (1995)
reviewed various publications and found
that organic matter increased under conser-
vation management systems, with rates ran-
ging from 0 to 1.15 t C ha
-
1
year
-
1
,
with the
highest accumulation rates generally occur-
ring in temperate conditions. In the tropics,
specifically in Brazil, the rate of C accumu-
lation has been estimated in the two main
regions under NT systems (the south and
central-western regions). In the southern re-
gion, Sá (2001) and Sá
et al
. (2001) esti-
mated sequestration rates of 0.8 t C ha
-
1
year
-
1
in the 0-20 cm layer and 1.0 t C ha
-
1
year
-
1
at 0-40 cm soil depth after
22
years under
NT compared to the same period under CT.
The authors mentioned that the accumu-
lated C was generally greater in the coarse
fraction (>
20
μm), indicating that most of
this additional C was relatively labile.
Bayer
et al
. (2000a,b) found a C accu-
mulation rate of 1.6 t ha
-
1
year
-
1
for a
9-
year
NT system compared with 0.10 t ha
-
1
year
-
1
for the CT system in the first 30 cm layer of
an Acrisol in the southern part of Brazil.
Corazza
et al
. (1999) reported an additional
accumulation of approximately 0.75 t C ha
-
1
year
-
1
in the 0-40 cm soil layer due to NT in
the savannah region located in the centre-
west. Estimates by Amado
et al
. (1998,
1999) indicated an accumulation rate of
2.2 t ha
-
1
year
-
1
of soil organic C in the first
10 cm layer. Other studies considering the
NT system carried out in the centre-west re-
gion of Brazil (Lima
et al
., 1994; Castro Fil-
ho
et al
., 1998; Riezebos and Loerts, 1998;
Vasconcellos
et al
., 1998; Peixoto
et al
., 1999;
Spagnollo
et al
., 1999; Resck
et al
., 2000) re-
ported soil C sequestration rates due to NT
varying from 0 up to 1.2 t C ha
-
1
year
-
1
for
the 0-10 cm layer. Bernoux
et al
. (2006) re-
ported that most studies of Brazilian soils
gave annual rates of carbon storage in the
top 40 cm of the soil varying from 0.4
to 1.7 t C ha
-
1
,
with the highest rates in
the Cerrado region. However, the authors
stressed that caution must be taken when
analysing NT systems in terms of carbon se-
questration. Comparisons should include
changes in trace gas fluxes and should not
be limited to a consideration of carbon stor-
age in the soil alone if the full implications
of global warming are to be assessed. The
adoption of NT management in subtropical
Brazilian soils has led to SOC accumulation
rates of 0.19-0.81 Mg ha
-
1
year
-
1
in the
0-20 cm layer (Bayer
et al
., 2006), due to
the less oxidative environment and the
physical protection mechanism imparted
by the stable aggregates in NT soils (Eiza
et al
., 2005).
The Importance of Cover Crops
for Sustainable Carbon Management
in Tropical and Subtropical
Agroecosystems
Soils in tropical and subtropical environ-
ments are often exposed to strong rain and
long-term drought events during the year.
Hence, either in sugarcane or annual crops,
any management that leaves more crop res-
idues covering the soil surface is beneficial
for soil protection and C sequestration.
The importance of residue cover to
avoid soil erosion or food web respiration,
and for maintaining SOM matter in annual
crops through grass-legume rotations has
been discussed by many authors (Lal
et al
.,
1998; Magdoff and Weil, 2004). In southern
Brazil, many experiments showed SOC ac-
cumulation due to the conversion of sys-
tems based on intensive tillage to NT with
crop rotation, with topsoil SOC gains of up
to 91% (Zanatta
et al
., 2007).
Dieckow
et al
. (2005a,b) evaluated soil
organic C and N losses during a period of
conventional cultivation (1969-1983) that
followed native grassland and the potential
of four long-term (
17
years) no-till cereal-
and legume-based cropping systems with
different N fertilization levels to increase
the C and N stocks of a southern Brazilian
Acrisol. The C content in the 0-17.5 cm soil
layer decreased by 22% (8.6 Mg C ha
-1
) and
N decreased by 14% (0.44 Mg N ha
-1
) dur-
ing the period of conventional cultivation.
Legume-based cropping systems increased
C and N stocks due to the higher residue in-
put. Although the major soil management
effects were found in the 0-100 cm layer,
up to 24% of the overall C losses and 63%
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