Agriculture Reference
In-Depth Information
(Alvarez, 2001). Considering the rolling
Pampas natural ecosystems (herbivore exclu-
sion zones) harboured a total SCS of
68
t C ha
-
1
(Andriulo
et al
., 1999), a century after an-
nual crops were established in the biome,
these stocks were reduced to
34
Mg C ha
-
1
(Alvarez, 2001). On the other hand, Piñeiro
et al
. (2006) used the Century model to esti-
mate SCS losses associated with herbivory
and concluded that
370
years after livestock
was introduced in the Pampas grasslands,
there was a 22% reduction in SCSs in the
0-20 cm depth. This is a dramatically dif-
ferent response to LUC.
LUC in the semi-arid Pampas caused
rapid soil C losses, with C half-lives just
above
10
years and no evidence of long-term
stabilized C in any of the soil fractions
(Zach
et al
., 2006). In this study, soils ori-
ginally under long-term pasture or natural
vegetation, with initial C contents between
24
and
33
mg C g
-
1
bulk soil, lost
33-
57% of
their original C within
12-18
years of con-
tinuous cultivation. In degraded soils that
had been restored with pasture, C accretion
was rapid, but levelled off well below the
original C levels (Zach
et al
., 2006).
The conversion of natural pastures to
cash cropping brings about similar carbon
stock losses, as does the deforestation of
native vegetation (Noellemeyer
et al
., 2008).
A 16% reduction was observed only
2
years
after converting pasture to agriculture, and
14
years later, soil C stocks were reduced by
32% after livestock was introduced.
Conservation or no-tillage agriculture
(NT) can improve the carbon sequestration
in soils under agricultural use in the semi-
arid and subhumid regions of the Pampas.
Long-term field experiments have shown
that under NT significantly higher amounts
of carbon were sequestered by the soil than
under conventional tillage (CT) (16.6 versus
13.2 Mg C ha
-
1
under NT and CT, respect-
ively,
9
years after establishment of crops).
This finding indicates that soils under NT
can act as a carbon sink, while those under
CT as a carbon source (Quiroga
et al
., 2009).
However, in NT production systems without
crop rotation this might not be the case (Díaz
Zorita
et al
., 2002; Steinbach and Alvarez,
2006; Álvarez
et al
., 2009).
LUC and the intensification of agricul-
tural systems also alter the biological activ-
ity in soils, with potential effects on soil
carbon stabilization and storage. Respiration
rates of different soil aggregate-size classes
are directly related to their carbon contents.
Both were substantially lower in agricul-
tural than in pasture soils, indicating higher
biological activity in the latter, which in-
creases the cycling rates and stabilization of
organic carbon in the soil as organo-mineral
complexes (Denef
et al
., 2009). This was
corroborated by the higher levels of silt- and
clay-associated carbon in these soils as
compared to their agricultural counterparts
(Noellemeyer
et al
., 2008). As indicated
above, soils under NT accrue more carbon
than those under CT, especially in the top-
soil. This provides more substrate for micro-
bial metabolism, and, subsequently, higher
respiration rates have been found under NT
when compared to CT in incubation experi-
ments, indicating an active soil biota and
conditions that might favour stabilization of
residue-derived carbon. Although one could
conclude that higher respiration would lead
to carbon loss, the enhanced microbial activ-
ity that processes the organic matter inputs
and the physical protection of soils under
no-tillage systems increase soil aggregation,
which leads to stabilization of soil organic
carbon decomposition products (Fernández
et al
., 2010).
Modern industrialized agricultural pro-
duction systems generally focus on very few
crops that only occupy the land surface dur-
ing short periods of the year. The average
growing season in the Southern Grasslands
is 5 months for soybean and many maize
hybrids and 4 months for sunflowers and
modern wheat hybrids. Thus, in most situ-
ations, the soil is fallow during more than
half of the year, exposing its surface to wind
and water erosion and causing important
soil water evaporation losses. Double crop-
ping, such as growing wheat during winter
and soybeans during summer, could effect-
ively counteract these effects by keeping the
soil protected. In Brazil, a lot of effort is being
placed on the development of integrated
crop-livestock-forestry systems, shown to be
economically and technically feasible in the
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