Principles of Soil Management in Neotropical Savannas - Principles of Sustainable Soil Management in Agroecosystems - page 72

Pennisetum or Mucuna spp. during the dry season after harvest of the main crop
provides additional mulch and enhances SOC levels (Nunes et al. 2011).
These tenets indicate that a successful management of Cerrado soils requires a
proper management of SOC and must result in its preservation, as stated earlier by
van Wambeke (1992) for tropical soils in general. This high importance of SOC is
attributed to the fact that (1) fresh organic residues and coarse/light SOC fractions
are immediate C sources for soil food webs and nutrient sources to plants; (2) humic
SOC is the main source of CEC; and (3) both SOC forms improve soil aggregation
and its resistance to compaction and other physical degradation processes, and also
enhance water retention at low suctions. In addition, about 90% of N in Cerrado soils
is in a NH + form, mostly bound to SOC (Frazão et al. 2010). Although these benefi-
cial effects of SOC are commonly observed in most soils throughout the world, they
are especially critical in highly weathered soils of the humid tropics, where nutrient
and CEC levels are remarkably low, and low bulk density due to large macropores
causes excessive internal drainage. However, SOC in the Cerrado and elsewhere
can be lost by mismanagement, excessive tillage, and erosion. Yet, it can also be
sequestered under best management practices (Bayer et al. 2006). Finally, there is
growing interest in mitigation of global warming by the sequestration of atmospheric
CO 2 as SOC in agricultural soils (Lal 2008). Therefore, choice of best management
practices in the Cerrado is inseparable from a discussion of SOC forms, functions,
and effects.
The most important combinations of land use systems and soil management tech-
niques for SOC management are discussed below.
3.2.2 p aStureS
The oldest settlements by European descendants in the Cerrado region were estab-
lished in the early eighteenth century by gold miners (Klink and Moreira 2002),
most of which used cattle as traction force for their wagons. When gold mining was
no longer profitable, these settlements enhanced the use of the “natural” pastures
to raise more cattle, which became their main activity. In fact, these were range
areas where most or many of the Cerrado trees were removed; thus, the underlying
herbaceous native vegetation could be grazed by cattle at low stocking rates, using
neither fertilizers nor other inputs. Thus, productivities were always low, which was
not a serious problem given the low human population for the entire region. This
extensive management could sustain most of the first urban settlements (e.g., Goiás,
Pirenópolis, Planaltina, Paracatu, etc.). Following an accelerated agricultural settle-
ment after 1960, many pastures were transformed to monocultures of exotic grass
species, more notably of Brachiaria decumbens Stapf. and B. brizantha Stapf. As in
the year 2002, Sano et al. (2010) reported that 54.1 M ha, slightly more than a quarter
of the Cerrado biome, was used for pastures, which is the most widespread land use
system. Most pastures are seeded with B. decumbens , making it the most commonly
cultivated species in the region, as well as the main feedstock for bovines, the pre-
dominant livestock species in the Cerrado and in Brazil. Pastures are predominant
in the Cerrado because they can be established even in an infertile soil (e.g., rocky
Entisols) and in climates too dry for annual crops.