Principles of Soil Management in Neotropical Savannas - Principles of Sustainable Soil Management in Agroecosystems - page 73

Traditionally, pastures were not seeded immediately after clearance of native veg-
etation, but after growing upland (rainfed) rice for 1 or 2 years (Klink and Moreira
2002). The upland rice was appropriate because of its low nutrient demand, which
could be met by the nutrients mineralized from decaying residues of native veg-
etation, even without liming or fertilization (Lilienfein et al. 2003). Fertilized pas-
tures of the Cerrado typically produce 5-11 Mg ha -1 year -1 of grass biomass, feeding
2-2.5 animals ha -1 with 500-800 kg ha -1 year -1 of liveweight gain (Lilienfein et al.
2003). Thus, most of these pasturelands are reasonably productive for the low inputs
they receive and have a relatively low environmental footprint compared to those of
annual crops. Further, pastures improve soil aggregation by their strong root system.
Salton et al. (2008) reported that pastures or crop-pasture rotations increased large
water-stable aggregates (>4.76 mm) in comparison to native vegetation, in two out
of three locations in the Cerrado in the State of Mato Grosso do Sul. Increases in
aggregate size were strongly correlated with the higher SOC stocks in 0.2-m depth.
For a sandy Haplustox in the southernmost Cerrado, SOC stocks for 0.3-m depth
increased from 30 Mg ha -1 under native vegetation to 37 Mg ha -1 after 80 years under
pasture (Maquere et al. 2008). Bustamante et al. (2006) reviewed SOC changes after
conversion to pastures and reported an average change of 1.3 Mg C ha -1 year -1 , with a
range of -0.87 to 3.0 Mg C ha -1 year -1 . Furthermore, some severely compacted soils
can be efficiently reclaimed by planting tropical grasses commonly used in pastures,
such as Cynodon spp. (Severiano et al. 2010).
Although improved management practices are being increasingly adopted, most
pastures in the Cerrado show some level of degradation. Therefore, productivity of
Brachiaria pastures in the Cerrado is much lower than that in Australia (Lilienfein et
al. 2003). Vendrame et al. (2010) analyzed >70 Oxisols under pasture in the Cerrado
and observed that about 90% had low or very low available P, and most contained
other nutrients below critical levels. Little or no nutrient replacement is often the
principal cause of pasture degradation, although physical degradation is also com-
mon. Some farmers intensify production by adding corrective and replacement
fertilization. However, degradation can still occur if the stocking rate exceeds the
optimum level, which varies widely according to local soil and climate conditions.
Overgrazing aggravates compaction and depletes SOC stocks by as much as 20% in
comparison to soils under native vegetation (Costa et al. 2011).
Effects of pasturelands on other environmental indicators have also been studied
in the Cerrado. Termite and ant mounds are a common sight in old and degraded pas-
tures. However, pasture restoration by tillage and fertilization may adversely affect
the macrofauna diversity and biomass (Benito et al. 2004), also decreasing some
ecosystem services such as pest control by natural enemies, and even water infiltra-
tion rate because of elimination of biochannels.
Fluxes of greenhouse gases (GHGs) from pastures are also an important environ-
mental issue. Using static chambers in situ on a clayey Haplustox, Siqueira Neto et
al. (2011) reported a strong contrast in methane (CH 4 ) emissions and its uptake under
a Brachiaria pasture and nearby native Cerrado vegetation and croplands. However,
fluxes of CO 2 and N 2 O were similar under land uses. Thus, conversion of Cerrado to
pastures has complex effects on the ecosystem and its environmental services, which
deserve further investigation.