Principles of Soil Management in Neotropical Savannas - Principles of Sustainable Soil Management in Agroecosystems - page 64

if on rock outcrops, Campo Rupestre in Portuguese), shrubby grasslands ( Campo
Sujo ), savanna woodland ( Cerrado stricto sensu ), and Cerradão, which is a forest
savanna, mostly tall-tree formation, with few grasses (Lopes and Cox 1977; Grace
et al. 2006). Riverine forests and palm-grass wetlands ( veredas ) occur along streams
and drainageways, and many dry forests with little or no grass strata occur amidst
the savannic formations. Typically, the dominance of grasslands is associated with
very low soil fertility and moisture, so treeless grasslands occur on the poorest soils,
whereas the Cerradão, the riverine, and dry forests indicate more fertile, less acidic
soils. The impact of vegetation biomass on soil properties can be unexpected, since
there appears to be no correlation between vegetation biomass and soil organic car-
bon (SOC) concentrations in surface soils (Lopes and Cox 1977). This effect is also
reflected in subsurface layers: Chapuis-Lardy et al. (2002) noted that SOC stocks
under treeless grasslands can be higher than under tall-tree savannas with much
higher aboveground biomass, because of stronger root biomass in the former veg-
etation. Since the classic studies on earthworms and soil bioturbation by Charles
Darwin (1882), the impact of soil fauna on soil genesis and properties has chiefly
been attributed to worms, moles, and other burrowing animals. Although large
earthworms and armadillos are abundant in the Cerrado, social arthropods exert
major influence on its soils not only by their sheer diversity and ubiquity but also
because of their overall biomass (Benito et al. 2004) and amount of soil disturbed
annually. Termites and leaf-cutting ants excavate large volumes of soils, disrupting
aggregates but also forming new ones, typically smaller and rounded, sometimes
spherical and welded (Figure 3.3), which can comprise a significant percentage of
total topsoil mass (Zinn et al. 2007b). The activity of termites over considerable geo-
logic time has been ascribed as the cause for the loose granular structure of many
Oxisols, especially in subsurface horizons (Schaefer 2001).
FIGURE 3.3 Typical granular microstructure of Cerrado Oxisols. The high macroporos-
ity is expressed in the form of packing voids, through which saturated flow is very rapid. A
cluster of biogenic, spherical aggregates is shown. Image is 2.3 mm wide.