Agriculture Reference
In-Depth Information
will give time for humification. Biological crusts can be an early casualty when
natural rangeland is grazed. Figure 4.13 demonstrates the erosion-resistant effects
of both the pasture and the biological crust, protecting the shallow loam topsoil over
a gravelly subsoil with massive structure in the Upper East Region of Ghana. With
continued grazing, erosion will continue; stock exclusion could help stabilization.
Removing vegetative cover leaves the soil fabric exposed to raindrop impact and
erosive forces of overland flow (Belnap et al. 2005). During rainfall, litter and bio-
logical crusts can protect soil structure from raindrop impact and compaction (Moss
1991a,b; Belnap 2006; Belnap et al. 2005). During overland flow, litter can form
microterraces (Figure 4.14), increasing surface hydraulic roughness and producing
longer durations for infiltration (Ellis et al. 2006).
Grazing affects surface soil structure, biological crusts, and microtopography by
either treading or reduction in litter and soil fauna (Lusby 1970; Bridge et al. 1983;
Williams and Bonell 1988; McIvor et al. 1995; Trimble and Mendel 1995; Holt et
al. 1996; Mwendera and Saleem 1997; Greenwood and McKenzie 2001; Roth 2004;
Belnap 2006; Drewry et al. 2008). These effects lead to reduced infiltration and soil
water storage.
In a semiarid South African environment, Materechera and Murovhi (2011) found
that bare land had the lowest of all the measured soil biological properties signify-
ing limited biological activity. They found significantly greater SOC and particu-
late matter under fallow, while microbial biomass carbon and nitrogen were greater
under grazing land use practice. The major contributing factor under grazing land
was considered to be the large amount of organic matter that is returned to soil, espe-
cially that from animal dung and turnover of grass roots. Soil biological properties
were further enhanced beneath the canopy of
Acacia erioloba
trees, owing to favor-
able microclimate and surface deposition of organic materials.
FIGURE 4.13
Degrading biological crust from an overgrazed granitic soil in the Upper
East Region of Ghana. This demonstrates the erosion resistant effects of both the pasture and
the biological crust, protecting the shallow loam topsoil over a gravelly subsoil with massive
structure. With continued grazing, erosion will continue; stock exclusion could help stabiliza-
tion. (Photographer: T.W. Ellis.)
