Agriculture Reference
In-Depth Information
however, it is reasonable to assume that closely grazed pastures will have shallower
root systems than lightly grazed or intermittently grazed systems.
Infiltration is a critical process of ecosystem water budgets, and grazing can degrade
it significantly. Perennial forages often improve soil organic matter (Magdoff and Weil
2004) and link surface and subsurface soil via continuous biopores, e.g., channels made
by roots (Elkins et al. 1977) and faunal activity (Shipitalo et al. 2000). These changes in
structure typically accompany soil regeneration by perennial pasture species, and basal
area and biomass have been used to develop soil infiltration indices in rangeland (Roth
2004; Tongway and Hindley 2004). Bardgett and Ward (2003) and McNaughton et al.
(1998) reported no reduction in belowground biomass on an annual timescale owing
to “natural” grazing on the African Serengeti. However, heavy grazing by sheep in
Mongolia reduced belowground biomass (Zhao et al. 2005), and other authors reported
similar biomass interactions and reduced faunal activity (Holt et al. 1996; Bardgett et al.
1998; Gross et al. 1991). If uncontrolled grazing results in excessive removal of living
and senesced vegetative cover, a process of soil degradation begins that threatens both
the water and nutrient supply to the ecosystem. To sustain a healthy condition, grazing
timing, intensity, and duration must be controlled to maintain vegetative cover on the
surface and reduce surface soil degradation by animal traffic (Figure 4.4). Herding has
been a method of control of stock movement to allow soil and vegetation regeneration.
Fencing is also an effective method for doing this but requires capital investment and
different management skills.
Years to decades
Decades to centuries
Litter
A-horizon
B-horizon
C-horizon/regolith
Root depth
Es
ET
Runoff
PAWC
Carbon
Stream flow
Drainage
FIGURE 4.4
Schematic of degradation and regeneration of vegetative cover and soil quality
with uncontrolled and controlled grazing.
