4.4.3.1. Influencing Degradation Processes
Up to 71 percent of the world's grasslands are degraded to some extent (Dregne
et al., 1991) as a result of overgrazing, salinization, alkalinization,
acidification, and other processes (Oldeman, 1994). Generally, degradation results
in reductions in perennial plant cover, increased erosion risk, and loss of
productive potential. Restoration of vegetation and hence productive capacity
results in increases in biomass, litter, and soil carbon pools (Table
4-6). There may also be a reduction in the intensity or frequency of water
and wind erosion, which removes carbon (some of which may be oxidized to CO2).
Wind and water erosion reduce carbon stocks and productivity at the location
of soil loss. The discussion in Section 4.2 of the uncertainty relating to the
net effect of erosion is relevant here as well. Grazing and burning reduce plant
cover, thereby increasing soil erosion rates (Scanlan et al., 1996) and
carbon loss (e.g., Ash et al., 1996). Increased erosion can reduce yields
(Tenberg et al., 1998) and thus carbon inputs into the soil, which depletes
soil carbon levels (e.g., Kelly et al., 1996; Lal, 2000b). Soil conservation
procedures that restore landscape patchiness (e.g., by using shrub branches
laid across the slope) can rapidly and substantially increase soil carbon (Tongway
and Ludwig, 1996). Activities that combat desertification more broadly are also
likely to result in substantial carbon storage (Fullen and Mitchell, 1994).
4.4.3.2. Grazing Management
Overgrazing is the single greatest cause of degradation in grasslands (Oldeman
et al., 1991) and the overriding human-influenced factor in determining
their soil carbon levels (Ojima et al., 1993a). Grazing influences carbon
storage through removal of biomass and nutrients-much of which are recycled
to soil pools but some of which are lost from the grassland (e.g., Haynes and
Williams, 1993). Grazing also influences partitioning of carbon to aboveground
or below-ground plant organs, changes the temperature and disturbance regimes
of the soil, and alters water infiltration and susceptibility to erosion (Follett
et al., 2000). Often, extensive heavy grazing practices result in decreases
in carbon pools of biomass and soil carbon (e.g., Naeth et al., 1991;
Ash et al., 1996; Li et al., 1997; McIntosh et al., 1997a,b).
Consequently, in many systems, improved grazing management (e.g., optimizing
stock numbers, rotational grazing) will result in substantial increases in carbon
pools (e.g., Ash et al., 1996; Eldridge and Robson, 1997; Table
4-6); in some cases, grazing can increase nutrient cycling, animal productivity
(e.g., Franzluebbers et al., 2000), and incorporation of litter into
the soil-thereby increasing soil carbon (Schuman et al., 1999). Where soil carbon
has been lost, however, the rate of recovery of carbon may be considerably slower
than the rate of loss (Northup and Brown, 1999). Additional human intervention
may be required where degradation is severe. Adoption of more sustainable grazing
practices that track climate variability and change is likely to reduce the
risk of degradation and hence carbon loss (McKeon et al., 1993).
In some grasslands, changes in species composition under grazing toward those
with large and dense root systems (e.g., Bouteloua gracilis) can increase
carbon levels in the surface soil layers (Dormaar and Willms, 1990; Frank et
al., 1995; Manley et al., 1995; Berg et al., 1997) but are associated
with reduced grazing utility. Where such species are already dominant, however,
heavy grazing will reduce soil carbon levels (Gardner, 1950). In other situations,
grazing intensity seems to have little or variable impact on soil carbon levels
(Milchunas and Lauenroth, 1993; Basher and Lynn, 1996; Chaneton and Lavado,
1996; Brejida, 1997; Tracy and Frank, 1998; McIntosh and Allen, 1998), provided
overgrazing does not occur. Heavy grazing can increase opportunities for establishment
of unpalatable woody shrubs, resulting in increased biomass carbon pools but
lower grazing utility (e.g., Boutton et al., 1998).
Grazing livestock are significant contributors of methane and nitrous oxide
globally (accounted for within the IPCC Guidelines), and their wastes can produce
ammonia (a precursor of oxides of nitrogen and subsequent ozone formation).
Improvements in the diet of livestock can substantially reduce methane emissions
per unit intake (Kurihara et al., 1999), but if livestock are fed grain or other
supplements, the emissions embodied in these feeds must be accounted for.
|