SOC is a function of the balance between inputs from primary production and
outputs through decomposition (Fig. 9.1). As a result of overgrazing, the quantity
of the inputs to the soil may be reduced due to the aboveground biomass removal
by animals. Also, the reduced plant biomass (i.e. less photosynthetic tissue) in
heavily grazed grasslands causes a reduction of the aboveground productivity in
comparison to the non-grazed grasslands (Ferraro and Oesterheld 2002). Although
grazing can stimulate aboveground plant productivity under light or moderate
grazing intensities through the so-called compensatory growth, it has been shown
that heavy defoliation may lead to substantial reduction of the aboveground production (Chen et al. 2006; Zhao et al. 2008). In this case, plants respond to defoliation by allocating more C aboveground and thus reducing root biomass and
productivity. The magnitude of this effect increases with the intensity of defoliation,
the nutrient availability and water availability (Zhao et al. 2008; Klumpp et al.
2009). So there are site-specific sustainable grazing regimes that allow the conservation of C stocks, but when that grazing pressures are exceeded, inputs into the
soil may be reduced (Georgaidis et al. 1989; Ferraro and Oesterheld 2002; Gao
et al. 2008).
Overgrazing causes an alteration in soil physical, chemical and biological
properties, resulting in changes in vegetation cover, a degradation of soil and a loss
of soil C stocks. A typical feature of grazing activity is spatial heterogeneity.
Animals tend to graze on areas with the most nutritious plants whereas select
particular landscapes features for resting and ruminating. As a consequence, different types of vegetation develop which, in turn, influences the subsequent
behaviour of the animals. Plant nutrient contents and soil nutrient availability
increase from grazed to resting areas (Badia et al. 2008). In highly grazed areas, in
comparison with only lightly grazed ones, the availability of P increases as a
consequence of cattle grazing and defecation, which may accelerate the P cycling
(Güsewell et al. 2005). An increase in fresh organic C (i.e. faeces) and nutrient
availabilities as a consequence of animal frequentation may increase the microbial
decomposition of native soil organic C. In addition, an excess of trampling and
continuous overgrazing increases the area of bare soil and the risk of soil erosion. In
the Tibetan plateau, the degradation of grasslands due to land-use change and
overgrazing caused relevant losses of soil organic C in the last 30 years (Xie et al.
2007). In the Alps, erosion rates can be considerably higher (4.4–20 Mg ha
−1
year
−1 ) on grasslands with clear signs of degradation of the vegetation cover
(Meusburger and Alewell 2014).
9.4.2 Grazing Abandonment
Abandonment of pasturelands and traditional farming practices is a widespread
phenomenon in the mountain areas of Europe (MacDonald et al. 2000). While the
impacts on several environmental and landscape values are evident (Tasser et al.
2007), the effects on C dynamics and soil C stocks are less apparent. The net effects
218
J. Garcia-Pausas et al.
outputs through decomposition (Fig. 9.1). As a result of overgrazing, the quantity
of the inputs to the soil may be reduced due to the aboveground biomass removal
by animals. Also, the reduced plant biomass (i.e. less photosynthetic tissue) in
heavily grazed grasslands causes a reduction of the aboveground productivity in
comparison to the non-grazed grasslands (Ferraro and Oesterheld 2002). Although
grazing can stimulate aboveground plant productivity under light or moderate
grazing intensities through the so-called compensatory growth, it has been shown
that heavy defoliation may lead to substantial reduction of the aboveground production (Chen et al. 2006; Zhao et al. 2008). In this case, plants respond to defoliation by allocating more C aboveground and thus reducing root biomass and
productivity. The magnitude of this effect increases with the intensity of defoliation,
the nutrient availability and water availability (Zhao et al. 2008; Klumpp et al.
2009). So there are site-specific sustainable grazing regimes that allow the conservation of C stocks, but when that grazing pressures are exceeded, inputs into the
soil may be reduced (Georgaidis et al. 1989; Ferraro and Oesterheld 2002; Gao
et al. 2008).
Overgrazing causes an alteration in soil physical, chemical and biological
properties, resulting in changes in vegetation cover, a degradation of soil and a loss
of soil C stocks. A typical feature of grazing activity is spatial heterogeneity.
Animals tend to graze on areas with the most nutritious plants whereas select
particular landscapes features for resting and ruminating. As a consequence, different types of vegetation develop which, in turn, influences the subsequent
behaviour of the animals. Plant nutrient contents and soil nutrient availability
increase from grazed to resting areas (Badia et al. 2008). In highly grazed areas, in
comparison with only lightly grazed ones, the availability of P increases as a
consequence of cattle grazing and defecation, which may accelerate the P cycling
(Güsewell et al. 2005). An increase in fresh organic C (i.e. faeces) and nutrient
availabilities as a consequence of animal frequentation may increase the microbial
decomposition of native soil organic C. In addition, an excess of trampling and
continuous overgrazing increases the area of bare soil and the risk of soil erosion. In
the Tibetan plateau, the degradation of grasslands due to land-use change and
overgrazing caused relevant losses of soil organic C in the last 30 years (Xie et al.
2007). In the Alps, erosion rates can be considerably higher (4.4–20 Mg ha
−1
year
−1 ) on grasslands with clear signs of degradation of the vegetation cover
(Meusburger and Alewell 2014).
9.4.2 Grazing Abandonment
Abandonment of pasturelands and traditional farming practices is a widespread
phenomenon in the mountain areas of Europe (MacDonald et al. 2000). While the
impacts on several environmental and landscape values are evident (Tasser et al.
2007), the effects on C dynamics and soil C stocks are less apparent. The net effects
218
J. Garcia-Pausas et al.