Defoliation induces an increase of root exudation (Paterson et al. 2005).
Therefore when grassland is abandoned a reduction of labile C inputs into the soil
can be expected. Also, when shrubs proliferate, their lower fine root density may
cause further reduction of C inputs by exudation. This decrease of labile C release
from roots may cause a significant reduction of microbial activity (Hamilton and
Frank 2001) and also a lower stimulation of soil organic matter mineralisation (i.e.
priming) that usually occurs in the presence of labile C (Kuzyakov et al. 2000).
Priming effect on soil organic matter mineralisation is particularly relevant in the
rhizosphere and, although its magnitude is variable, it increases with the rate of
rhizospheric C inputs (Paterson and Sim 2013) and may account for a substantial
fraction of the SOM-derived CO 2 efflux (Cheng and Kuzyakov 2005). In addition,
this reduction of priming effect may cause in turn a reduction of the nutrient
availability for plants (Hamilton and Frank 2001).
The change of root exudates, as well as the fate of particulate organic matter also
promotes a change in the microbial community composition (Grayston et al. 2004).
Indeed, fungal growth and activity seems to be generally favoured in surface
horizons after grazing or agricultural abandonment (Zornoza et al. 2009;
Lopez-Sangil et al. 2011) and a higher fungal-to-bacterial activity ratio seems to
promote a conservative cycling of nutrients in soil and C accumulation (Wardle
et al. 2004; Gordon et al. 2008). Therefore a reduction in soil organic C decomposition in abandoned sites is expected.
9.4.4.4 Soil C Stocks in Shrub-Encroached Grasslands
In summary, shrub encroachment into mountain grasslands increases soil organic
carbon in the upper soil mineral profile compared to the grassland soil (Montané
et al. 2007). This net C increase may be explained by lower aboveground and
belowground litter decomposition after shrub proliferation due to lower soil temperatures and lower biochemical quality of shrub organic matter. Lower litter
quality may promote a shift in the composition of the microbial community to a
slow-growth strategy, typical of a fungal-dominated microbial community
(Bardgett et al. 2005), which may contribute to explain lower decomposition. The
reduction of fine root density with the proliferation of shrubs may also reduce the
rates of root exudation, which may decrease the priming effect on soil organic
matter mineralisation thus contributing to the conservation of soil C stocks.
9.4.4.5 Shrubland Management and Soil C Stocks
Shrub encroachment into grasslands involves the replacement of one dominant
growth form by another one, and it is likely to impact on ecosystem structure and
functions (Lett and Knapp 2005). In the Pyrenees, a decrease in diversity
9 Are Soil Carbon Stocks in Mountain Grasslands …
223
Therefore when grassland is abandoned a reduction of labile C inputs into the soil
can be expected. Also, when shrubs proliferate, their lower fine root density may
cause further reduction of C inputs by exudation. This decrease of labile C release
from roots may cause a significant reduction of microbial activity (Hamilton and
Frank 2001) and also a lower stimulation of soil organic matter mineralisation (i.e.
priming) that usually occurs in the presence of labile C (Kuzyakov et al. 2000).
Priming effect on soil organic matter mineralisation is particularly relevant in the
rhizosphere and, although its magnitude is variable, it increases with the rate of
rhizospheric C inputs (Paterson and Sim 2013) and may account for a substantial
fraction of the SOM-derived CO 2 efflux (Cheng and Kuzyakov 2005). In addition,
this reduction of priming effect may cause in turn a reduction of the nutrient
availability for plants (Hamilton and Frank 2001).
The change of root exudates, as well as the fate of particulate organic matter also
promotes a change in the microbial community composition (Grayston et al. 2004).
Indeed, fungal growth and activity seems to be generally favoured in surface
horizons after grazing or agricultural abandonment (Zornoza et al. 2009;
Lopez-Sangil et al. 2011) and a higher fungal-to-bacterial activity ratio seems to
promote a conservative cycling of nutrients in soil and C accumulation (Wardle
et al. 2004; Gordon et al. 2008). Therefore a reduction in soil organic C decomposition in abandoned sites is expected.
9.4.4.4 Soil C Stocks in Shrub-Encroached Grasslands
In summary, shrub encroachment into mountain grasslands increases soil organic
carbon in the upper soil mineral profile compared to the grassland soil (Montané
et al. 2007). This net C increase may be explained by lower aboveground and
belowground litter decomposition after shrub proliferation due to lower soil temperatures and lower biochemical quality of shrub organic matter. Lower litter
quality may promote a shift in the composition of the microbial community to a
slow-growth strategy, typical of a fungal-dominated microbial community
(Bardgett et al. 2005), which may contribute to explain lower decomposition. The
reduction of fine root density with the proliferation of shrubs may also reduce the
rates of root exudation, which may decrease the priming effect on soil organic
matter mineralisation thus contributing to the conservation of soil C stocks.
9.4.4.5 Shrubland Management and Soil C Stocks
Shrub encroachment into grasslands involves the replacement of one dominant
growth form by another one, and it is likely to impact on ecosystem structure and
functions (Lett and Knapp 2005). In the Pyrenees, a decrease in diversity
9 Are Soil Carbon Stocks in Mountain Grasslands …
223