Keywords Grassland abandonment Á Land-use changes Á Mountain grasslands Á
Grazing intensification Á Shrub encroachment Á Soil organic carbon dynamics Á
Soil organic carbon stocks
9.1 Introduction
Soil organic matter plays essential roles in terrestrial ecosystems. It maintains the
soil structure, favours water infiltration and reduces the risk of soil erosion. It also
increases the water holding capacity of soils and, through its decomposition by soil
biota, provides nutrients to the plants.
Carbon (C) comprises about 45% of the mass of soil organic matter. Plant
photosynthetic activity produces organic matter using atmospheric CO 2 , which is
then accumulated in soil mainly by incorporating plant residues into the soil
organic matter. Although this is the primary pathway by which atmospheric CO 2 –C
is incorporated into the soil, some additional atmospheric CO 2 –C can also be
sequestered in soil in inorganic forms by rock weathering and precipitation of Caand Mg-carbonates. Then, the oxidation of organic matter by soil microorganisms
is the main process causing a release of carbon as CO 2 to the atmosphere, leaving
less decomposable organic compounds, which are accumulated in the soil.
Together with this biotic process, a significant amount of soil C can also be
exported from the soil by leaching. Overall soil C sequestration results from the
balance between the C flux from the atmosphere into the soil and the C release back
to the atmosphere through microbial decomposition. This balance determines if soil
behaves as a net sink for removing CO 2 from the atmosphere or a net source that
contributes to rising atmospheric CO 2 .
Soils represent the main compartment of organic C in most terrestrial ecosystems, containing globally about 1550 Pg C (1 Pg = 10
15 g), which roughly is twice
the amount of C in the atmosphere (760 Pg C) and three times the amount in the
biomass (550 Pg C) (Lal 2008). Given the large magnitude of these soil C stocks,
potential reductions as little as 10% of the soil C content would equal to the
anthropogenic CO 2 emitted over 30 years (Kirschbaum 2000), meaning significant
changes in the atmospheric CO 2 concentrations and the reinforcement of the current
global warming trend. So, there is a strong interest in avoiding C losses from soils
and, if possible, to promote the C sequestration to mitigate the current greenhouse
gases (GHG) emissions.
In this chapter, we summarise the special features of mountain alpine soils that
contribute to explaining the organic C content and explore the challenges for soil C
conservation due to changes in land management and use.
9.2 Mountain Soils and Their C Stocks
Mountain soils are generally steep, shallow, with relatively high erosion rates and
influenced by harsh climatic conditions. Despite mountain ecosystems have much
in common with those in high latitude, mountain soils are markedly different. These
208
J. Garcia-Pausas et al.
Grazing intensification Á Shrub encroachment Á Soil organic carbon dynamics Á
Soil organic carbon stocks
9.1 Introduction
Soil organic matter plays essential roles in terrestrial ecosystems. It maintains the
soil structure, favours water infiltration and reduces the risk of soil erosion. It also
increases the water holding capacity of soils and, through its decomposition by soil
biota, provides nutrients to the plants.
Carbon (C) comprises about 45% of the mass of soil organic matter. Plant
photosynthetic activity produces organic matter using atmospheric CO 2 , which is
then accumulated in soil mainly by incorporating plant residues into the soil
organic matter. Although this is the primary pathway by which atmospheric CO 2 –C
is incorporated into the soil, some additional atmospheric CO 2 –C can also be
sequestered in soil in inorganic forms by rock weathering and precipitation of Caand Mg-carbonates. Then, the oxidation of organic matter by soil microorganisms
is the main process causing a release of carbon as CO 2 to the atmosphere, leaving
less decomposable organic compounds, which are accumulated in the soil.
Together with this biotic process, a significant amount of soil C can also be
exported from the soil by leaching. Overall soil C sequestration results from the
balance between the C flux from the atmosphere into the soil and the C release back
to the atmosphere through microbial decomposition. This balance determines if soil
behaves as a net sink for removing CO 2 from the atmosphere or a net source that
contributes to rising atmospheric CO 2 .
Soils represent the main compartment of organic C in most terrestrial ecosystems, containing globally about 1550 Pg C (1 Pg = 10
15 g), which roughly is twice
the amount of C in the atmosphere (760 Pg C) and three times the amount in the
biomass (550 Pg C) (Lal 2008). Given the large magnitude of these soil C stocks,
potential reductions as little as 10% of the soil C content would equal to the
anthropogenic CO 2 emitted over 30 years (Kirschbaum 2000), meaning significant
changes in the atmospheric CO 2 concentrations and the reinforcement of the current
global warming trend. So, there is a strong interest in avoiding C losses from soils
and, if possible, to promote the C sequestration to mitigate the current greenhouse
gases (GHG) emissions.
In this chapter, we summarise the special features of mountain alpine soils that
contribute to explaining the organic C content and explore the challenges for soil C
conservation due to changes in land management and use.
9.2 Mountain Soils and Their C Stocks
Mountain soils are generally steep, shallow, with relatively high erosion rates and
influenced by harsh climatic conditions. Despite mountain ecosystems have much
in common with those in high latitude, mountain soils are markedly different. These
208
J. Garcia-Pausas et al.