CO 2 efflux from these soils, but the release of soluble cell constituents from dead
fine roots is the main factor that contributes to the observed short-term CO 2 and
N 2 O pulse after thawing (Matzner and Borken 2008), and the higher concentrations
of N and P in the soil solution (Fitzhugh et al. 2001; Freppaz et al. 2007; Wipf et al.
2015) after freeze–thaw events. The third source of the CO 2 flushed upon a freeze–
thaw event is the change in the soil structure. Soil freezing causes a disruption of
soil aggregates because the ice crystals expand breaking the bonds between mineral
particles. This breakdown of the aggregates makes the organic matter protected
within the aggregates available for microorganisms. Macroaggregates are more
susceptible to these disruptive forces than microaggregates, and their vulnerability
is enhanced with increasing soil water content, while high clay, organic matter and
Fe-oxide concentrations reduce the disruptive effects of freezing (Six et al. 2004).
9.3.3 Plant Community Composition
The effect of abiotic environment on soil C content and its stability is in part
mediated by its effect on plant communities. It is well known that abiotic factors
determine the composition and characteristics of the plant community, which in
mountain ecosystems is also strongly related to topography (Sebastià 2004) and
bedrock type. The composition of plant communities determines some functional
characteristics that can be relevant for the organic matter production and allocation.
For instance, although most of the root characteristics are species-specific, Pohl
et al. (2011) showed that in alpine ecosystems graminoids usually have a large
proportion of fine roots compared to forbs or shrubs. This feature may enhance
topsoil aggregate stability under grasses (Pohl et al. 2009), which could be relevant
for stabilising C in soils. Other characteristics of the vegetation such as above- and
belowground productivity and allocation, rooting depth, horizontal root expansion
may influence the C inputs and its persistence in the soil.
Plant community composition is in turn related to the quality of soil organic
matter and, consequently to its decomposition rates. The low quality for decomposition of the organic matter produced by woody shrubs, with high lignin and
polyphenol content, greatly differs from that produced by herbaceous plants, which
is much more readily decomposable. But also among herbaceous plants, there can
be significant differences, as occurs with the N-enriched organic matter produced by
legumes. Thus plant communities differing in the biochemical characteristics of the
biomass can lead to differences in the biochemical quality of soil organic matter.
Indeed, Eskelinen et al. (2009) indicated that high proportion of forbs in an alpine
tundra of northern Europe were related to low soil C/N ratios and high soluble
N/phenolics ratios, causing in turn differences in the associated microbial
communities.
9 Are Soil Carbon Stocks in Mountain Grasslands …
215
fine roots is the main factor that contributes to the observed short-term CO 2 and
N 2 O pulse after thawing (Matzner and Borken 2008), and the higher concentrations
of N and P in the soil solution (Fitzhugh et al. 2001; Freppaz et al. 2007; Wipf et al.
2015) after freeze–thaw events. The third source of the CO 2 flushed upon a freeze–
thaw event is the change in the soil structure. Soil freezing causes a disruption of
soil aggregates because the ice crystals expand breaking the bonds between mineral
particles. This breakdown of the aggregates makes the organic matter protected
within the aggregates available for microorganisms. Macroaggregates are more
susceptible to these disruptive forces than microaggregates, and their vulnerability
is enhanced with increasing soil water content, while high clay, organic matter and
Fe-oxide concentrations reduce the disruptive effects of freezing (Six et al. 2004).
9.3.3 Plant Community Composition
The effect of abiotic environment on soil C content and its stability is in part
mediated by its effect on plant communities. It is well known that abiotic factors
determine the composition and characteristics of the plant community, which in
mountain ecosystems is also strongly related to topography (Sebastià 2004) and
bedrock type. The composition of plant communities determines some functional
characteristics that can be relevant for the organic matter production and allocation.
For instance, although most of the root characteristics are species-specific, Pohl
et al. (2011) showed that in alpine ecosystems graminoids usually have a large
proportion of fine roots compared to forbs or shrubs. This feature may enhance
topsoil aggregate stability under grasses (Pohl et al. 2009), which could be relevant
for stabilising C in soils. Other characteristics of the vegetation such as above- and
belowground productivity and allocation, rooting depth, horizontal root expansion
may influence the C inputs and its persistence in the soil.
Plant community composition is in turn related to the quality of soil organic
matter and, consequently to its decomposition rates. The low quality for decomposition of the organic matter produced by woody shrubs, with high lignin and
polyphenol content, greatly differs from that produced by herbaceous plants, which
is much more readily decomposable. But also among herbaceous plants, there can
be significant differences, as occurs with the N-enriched organic matter produced by
legumes. Thus plant communities differing in the biochemical characteristics of the
biomass can lead to differences in the biochemical quality of soil organic matter.
Indeed, Eskelinen et al. (2009) indicated that high proportion of forbs in an alpine
tundra of northern Europe were related to low soil C/N ratios and high soluble
N/phenolics ratios, causing in turn differences in the associated microbial
communities.
9 Are Soil Carbon Stocks in Mountain Grasslands …
215