is attributed to the harsh conditions for residue decomposition, the low soil pH and
nutrient limitations. However, in areas where environmental conditions are unfavourable for decomposition, the degree of physicochemical stabilisation of organic
matter, as measured by incubation under standard conditions, is lower (Garcia-Pausas
et al. 2008). This feature could make these C pools particularly vulnerable to future
climate and land-use changes.
The temperature variations in altitude and aspect, as well as the microtopography
and the predominant wind direction largely determine the distribution and duration
of the snowpack cover. The duration of the snowpack cover has important implications for the soil organic C dynamics, as it determines not only the temperature
and moisture of the underlying soil but also the length of the plant growing season,
the plant community composition, the microbial activity and nutrient dynamics.
Indeed, snow cover maintains soil temperature relatively high compared to the air
temperatures during winter, allowing the maintenance of unfrozen conditions
(Edwards et al. 2007). This isolation is because of the low thermal conductivity of
the snow, particularly when it is fresh and non-compacted (Körner 2003).
Consequently, topsoil temperature under the snow is usually stable around 0 °C,
even when air temperatures are far below zero. This allows the microbial processes
to continue in winter (Schmidt and Lipson 2004), causing an increase in the winter
CO 2 efflux (Walker et al. 1999) and also a faster decomposition of the leaf litter
(Baptist et al. 2010; Saccone et al. 2013) under the snow than in non-covered soils.
However, when the snowpack melts in late winter and before the snowfall in late
autumn, soils are usually exposed to temperatures well below 0 °C, undergoing
frequent episodes of freezing and thawing.
Soil frost does not allow the belowground plant production, but an earlier peak
in fine root production during the subsequent growing season has been observed by
Tierney et al. (2001) after an experimental snow removal in forest ecosystems. They
also reported significant increases in fine root mortality, resulting in an increased
root turnover. In grasslands, Kreyling et al. (2008) indicated that recurrent freeze–
thaw events reduced root length during the subsequent growing season, but also
increased aboveground productivity.
Freeze–thaw events alter C and N dynamics, affecting root production and
turnover, soil microbial activity, soil C and N availability and its mineralisation. It
has been observed that repeated freeze–thaw cycles as well as prolonged frost
increase C and nutrient concentration in the soil solution, which can eventually be
lost by leaching (Fitzhugh et al. 2001; Freppaz et al. 2007; Wipf et al. 2015). Also,
a burst of CO 2 and N 2 O emissions from thawing soils has often been observed
(Nielsen et al. 2001; Teepe et al. 2001; Matzner and Borken 2008), as well as
higher emissions during the subsequent growing season (Blankinship and Hart
2012). This response is partially explained by the decomposition of the microbial
necromass (Herrmann and Witter 2002), but a reduction of microbial biomass in
thawing episodes has not been detected in alpine soils (Lipson et al. 2002; Freppaz
et al. 2007). Another source of the CO 2 flush when soil thaws is the death of fine
roots due to the soil frost. The decomposition of the fine root litter increases the
214
J. Garcia-Pausas et al.
nutrient limitations. However, in areas where environmental conditions are unfavourable for decomposition, the degree of physicochemical stabilisation of organic
matter, as measured by incubation under standard conditions, is lower (Garcia-Pausas
et al. 2008). This feature could make these C pools particularly vulnerable to future
climate and land-use changes.
The temperature variations in altitude and aspect, as well as the microtopography
and the predominant wind direction largely determine the distribution and duration
of the snowpack cover. The duration of the snowpack cover has important implications for the soil organic C dynamics, as it determines not only the temperature
and moisture of the underlying soil but also the length of the plant growing season,
the plant community composition, the microbial activity and nutrient dynamics.
Indeed, snow cover maintains soil temperature relatively high compared to the air
temperatures during winter, allowing the maintenance of unfrozen conditions
(Edwards et al. 2007). This isolation is because of the low thermal conductivity of
the snow, particularly when it is fresh and non-compacted (Körner 2003).
Consequently, topsoil temperature under the snow is usually stable around 0 °C,
even when air temperatures are far below zero. This allows the microbial processes
to continue in winter (Schmidt and Lipson 2004), causing an increase in the winter
CO 2 efflux (Walker et al. 1999) and also a faster decomposition of the leaf litter
(Baptist et al. 2010; Saccone et al. 2013) under the snow than in non-covered soils.
However, when the snowpack melts in late winter and before the snowfall in late
autumn, soils are usually exposed to temperatures well below 0 °C, undergoing
frequent episodes of freezing and thawing.
Soil frost does not allow the belowground plant production, but an earlier peak
in fine root production during the subsequent growing season has been observed by
Tierney et al. (2001) after an experimental snow removal in forest ecosystems. They
also reported significant increases in fine root mortality, resulting in an increased
root turnover. In grasslands, Kreyling et al. (2008) indicated that recurrent freeze–
thaw events reduced root length during the subsequent growing season, but also
increased aboveground productivity.
Freeze–thaw events alter C and N dynamics, affecting root production and
turnover, soil microbial activity, soil C and N availability and its mineralisation. It
has been observed that repeated freeze–thaw cycles as well as prolonged frost
increase C and nutrient concentration in the soil solution, which can eventually be
lost by leaching (Fitzhugh et al. 2001; Freppaz et al. 2007; Wipf et al. 2015). Also,
a burst of CO 2 and N 2 O emissions from thawing soils has often been observed
(Nielsen et al. 2001; Teepe et al. 2001; Matzner and Borken 2008), as well as
higher emissions during the subsequent growing season (Blankinship and Hart
2012). This response is partially explained by the decomposition of the microbial
necromass (Herrmann and Witter 2002), but a reduction of microbial biomass in
thawing episodes has not been detected in alpine soils (Lipson et al. 2002; Freppaz
et al. 2007). Another source of the CO 2 flush when soil thaws is the death of fine
roots due to the soil frost. The decomposition of the fine root litter increases the
214
J. Garcia-Pausas et al.