temperatures under shrub canopies likely reduces above- and/or belowground litter
decomposition which may be the predominant mechanism behind higher SOC after
shrub encroachment (Smith and Johnson 2004). For instance, a reduction of soil
CO 2 efflux after grazing exclusion in the Tibetan Plateau has been attributed in part
to its lower soil temperature (Chen et al. 2016). In the Pyrenees, Festuca eskia roots
incubated for 1 year in buried litterbags in a subalpine soil decomposed slightly
slower under shrubs than in paired grasslands (20.1 ± 0.42% and 22.4 ± 1.44%
mass loss, respectively) (Casals et al. 2010). In addition, using buried labelled
wheat roots mixed with soil, Casals et al. (2010) showed that
13 C loss was about
four percent units lower in root bags incubated for 1 year in non-grazed grassland
plots and seven percent units lower under shrubs than in paired grazed grasslands.
As these results derived from the incubation of standard labelled material, they
mainly reflect a change to a less favourable soil environment for root decomposition
due to either grazing exclusion or shrub encroachment. Therefore, a decrease in soil
temperature may contribute to explain lower root decomposition rates after grassland abandonment and shrub encroachment.
9.4.4.3 Biochemical Quality and Microbial Activity
It is widely known that litter nutrient concentration and organic matter quality (e.g.
lignin content) are the main factors determining litter decomposition rates
(Cornwell et al. 2008). Grasslands typically have a high density of fine roots that are
poorly lignified and with high turnover rates, thus providing a relatively labile C
substrate for microbial activity. In contrast, the proliferation of shrubs may increase
the presence of lignified roots with lower turnover rates. After shrub encroachment,
low quality of litter inputs, with large amounts of secondary compounds such as
lignin or polyphenolic substances, may hinder decomposition and promote C
accumulation (Pérez-Harguindeguy et al. 2000; Shaw and Harte 2001; McCulley
et al. 2004; Liao and Boutton 2008).
The biochemical quality of litter may differ between species. In the Pyrenees, an
aboveground litter of grasses showed marked differences in the chemical composition from that of the two main invading shrubs of that area (Cytisus balansae and
Juniperus communis). The litter of both shrubs had higher concentrations of
recalcitrant compounds (e.g. lignin, lipids, suberin) and a low concentration of
either N (conifer) or P (legume) relative to grass litter (Montané et al. 2010).
Consequently, the higher organic C found in the upper mineral soil layer under
shrubs compared to the grassland was mainly attributed to the slower decomposition of shrub litter and the transfer of litter-derived C into the soil. However, the
presence of grass litter, with high N and P concentrations, may enhance microbial
activity and prime the decomposition of recalcitrant shrub litter. As a result, at least
in the short term after shrub proliferation when both shrub and grass litters coexist,
the shrub litter accumulation pattern is altered (Montané et al. 2013).
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J. Garcia-Pausas et al.
decomposition which may be the predominant mechanism behind higher SOC after
shrub encroachment (Smith and Johnson 2004). For instance, a reduction of soil
CO 2 efflux after grazing exclusion in the Tibetan Plateau has been attributed in part
to its lower soil temperature (Chen et al. 2016). In the Pyrenees, Festuca eskia roots
incubated for 1 year in buried litterbags in a subalpine soil decomposed slightly
slower under shrubs than in paired grasslands (20.1 ± 0.42% and 22.4 ± 1.44%
mass loss, respectively) (Casals et al. 2010). In addition, using buried labelled
wheat roots mixed with soil, Casals et al. (2010) showed that
13 C loss was about
four percent units lower in root bags incubated for 1 year in non-grazed grassland
plots and seven percent units lower under shrubs than in paired grazed grasslands.
As these results derived from the incubation of standard labelled material, they
mainly reflect a change to a less favourable soil environment for root decomposition
due to either grazing exclusion or shrub encroachment. Therefore, a decrease in soil
temperature may contribute to explain lower root decomposition rates after grassland abandonment and shrub encroachment.
9.4.4.3 Biochemical Quality and Microbial Activity
It is widely known that litter nutrient concentration and organic matter quality (e.g.
lignin content) are the main factors determining litter decomposition rates
(Cornwell et al. 2008). Grasslands typically have a high density of fine roots that are
poorly lignified and with high turnover rates, thus providing a relatively labile C
substrate for microbial activity. In contrast, the proliferation of shrubs may increase
the presence of lignified roots with lower turnover rates. After shrub encroachment,
low quality of litter inputs, with large amounts of secondary compounds such as
lignin or polyphenolic substances, may hinder decomposition and promote C
accumulation (Pérez-Harguindeguy et al. 2000; Shaw and Harte 2001; McCulley
et al. 2004; Liao and Boutton 2008).
The biochemical quality of litter may differ between species. In the Pyrenees, an
aboveground litter of grasses showed marked differences in the chemical composition from that of the two main invading shrubs of that area (Cytisus balansae and
Juniperus communis). The litter of both shrubs had higher concentrations of
recalcitrant compounds (e.g. lignin, lipids, suberin) and a low concentration of
either N (conifer) or P (legume) relative to grass litter (Montané et al. 2010).
Consequently, the higher organic C found in the upper mineral soil layer under
shrubs compared to the grassland was mainly attributed to the slower decomposition of shrub litter and the transfer of litter-derived C into the soil. However, the
presence of grass litter, with high N and P concentrations, may enhance microbial
activity and prime the decomposition of recalcitrant shrub litter. As a result, at least
in the short term after shrub proliferation when both shrub and grass litters coexist,
the shrub litter accumulation pattern is altered (Montané et al. 2013).
222
J. Garcia-Pausas et al.