4.2 Sequestration of soil organic carbon by vegetation
Sequestration of soil C implies the transfer of atmospheric
CO2 into the soil of a land unit through plants grown on
that land (Lal et al. 2015). Although the effects of afforestation on
soil C storage in arid and semi-arid areas have been examined in the
past, the results continue to be uncertain: some found that
afforestation of grasslands had no effect on SOC from 5 to 30 years
after planting (Davis and Condron 2002,Perez-Quezada et al. 2010,
Cunningham et al. 2012) whereas Paul et al. (2002) found that following
such afforestation, soil C decreased during the first 5 years but
recovered later so that after about 30 years, the difference was either
minimal or that soil C was slightly more and continued to increase
gradually. Hu et al. (2008) suggested that the loss of C in the mineral
component of soil was partly compensated for by the increased stocks of
C in roots. Jackson et al. (2002) found a clear negative relationship
between precipitation and changes in soil organic matter in grasslands
that had been invaded by woody vegetation, with drier sites gaining SOC
and wetter sites losing it. In contrast with these studies, our results
suggest that planting trees in grasslands could increase SOC
significantly, a finding supported by Li et al. (2013), who maintain
that the accumulation of C in soil may be due to annual inputs through
net primary production that exceeds the amount lost by decomposition.
However, these effects were found to vary with the region, because they
are influenced by soil texture, tree species, age of the stand, and
management practices. Therefore, more empirical data are needed to
identify the point in time after which soil C begins to be sequestered
as a result of afforesting a desertified grassland, such as the one
examined in the present study.
Storage of soil C decreased with depth, an observation consistent with
that made by Grandy and Robertson (2007), the storage being maximum in
the 10–20 cm layer. The concentration of soil C across all depths
(except the 90–100 cm layer) differed significantly between the three
treatments and the control. Among the three treatments, the
concentration in the 0–40 cm layer was significantly higher in FG than
in UG and MG. Total C in the uppermost layer (0–5 cm) was found to
increase substantially after tree planting, but that in the 0–30 cm
layer remained the same (Cunningham et al. 2012). Noble et al. (1999)
also found higher SOC in the 0–5 cm layer of a plantation than that of
a pasture, but not at greater depths (5–50 cm). Similarly, at most of
the 28 sites studied by Davis and Condron (2002), afforestation showed
no pronounced influence on soil C at depths greater than 10 cm.
The total estimated pool of SOC up to a depth of 100 cm was 23.63–79.98
t/ha, which amounted to 80.12% of the total pool in the ecosystem,
showing that terrestrial C is sequestered mainly in soil. This finding
is consistent with the findings of Fang et al. (2001) and Li et al.
(2007). Dixon et al. (1994) reported that more than a third of the C
pool in terrestrial ecosystems is contributed by soil organic matter.
Detwiler (1986) reported that 35%–80% of the C stored up to a depth
of 100 cm is concentrated in the 0–40 cm layer in tropical and
subtropical soils; we obtained similar results, with 41.81%–60.13%
stored in the 0–30 cm layer and 59.42%–80.80% in the 0–50 cm layer.
The accumulation of SOC is controlled by the rates of biomass formation
and decomposition (Lal et al. 1995). The turnover of fine roots is
another major component of the dynamics of SOC; the size of this
turnover matches that of leaf litter (Rasse et al. 2005). In the present
study, the lowest amount of soil C was in the control, probably because
of much lower input of litter, greater abundance of woody and herbaceous
vegetation, different patterns of distribution of roots at different
depths, and the differences in the availability of soil moisture and
soil temperature, all of which accelerated the decomposition of organic
matter (Covington 1981, Jackson et al. 2000). The storage of soil C was
significantly greater in MG than in FG and UG, an observation consistent
with earlier observations on the enrichment of soil organic matter as a
result of greater vegetation cover ensured by appropriate intervention
(Fearnehough et al. 1998, Wezel et al. 2000).