4. DISCUSSION
4.1. Response ratio of SOC and TN storage due
to deforestation
The soil organic matter (SOM), carbon and nutrients are
deemed
as key attributes to soil fertility, providing ecosystem services,
environmental quality
and
sustainable land use (Kucharik et al., 2001; Lal, 2016). The increased
SOC storage is also viewed to be a potential source to offset the
enhanced global atmospheric CO2 concentration (Smith,
2016). In this study, significant losses of SOC and TN storage happened
after deforestation (Fig. 2, Fig. 3, Table 3, Fig. 4), which has become
an urgent issue to improve the fragile ecological environment in Karst
areas (Hu et al., 2018). Similar results were found in the Loess
Plateau, another ecologically fragile region in the northwest China
(Zhang et al., 2014). The decreased SOC and TN are probably due to
deforestation that destroyed soil structure (i.e., aggregation) leading
to releasing the previously protected organic matter (Jiang et al.,
2005; Xiao et al., 2009) and reduced terrestrial litter inputs (Xie et
al., 2004; Luo et al., 2009). On the other hand, the changed vegetation
types may influence soil moisture and temperature conditions and thus
accelerate SOM decomposition
(Luo
et al., 2009; Fei et al., 2015). Besides, because of the fragile land
scape and soil properties in the Karst areas, deforestation may
greatly
aggravate soil erosion, resulting in massive loss of soil nutrients
(e.g., C, N). When native forests are converted to plantations, they are
also affected by tree species and human management (Zhu et al., 2004).
Overall, our meta-analysis revealed the greatly reduced SOC and TN
storages due to deforestation. This
depleted
SOM may
hamper
soil quality and subsequent serious ecological and environmental
problems, posing big challenges
under global climate context. We should propose some effective
ecological measures to restore SOM in this fragile Karst areas.