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.