4.2. Response ratio of SOC and TN storage due to land restoration
Our meta-analysis demonstrated that land restoration could increase overall SOC and TN storages by 27.9% and 25.7% respectively (based on ESM calculation, Fig. 2). It should be noted that the SOC and TN gains have large variations within specific LUCCs and sampling depths (Fig. 3, Fig. 5, and Table 3). Our data and other studies have confirmed that the lost SOC and TN could be reversed by adopting land restoration strategy (especially conversion from farmland to grassland or forest) (Hu et al., 2018; Deng et al. 2016). The underlying reasons may be attributed to the increased quantity and quality of litter inputs (Deng and Shangguan, 2017) and the maintained soil moisture after vegetation coverage and land restoration, thereby reducing the decomposition of SOM (Jerome et al., 2010).
In our study, the SOC and TN storages did not significantly increase upon restoring from the cropland to other types, such as grassland, fruit trees and plantations (Fig. 3). This case may be associated with the depleted SOM from these land-use types previously subjected to intense human disturbance (e.g., mowing, fertilization and tillage, etc.) are unable to recover in short term (Post and Kwon, 2000). In addition, the positive effect of land restoration on SOC and TN gains showed spatial heterogeneity among different LUCCs (Fig. 3, Fig. 5). When the cropland returned to the natural forest and shrub, higher SOC and TN accumulated, which may be due to higher species diversity and litter detritus (Wei et al., 2009; Hu et al. 2018). A study in the Loess Plateau of northwest China by Jin et al. (2014) found that natural vegetation restoration benefited for soil surface C sequestration than tree plantation. On the one hand, lower fertilizer input may result in less carbon sequestration after converting from farmland to plantation (Jiang, 2006). Secondly, the planted trees (e.g., eucalyptus) are harvested regularly (once every five years), which probably limit C input in the soils.
Our data have implications that to protect and improve the fragile ecological environment in the Karst area of southwest China, we should urgently implement ecosystem restoration project. Further, we should fully consider the variations of SOM stabilization mechanisms under different land use patterns in future study.