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.