4.4 Effect of calculation methods on the SOC and TN storage
Our meta-analysis showed that calculation methods (the FD and ESM) have unavoidable impacts on SOC and TN Storage estimation (Fig. 2). After deforestation, the response values of SOC storage calculated by the FD and ESM were - 0.43 and - 0.66, and that of TN storage were - 0.29 and - 0.62, respectively. There was a significant difference between the two calculated methods (Fig. 2a, P < 0.01). In addition, the responses of SOC and TN storages calculated by the FD and ESM to land restoration were 0.23, 0.25, 0.16 and 0.23, respectively.
The results clearly indicated that the FD method overestimated SOC and TN storage due to the increased soil BD after deforestation. Instead, the FD underestimated the SOC and TN storage after land restoration, likely owing to the reduced soil BD (Fig. 2). Therefore, the FD method may produce greater variations and even lead to wrong judgement (Don et al., 2011; Du et al., 2017; Hu et al., 2016). The variation between the two methods may be mainly due to the changed BD in the collected soil profile (Fig. 2c). In recent years, SOC estimation bias from soil BD has received more attention (Lee et al., 2010; Schrempf et al., 2011; Toledo et al., 2013; Gulab et al., 2019). Therefore, it is critical important to select the accurately method (e.g., ESM) to estimate soil carbon and nitrogen storage in future study.