Spatial–temporal dynamics and recovery mechanisms of dried soil layers under Robinia pseudoacacia forest based on in-situ field data from 2017 to 2020

Planting Robinia pseudoacacia in water-limited regions can promote soil and water conservation and ecological service functions. However, it can cause the formation of below ground dried soil layers (DSLs), causing land degradation and tree mortality. To ascertain the spatial–temporal dynamics and recovery processes of DSLs, we monitored the deep soil water content (SWC) to a depth of 500 cm at 27 sites on a typical R. pseudoacacia forest from 2017 to 2020 and calculated the evaluation indices of DSLs based on plant-available SWC (PASWC) and stable field capacity (SFC). Compared with PASWC-identified DSLs, the degree of SFC-identified DSLs was more severe, although the spatial–temporal characteristics were similar. Severe soil desiccation was identified as 79% of the 500 cm profile dried out below 101 cm. The mean thickness of DSL and mean SWC within a DSL were 396 cm and 9.0%, respectively, and the quantitative index reached Grade III (severe) DSLs. All DSL indices demonstrated weak or moderate variability in space and strong variability in time. The heterogeneity of rainfall events and microtopographic positions explain the spatial–temporal variation of DSL indices well. A heavy rainfall event (>50 mm) triggered the disappearance of DSLs at some sites, indicating the possibility of DSLs recovery based on field observations under natural condition. Microtopography (such as the swales) provides advantageous conditions to the recovery of DSLs by redistributing rainfall flow at the local scale. These results provide insights into DSLs recovery, which is useful for land development and rehabilitation in planted forests.