Effects of restoration age on water conservation function and soil fertility quality of restored woodlands in phosphate mined-out areas

The age of vegetation restoration has significant influences on near-surface hydrological function and soil quality in mining areas. This study analyzed the effects of restoration age and tree species on water conservation function (reflected by the water-holding characteristics of litter and soil) and soil fertility quality [reflected by soil bulk density (BD), soil organic carbon (SOC), total N (TN), available N (AN), available P (AP), and available K (AK)] of restored woodlands in phosphate mined-out areas. A primary forest (the control) and six woodlands restored for 4–25 years were selected as test sites. The results showed that total litter storage capacity, litter water-holding capacity, soil effective water-holding capacity, SOC, TN, AN, AP, and the comprehensive soil fertility index (SFI) increased with the restoration age, while small changes were also observed in soil total water-holding capacity, BD, and AK with restoration age. Compared to the control, the litter-modified interception amount, soil total water-holding capacity, and SFI in woodlands restored for 25 years were restored on average by 78%, 77%, and 92%, respectively. Furthermore, the litter water-holding capacities, soil capillary water-holding capacity, and AN in the Eucalyptus robusta woodlands restored for 10 years were significantly lower than those in woodlands of Alnus nepalensis restored for < 10 years. Our findings highlight that the restoration age has positive impacts on improving the water conservation function and soil fertility quality of restored woodlands, and the influence of tree species on the water conservation function should be specifically considered while carrying out vegetation restoration in phosphate mined-out areas.