Forest restoration in tropical forests recovers topsoil water retention but does not improve deep soil layers

Tropical primary forests have been rapidly reduced in recent decade owing to slash-and-burn, leading to the formation of tropical secondary forests in different recovery stages. However, it is still unclear whether the soil water retention capacity in secondary forests can recover to the level of soil water retention in old-growth forest. In this study, three recovery stages of tropical secondary forests (i.e. early recovery stage, middle recovery stage, late recovery stage), and old-growth forest were selected for comparison in tropical forests on Hainan Island. By using spatiotemporal substitution method, we investigated the variation of soil water retention in three recovery stages and old-growth forest, and revealed its dominant controlling factors. The results showed that 0-60 cm soil water retention was improved as recovery stage progresses. Specifically, the topsoil (0-10 cm) soil water retention in later stage almost could recover the level of old-growth forest, whereas the deep soil (10-60 cm) water retention may not recover the level of old-growth forest as recovery stage progresses. Additionally, the soil structure and soil nutrients also improve while the soil aggregates stability reduced as recovery stage progresses. Among these properties, total porosity was found to be the most important factor controlling soil water retention, accounting for 27.44%, followed by bulk density (19.62%) and capillary porosity (16.83%), but soil particle size composition had a weakly effect on soil water retention. Overall, our results suggested that forest restoration is effective measures improve topsoil water retention capacity, but the deep soil water retention capacity may need more years to recovery. These findings have implications for the management and retention of primary forests and the restoration of secondary forests.