Deep soil biota drive trade-offs between above and belowground functioning during dryland restoration

Afforestation can enhance carbon sequestration in global drylands but may impair ecosystem functioning via deep-soil water depletion. However, it remains unclear how afforestation-driven turnover in soil biota influences aboveground vegetation status and soil multifunctionality, particularly in deep soil. Here, we conducted a ~500-km transect survey across four precipitation regions on the Loess Plateau, China, comparing 25-year-old plantations with adjacent croplands. We characterized soil biota (bacteria, fungi, protists, and invertebrates) using amplicon sequencing and quantified soil multifunctionality in topsoil (0–20 cm) and deep soil (160–200 cm). We found that afforestation was linked to stronger effects in deep versus topsoil, and the magnitude of these effects varied across the precipitation gradient. Afforestation consistently reduced deep-soil water and multifunctionality, whereas topsoil responses became increasingly negative at the drier range of the precipitation gradient. Soil biotic change was driven primarily by community turnover rather than diversity, and turnover responses across all biotic groups weakened with reduced precipitation. Turnover patterns further supported a trade-off between aboveground greening and belowground functioning. Soil biota that established after afforestation were positively associated with canopy greenness but negatively associated with soil multifunctionality, whereas those that disappeared showed the opposite linkages. Biota that persisted before and after afforestation were positively associated with both canopy greenness and multifunctionality. Overall, our results show that gains in aboveground greenness can mask persistent deep-soil functional losses in dryland afforestation, emphasizing that restoration success should be evaluated with explicit deep-soil indicators.