Rooting depth governs groundwater recharge in agroforestry systems: Implications for water sustainability under dryland afforestation

In dryland agroforestry systems, large-scale afforestation for ecological restoration often compromises groundwater sustainability, yet the roles of tree species traits (e.g., rooting depth) and soil-vegetation interactions remains unclear. We conducted a field study on the Loess Plateau, comparing 14 deep-rooted afforestation species (13 trees, 1 shrub) with adjacent shallow-rooted crops (maize, wheat, millet, etc.) across 32 paired sites. Using soil water profiling, root mapping, and tritium tracing, we found that deep rooting (4.8–28 m) reduced soil water storage by 0–2131 mm and groundwater recharge by at least 67.2 % compared to shallow-rooted vegetation. Rooting depth explained 72 % of recharge variation, outperforming climate and soil factors. Moreover, the adverse effect of rooting depth on recharge was strongly modulated by soil texture, as coarser-textured soils weaken this impact by enhancing deep percolation. These findings provide a scientific basis for selecting low-water-demand tree species (e.g., shallow-rooted Ziziphus jujuba) to maintain groundwater recharge in dryland agroforestry systems. Our results quantify the trade-off between afforestation goals and groundwater security, offering actionable insights for climate-smart land-use planning.