Background and aims <p>The combined structures of soil and rock directly influence root distribution, and root zone water movement and storage. However, it remains unclear how these processes jointly affect plant water-use strategies. We aimed to examined how subsurface structure-induced root architecture govern water acquisition strategies and their implications for species coexistence.</p> Methods <p>We conducted combined root traits and stable isotopes measurements on four dominant species across slope positions in a subtropical soil-limited environment. Electrical resistivity tomography was employed to investigate the soil-rock structures at the study plots. The vertical and temporal origin (water age) of root water uptake were estimated to determine water acquisition strategies of co-existing species.</p> Results <p>Slope positions had less effect on root architecture, but significantly influenced average soil thickness and water storage capacity. Diverse soil-rock combinations created multiple root niche spaces (soil layers, soil-rock interfaces, and bedrock fissures). Although plant rooting depths were generally shallow (&lt;40 cm), these species achieve coexistence through differential occupation of these three niche spaces, enabling access to water sources of varying ages (reflected by seasonal fluctuations in water age of root water uptake, 2-63d). Shrubs develop more extensive lateral root systems to enhance water uptake capacity and anchorage, supporting larger plant structures than herbaceous species.</p> Conclusion <p>Our study reveals that in soil-limited environments, plants share water sources within the soil layer but differentially utilize water resources available at the soil-rock interfaces and within fissures. These results demonstrate that bedrock-controlled substrate heterogeneity promotes species coexistence via spatiotemporal ecohydrological niche differentiation.</p>