Plants Uptake Substantial Water From the Surface Organic Layer in Highly Permeable Karst Critical Zones

Abstract Plants growing in highly permeable karst critical zones are exposed to shallow soils and rapid hydrological turnover. The surface organic layer is recognized as a key ecohydrological interface in these environments, yet its direct contribution to plant water uptake has rarely been quantified. Using δ 2 H and δ 18 O stable isotopes, this study quantified the relative contributions of soil water, surface organic layer water, and shallow epikarst water to plant water use in a karst critical zone. The results reveal that shallow soil water dominated plant uptake (43%–67%). Notably, the thin surface organic layer contributed more than 10%, with the highest proportion observed in herbs (15%), followed by shrubs (11%) and trees (10%). In contrast, epikarst water, traditionally considered an important reservoir, contributed <10%. Scenario analysis further revealed that excluding surface organic layer water as a distinct source systematically increased estimated shallow‐soil water contributions by approximately 10%–15% in herbaceous species, highlighting the risk of misattribution in isotope‐based studies. This indicates that even a thin surface organic layer can represent a functionally relevant water source in highly permeable karst systems. Our study provides the first systematic quantitative assessment of surface organic layer water uptake across multiple plant functional types in a highly permeable critical zone. These findings underscore the ecological significance of surface organic layer‐mediated water pathways. Explicitly incorporating surface organic layers into ecohydrological models and Earth system simulations can improve predictions of vegetation‐water interactions under changing climate and land cover.