Isotopic fractionation induced by a surface effect influences the estimation of the hydrological process of topsoil

Isotopic heterogeneity in soil water has hindered the application of isotope compositions (δ18O and δ2H) in soil water dynamics. This heterogeneity has been suggested to be caused by soil properties such as organic matter (OM) and clay content. However, this is yet to be verified in field soil. We sampled the organic layer (O‐horizon soil) with highly decomposed organic material and the A‐horizon soil in western Sichuan, China, and equilibrated these samples with vapour created by unconfined labelling water. The relationship between soil properties and isotopic fractionation (εT/U) between unconfined water and the total soil water was used to determine the line‐conditioned excess (lc‐excess) and source rain of A‐horizon field soil by removing the influence of confined water. Equilibration experiments demonstrated a significant isotopic difference between the εT/U levels in the A‐horizon and O‐horizon soils, indicating that OM plays an important role in isotopic fractionation. In field samples, the lc‐excess of the unconfined A‐horizon water was, on an average, 2.5‰ higher than that of bulk soil water. The average offsets between the annual rain and the estimated source rain of soil water decreased by 5.0 and 0.5‰ for hydrogen and oxygen after removing the influence of confined water. Isotopic heterogeneity should not be ignored while examining the evaporation of soil water, soil source rain, and hence the recent ‘two water worlds’ hypothesis, which is especially true for cases in which the soils contain high levels of OM. Organic matter content in soil was a key factor determining the isotopic inhomogeneity of soil water. Excluding the surface effect induced by organic matter may improve the application of δ18O and δ2H in hydrological study on soil.