Climatic and biotic influences on isotopic differences among topsoil waters in typical alpine vegetation types

Abstract Hydrogen and oxygen isotopes have been widely used to investigate global hydrologic cycles. However, the most studies of soil water isotopes have overlooked the importance of O-horizon that may potentially influence the accurate evaluation of hydrologic processes, especially in the alpine ecosystem with thin soil layers. To address this knowledge gap, soils were sampled from the O-horizon and the 0–10 cm soil layer from 107 sites in the grasslands and woodlands of the Western Sichuan alpine ecosystem in order to evaluate the influence of climatic and biotic factors on the isotopic differences (expressed as the logarithmic of effect sizes, lnRR), which represented the difference of δ2H and δ18O between the O-horizon and the 0–10 cm soil layer. The isotopic differences between the O-horizon and the 0–10 cm soil layer were significantly different in the grasslands, but not in the woodlands. The influence of climatic factors on the lnRR was limited relative to the biotic factors, and the influence of climate contrasted with expectations based on an evaporation principle. Rather, aboveground biomass (AGB) was significantly correlated with the lnRR between and within the soil water from different vegetation types. Consequently, the observed differences were mainly related to the vegetation conditions that influence the microclimates within canopies. Therefore, the investigations of hydrological processes may inaccurately estimate their influences when not separately considering the especially high stable isotope values of the O-horizon in grasslands of alpine regions with thin soil layers. In particular, the influence of O-horizon should be considered in the areas where AGB is less than 100 t/hm2.