The influences of climatic and biotic parameters on the isotopic offset among topsoil waters in typical vegetation types in alpine

Hydrogen and oxygen stable isotope compositions in soil waters have been widely used to investigate hydrologic cycles, particularly for understanding plant water usage. However, most studies of soil water isotopes have traditionally ignored the importance of O-horizon that may potentially influence the accurate evaluation of hydrologic processes, especially in alpine regions where O-horizon are thick due to low temperatures. Therefore, we investigated the isotopic differences (via mean effect size, lnRR) of waters from O-horizon and 0–10 cm soil layer in grasslands and woodlands of Western Sichuan alpine regions and evaluated the influences of climatic and biotic factors on observed differences. The results indicated that the δ²H and δ¹⁸O of O-horizon water were significantly higher than those of the 0–10 cm soil layer in grasslands, but these differences were not significant in woodlands. The influence of climatic factors on lnRR was limited relative to biotic factors, and the influence of climate contrasted with expectations based on evaporation principles. Rather, above ground biomass (AGB) was the most important factor associated with lnRR and it was significantly correlated with lnRR between and within soil waters from two vegetation types. Consequently, the observed differences were mainly related to vegetation conditions that influence microclimates within canopies. Therefore, investigations of hydrological processes may inaccurately estimate their influences when not separately considering the high stable isotopes values of O-horizon in grasslands of alpine regions with thin soil layers. In particular, the influence of O-horizon should especially be considered when AGB was lower than 100 t/hm² not only in grassland but also in other vegetation types.