Illuminating isotopic offset between bulk soil water and xylem water under different soil water conditions

Abstract

The temporal and spatial heterogeneity of soil water content affects the isotopic composition of bulk soil water and xylem water. However, there is a lack of systematic assessment of the impact of soil water conditions on the widely reported bulk soil–xylem water isotopic offset. For this purpose, we measured δ2H and δ18O values of xylem water of the shrub plant Caragana korshinskii and bulk soil water, for two consecutive years at two sites with contrasting water conditions. At Site 1, with moister conditions, xylem water showed a clear isotopic deviation towards more negative values relative to the bulk soil water values in both years (< 20% overlap), with a mean of 10.7‰ for the bulk soil–xylem water line-conditioned offset (Δlc-excess) and −9.1‰ for the soil water line conditioned excess (SW-excess). However, xylem water was strongly matched with immobile water in the dry year. At Site 2, with drier conditions, an isotopic offset occurred in the wet year, with a 20% overlap between xylem water and bulk soil water. Mean Δlc-excess and SW-excess were 13.7‰ and −11.8‰, respectively. Interestingly, the overlap reached 97% in the dry year, with the xylem water falling within the isotope space of bulk soil water. At Site 2, soil water content was positively correlated with Δlc-excess and negatively correlated with SW-excess. These results suggest that the isotopic signatures of xylem water and immobile water tend to overlap, supporting the “two water worlds” hypothesis. The bulk soil–xylem water isotopic offset is likely to be closely associated with the heterogeneity of soil water isotopes driven by soil water content. Our results elucidate changes in the isotopic signals of xylem water and bulk soil water under different water conditions and help better understand how plants take up water in a heterogeneous soil.