Does isotopic fractionation occur during root water uptake?. Reporting a global divergence in the isotopic composition of plant water and its sources

Abstract

The analyses of water isotopic composition serve to investigate plant water sources under the assumption that root water uptake does not entail isotopic fractionation, i.e. the isotopic composition of the plant water reflects that of the root-accessed sources. However, a growing number of studies challenge this assumption by reporting plant-source offsets in water isotopic composition, for a wide range of ecosystems. We conducted a meta-analysis to quantify the magnitude of this plant-source offset in water isotopic composition world-wide and its potential explanatory factors. We compiled 77 studies reporting dual water isotopic composition (δ2H and δ18O) and extracted plant and source (soil) δ2H and δ18O for 141 species. To calculate the offset, first, we fit a soil water isotopic evaporation line (δ2H vs. δ18O) for each study and sampling campaign. Then, we calculated our offset with respect to this line (SW-excess) as the difference between the observed and predicted δ2H plant values. Effects of climate and plant functional traits on SW-excess were assessed using linear mixed models. Overall SW-excess was significantly negative: plant water was systematically more depleted in the heavier water isotopes than soil water, for δ2H. The sign and magnitude of the SW-excess differed among plant functional types: SW-excess was. more negative in angiosperms, deciduous and broadleaved species. The SW-excess increased with mean annual precipitation. Additionally, ~90% of cases where SW-excess was negative, the estimated offset with respect to alternative water sources (precipitation and groundwater) was also negative. Thus, we conclude that this overall significant soil-plant offset in water isotopic composition cannot be attributed to alternative water sources. A consistent negative offset between plant and potential water sources could introduce biases when estimating water sources accessed by the vegetation, particularly in broadleaved forests in temperate and humid regions. So, isotopic analyses to estimate water use should be revisited