Soil water uptake by trees using water stable isotopes (δ2H and δ18O)−a method test regarding soil moisture, texture and carbonate

Abstract

Aims Stable isotopes of oxygen and hydrogen are often used to determine plant water uptake depths. We investigated whether and to what extend soil moisture, clay content, and soil calcium carbonate influences the water isotopic composition. Methods In the laboratory, dried soil samples varying in clay content were rewetted with different amounts of water of known isotopic composition. Further, we removed soil carbonate from a subset of samples prior to rewetting. Water was extracted from samples via cryogenic vacuum extraction and analysed by mass spectrometry. Results The isotopic composition of extracted soil water was similarly depleted in both 18O and 2H with decreasing soil moisture and increasing clay and carbonate content. Soil carbonate changed the δ18O composition while δ2H was not affected. Conclusions Our results indicate that soil carbonate can cause artifacts for 18O isotopic composition of soil water. At low soil moisture and high carbonate content this could lead to conflicting results for δ18O and δ2H in plant water uptake studies.