Effects of soil‐bound water exchange on the recovery of spike water by cryogenic water extraction

Rationale The effects that alter the isotopic composition of water added to soil samples that have been oven‐dried previously are not fully understood. Methods Oven‐dried clay‐rich soil was repeatedly rewetted with two waters of strongly differing isotopic composition. This approach allowed the determination of the amount and the isotopic composition of soil‐bound water that exchanges with spike water after rewetting. Results After oven drying, 1.8% and 1.4% water (gravimetric content) exchanged with the added spike water, for δ 2 H and δ 18 O values, respectively. The isotopic composition of this soil residual water was depleted by 89.7 mUr and 5.42 mUr relative to the water extracted from field‐fresh soil for H and O, respectively. The cryogenic extraction method was more efficient than oven drying in terms of water removal from soil. Conclusions Our results show that the isotopic difference between extractable (mobile) water and non‐extractable (soil‐bound) water explains the isotopic effects observed in spike water experiments. This difference, however, can also lead to a considerable isotopic offset between extractable and total soil water.