Tritium analysis shows apple trees may be transpiring water several decades old

Recent work has shown evidence of ecohydrological separation whereby plants appear to use a less mobile soil water pool that does not mix with more mobile soil water, groundwater and streamflow. While many elements of this two water worlds hypothesis remain to be tested and challenged, one key question is ‘how old might the less mobile water used by plants be’? Such a question is methodologically difficult to answer: stable isotope tracing makes it difficult to resolve any water age over than a few years since the signal gets so damped. Tritium—a useful radiogenic isotope and age dating tool, is now difficult to use in natural systems since most bomb tritium has washed out of soil profiles. Here we leverage new data from an unusually deep, homogenous soil profile that preserves the mid-1960s tritium bomb signal. We the sample Fuji apple trees (Malus pumila Mil) growing on this site that have root systems that penetrate over 15 m and utilize water from within the bomb peak soil water distribution (extracted via cryogenic extraction). Our data shows that water used by these trees is on average 29 years old. Bayesian mixing analysis suggests that 40 ± 30% of fruit tissue water came from depths between 4 and 9 m depth within the soil profile (36 ± 9 years old); 60 ± 29% was equally divided between 0-4 m and 9-15 m ranges (13 ± 5 years old). These findings suggest that trees can use quite old less mobile water, highlighting the separation in ages between more mobile soil water and water in transit in sap flow.