Quantitative tracing of precipitation in the critical zone: The Spike II experiment

Several ecohydrological problems such as when and where precipitation becomes the source of plant uptake are usually tackled through stable isotope measurements. Our ability to go after these questions is often limited by field conditions that cannot be controlled, but targeted manipulation experiments can go beyond some of these limitations by imposing known boundary conditions and allowing the experimental closure of the isotope balance. This contribution presents the Spike II experiment, which was carried out on a large vegetated lysimeter within the EPFL campus (CH) in 2018, with the goal of tracing one precipitation event through the entire water balance. We applied 40 mm of isotopically-enriched water on top of the lysimeter and tracked it for 40 days through the soil water, the lysimeter bottom drainage and the plant xylem before destructively sampling the system to quantify the remaining isotope mass. A total of more than 900 water samples were collected to reconstruct the "story" of the labeled precipitation. Results include tracer breakthrough curves in xylem water and soil water at different depths, and offer insights on tree status variations throughout the experiment.