Evaluating titanium isotope fractionation processes between soil, plants and waters

This study investigates the Ti isotopic behavior across major reservoirs of the critical zone, including waters and plants. We present Ti concentration and isotope data from bedrocks, soils, the forest floor (i.e. mix of litter with soil particles), groundwater, river water, soil solutions and plant tissues from the Strengbach Catchment Critical Zone Observatory in France, as well as from soils and respective plant roots and shoots from a greenhouse experiment. Despite the low solubility of Ti, we findthat some Ti is mobile in the critical zone and isotopes undergo fractionation during their transport through water and biotic systems. Our results show that groundwater, river water, and soil solutions have lighter Ti isotopic compositions (δ49Ti) compared to the average bedrock, with average Δ49TiSample-Bedrock values of -0.80 ‰, -0.58 ‰, and -0.71 ‰, respectively, indicating that waters define an isotopically light Ti reservoir in the critical zone. While the Ti isotopic composition of bottom soil (30–90 cm) is similar to that of the bedrock, topsoils (0–30 cm) are on average 0.07 ‰ lighter than the bedrock. In contrast, plant samples like greenhouse grown maize roots, tobacco roots and aboveground plant tissue, as well as field-collected spruce needles and beech leaves show between 0.09 ‰ and 0.22 ‰ heavier δ49Ti than the bedrock or soil, suggesting that vegetation and litter form an isotopically heavy Ti reservoir. This distribution of Ti isotopic compositions suggests that plants actively modify their environment by preferentially incorporating heavy Ti isotopes, thereby influencing the isotopic signatures of surrounding soils and waters. The here described isotope fractionation between soil, plants, and water highlights the potential of Ti isotopes for tracing interactions among these ecosystem reservoirs.