Plants’ influence on the export of dissolved matter from rivers

Plants play a major role in the water cycle of our planet due to evapotranspiration, through which water is taken up by roots and returned to the atmosphere. However, root uptake does not only concern water: during this uptake, nutrients essential for plant growth are also taken from the soil water (nitrates, potassium, phosphorus, magnesium, silicon…); while other elements, sometimes toxic for plant physiology (sodium, aluminum), are excluded. All these effects modify the chemistry of water as it passes through the root zone, which influences the chemistry of groundwater and rivers. Our hypotheses are that the intensity of root uptake controls the export of dissolved matter to rivers via (i) element cycling and (ii) the formation or dissolution of secondary phases.

In order to test these hypotheses, our approach combines measurements of different geochemical tracers, including some isotopic ones on different compartments (rocks, soils, plants, rivers) of the Critical Zone on small instrumented watersheds; quantification of some key parameters of ecosystem functioning (biomass, evapotranspiration), as well as their temporal variations using LiDAR and sap flow sensors; and modeling of chemical reactions (including those related to the cycling of nutrients through ecosystems) in the Critical Zone using reactive transport codes such as CrunchFlow. This question and methods are applied to mountainous Mediterranean catchments located in Mont Lozère, located Mediterranean Hydrometeorological Observatory Cévennes-Vivarais (OHMCV), which is part of the French Network of Critical Zone Observatories (OZCAR).