Stable isotopes in the water within soil vegetation and atmosphere

Francesca Scandellari [Libera Università di Bolzano-Bozen];
Daniele Penna [Università di Firenze]

This work aims to present the state of the art regarding the distribution of stable isotopes of oxygen and hydrogen in the water flowing within the soil-vegetation-atmosphere continuum. A special attention will be devoted to the fruit crops, although less research has been done in this field relative to other ecosystems. The most up-to-date researches on climate change highl ight the risk of extended drought periods throughout the European continent, and especially in the Mediterranean region. For this reason, the European community has already activated a series of actions to improve the resilience of the European society to future water scarcity. Although much is known already about the water fluxes on Earth, much more has to be unveiled, especially regarding water permanence in the soil and its flux from the soil to the atmosphere through plants. This is because many effects of the complex interactions between water and its environment still need to be fully comprehended. Many techniques are available to researchers to study water fluxes in the environment. The analysis of stable isotopes of oxygen and hydrogen in water is one of them and is getting particularly popular because of the development of new, powerful techniques that allow continuous measurements directly in the field. We shortly introduce the topic reporting the background information regarding the stable isotopes, including the most important chemical-physical processes affecting the isotopic composition of water and the most common techniques to measure them. We explain how and why use stable isotopes in ecophysiology, agriculture and agroecological research. Stable isotopes are naturally occurring alternative forms of a specific element. Hydrogen has two stable isotopes, 1H and 2H, while oxygen has three, 16O, 17O and 18O, though only the former and the latter are commonly used for research. Because of their ubiquity and because they are natural constituents of water, they represent the perfect trace to follow its flow within ecosystems, even when it is not possible to directly measure the flux rate. We then dissect the agroecosystem presenting the main processes affecting the isotopic composition of water entering in, flowing through and exiting from the ecosystem. In particular, we discuss the dynamics of water in soil, by showing how the forces that push or pull water out of the soil and those that keep it in the system affect its isotopic composition. This is, probably, the environmental compartment that is known the least, not only because it is difficult to study, but also because of the high number of chemical, physical and biological factors that may have an effect on soil water isotopic composition. From the soil, the water is taken up by roots and transferred in huge amounts in the atmosphere. This flux does not only cause a huge withdrawal of water from the soil, pushing the farmers to artificially import it into the system by irrigation, but also creates air turbulences, which can deeply affect the local climatic conditions. For space reasons, we only report the most important fluxes of water. Many more aspects should have been explained, but this would have produced a book instead of a review paper. Nonetheless, with this work, we hope to stimulate the interest of the reader to know more about the distribution of stable isotopes in agroecosystems and the usefulness of using them as a tool to study water fluxes in these systems.

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