Hydrological constraints affect structures and functions of ecosystems - particularly in arid and semi-arid environments. This session particularly addresses biogeochemical processes and element cycling in arid and semi-arid environments, where knowledge is still widely lacking. The session will focus on two topics: 1) Long-term data sets and 2) the impact of land use change on ecosystem function in this environment. Contributions are also welcome concerning (a) soil-vegetation-atmosphere interactions and (b) biogeochemical and ecohydrological controls on water and nutrient availability of soils and plants and (c) adaptation mechanisms at different scales.