Biogeochemical, in particular hydrological constraints affect biotic processes on the distribution, structure, and function of ecosystems, and on the cycles of water and elements (C,N,P,S). The session particularly addresses arid and semi-arid environments, where knowledge is still widely lacking. The session will provide information on (a) background and synthesis of new research approaches, and (b) recent results in the study of biogeochemical, ecological, and hydrological processes. Contributions are welcome on studies in arid and semi-arid ecosystems concerning (a) soil-vegetation-atmosphere interactions and (b) biogeochemical and ecohydrological controls on water and nutrient availability of soils and plants and (c ) on the impact of land use change or land degradation on these processes.