Influence of plant water relation on ecosystem function under water stress In this thesis we study the interaction between biodiversity and near surface groundwater during water stress by first characterizing the study area in terms of groundwater and soil water geochemical processes and hydrogeology in a large scale biodiversity experiment. We also investigated the spatial and seasonal variation of groundwater chemical composition and the effect of the river Saale on the groundwater of the main experimental field before relating to diversity. Interpretation of the groundwater hydrochemical data using statistical technique showed that groundwater geological matrix interaction and redox and redox sensitive elements are the dominant factors that resulted in variation of the chemical data set. Calcite dolomite quartz and siderite are found to be reactive minerals and are responsible for changes in chemical composition. Analysis of the spatial variation of the groundwater chemical composition of the experimental field showed that sampled plots BIIIA02 BIIIA13 have relatively different chemical composition from other plots. The comparison of chemical composition of the groundwater of our study area with the groundwater hosted in various hydrogeologic formation shows that the groundwater in most of the plots of the study area have similar hydrochemical signature with the Upper Buntsandstein. Water isotope of the groundwater results also shows that most of the ground waters in wells of plots BIIIA13 and BIIIA02 are more enriched than that in other plots. PHREEQC results showed that river Saale at mean flow did not have significant effect on the groundwater chemistry. Chloride concentration measured at the inlet of the channel and in groundwater shows that the channel did not leak and therefore has no effect on the groundwater. The upward chloride transport during water stress could not show a clear correlation with the aboveground diversity However we could see a positive correlation of leaf area index and chloride accumulation.