The influence of the wetland root zone on nitrification and denitrification processes is studied by lysimeter experiments. Microbiological investigations on soil samples and soil water analysis will be performed in order to develop a numerical process model. The paper introduces the technical set-up for online chemical analysis of soil waters from various lysimeter sampling points, developed during the last years. The computer- controlled set-up consists of a hydraulic interface, a basis unit and a sample collector. A peristaltic pump, a sensor set and some valves build up the basis unit. This unit controls the fluid movement inside the whole set-up. A combined pressure and electric conductivity control enables automated sampling of soil waters from the unsaturated zone of the lysimeter. The hydraulic interface allows an addressing of fluids (1) from a sampling point, calibration fluids and cleaning fluids to the basis unit (and back), (2) from the basis unit to different commercial analysis devices, (3) to the sample collector and (4) to a waste vessel, respectively. The setup is completely air-proof to minimize the influence of oxygen. Components containing gas permeable tubes are flushed by inert gas (e.g. N2).