The University of Giessen Free-Air Carbon dioxide Enrichment study: Description of the experimental site and of a new enrichment system

The University of Giessen long-term Free Air Carbon dioxide Enrichment experiment (GiFACE) started in 1998 to study responses of a semi-natural grassland to elevated CO 2 . Permanent grasslands, managed as hay meadows, represent about 13% of the agricultural and approximately 6% of the total land area in Germany. The GiFACE grassland site has not been ploughed for at least 100 years. During the last decades, it has been mown twice a year and fertilized with 50-80 kg N ha -1 a -1 . Since 1995, the fertilizer has been reduced to 40 kg N ha -1 . The vegetation is an Arrhenatheretum elatioris (Br.-Bl.) Filipendula ulmaria sub-community with approximately 60 species. The Giessen Free Air Carbon dioxide Enrichment technique (PlumeX) was developed based on the classical FACE technique in order to adapt it to local site requirements. The main features of the PlumeX are: (1) A circular active plenum (CAP) which consists of a modular structure with individually controlled exhausting and sucking vent pipes; (2) the symmetric layout without any external blower providing wind-direction independent CO 2 enrichment control; (3) the regulation of the CO 2 target concentration with an electronic controller (PID), which leads to CO 2 concentrations of 22% above ambient in a height of 40 cm above ground; (4) low maintenance requirements and (5) a reduction of the CO 2 usage by approximately 5%. The CO 2 enrichment is applied throughout the year during daylight hours. Since the start of fumigation the system worked continuously with the exception of 5 days. The homogeneity of the CO 2 enrichment in 40 cm above ground in the central 16m 2 large ring area was for instance in the vegetation period 1999 (April to September) 122.1±2%. The GiFACE is currently the oldest and only running long-term FACE experiment on a semi-natural grassland ecosystem in Europe.