The main source of nitrogen in organic farming is biological nitrogen fixation, the result of a symbiosis between legumes and nodulating bacteria. Lucerne (Medicago sativa L.) is the most biomass efficient forage legume under the semiarid conditions in East-ern Austria. Farmers inquired about information on the best site-adapted lucerne variety, but knowledge about water use and productivity of different lucerne varieties in organic farming is sparse. From 2005-2006, four lucerne varieties were studied with respect to above- and below-ground biomass production, biological nitrogen fixation and water use. The aim of this study was to find first practical criteria for farmers to choose lucerne varieties for green manure use, adapted to the dry region in the Marchfeld region. The years 2005 and 2006 were characterised by sufficient soil water availability in spring and summer. The shoot biomass production ranged from 4.6 to 5.8 t DM ha-1 per year, the below-ground biomass yield varied from 9.8 to 12.0 t DM ha-1 at the second harvest. Biological nitrogen fixation of lucerne amounted to 193-281 kg N ha-1. Generally, the tested lucerne varieties did not differ in their performance during both vegetation periods when only little drought occurred. Additional research is needed, for example comparison of the varieties under conditions of more severe drought, e.g. by induced water stress in the greenhouse
