Nitrate leaching and N2-fixation in grasslands of different composition, age and management

Cut grassland systems usually have high N efficiency and consequently low nitrate leaching, whereas the introduction of grazing animals increases the loss potential dramatically (Jarvis, 2000). It has been demonstrated that nitrate leaching from unfertilised grass-clover swards is lower than from mineral-N fertilised swards grazed by cattle. The possible explanation for this is that the N2 fixation by pasture legumes is regulated by a natural feedback mechanism driven by soil inorganic N levels. The feedback mechanism acts as a limit to N inputs from legumes and consequently regulates the potential for N losses (Ledgard, 2001). In this paper we report 5 years (1997-2002) of nitrate leaching from four cropping sequences with different grassland frequency and management for both unfertilised grass-clover and fertilised monoculture grass. Furthermore, one year of N2 fixation in 1, 2 and 8-year-old grass-clover pastures is reported