Management effects on nitrogen fixation and water use of lucerne under dry site conditions

Summary

Biological nitrogen fixation is the main source of nitrogen in organic farming systems. There is little known about the impacts of cultivation techniques in pannonic regions on the capacity of biological nitrogen fixation (BNF) and the water use efficiency (ETC: evapotranspiration coefficient) of lucerne. From 1999 to 2001, pure lucerne crops and lucerne-grass mixtures were investigated with respect to the effect of the utilization system (harvested = forage production; mulched = green manure; pure lucerne crops versus lucerne-grass mixtures) on yield, BNF, soil N balance and water use efficiency. The amount of total fixed N was 124-150 kg N ha-1 in 2000 and 178-197 kg N ha-1 in 2001. The proportion of N derived from the atmosphere (% Ndfa) ranged from 26 to 79%. When the proportion of grass yield in lucerne-grass mixtures was high enough (exceeding 20%), mixtures showed a higher water use efficiency than pure crops by using site resources more efficiently than the pure lucerne crops. The release of nitrogen from the legume mulch was inhibited due to unfavourable conditions for mineralization in both years. Therefore green manure legumes did not decrease nitrogen fixation and the proportion of nitrogen derived from the atmosphere. Nevertheless, mulching the legumes reduced the water consumption of the crops compared to the evapotranspiration of the forage legumes.