Narrow-leaved lupine as an N source alternative to grass-clover swards in organic vegetable rotations

Legumes represent an important N source in organic vegetable rotations. Since the amount of N-2 fixed as well as N mineralisation from legume biomass are highly variable, N availability does often not match the requirements of following crops. The production of grain legume seeds followed by their temporary storage and application as N fertilizer may represent a flexible and controllable alternative to traditional legume green manures. In two field experiments, narrow-leaved lupine was investigated as N source for subsequent white cabbage and beetroot and was compared to two grass-clover treatments, in which cut herbage was either removed from the sward or remained as mulch. Symbiotic N-2 fixation was highest for cut grass-clover, averaging about 350kgha(-1). Mulching reduced N-2 fixation by on average 57%. Lupine N-2 fixation was largely comparable to that of mulched grass-clover swards. Net N mineralisation from grass-clover residues within the year of their incorporation was positively related to the percentage of clover (R-2=0.93). Application of coarsely milled lupine seeds on plots previously cropped with lupines resulted in an N supply for cabbage being higher or equal to that after mulched grass-clover. Residual effects on the N supply for beetroot, grown in the third rotational year following white cabbage, averaged -2 and 28kgNha(-1) for lupine and grass-clover, respectively. In conclusion, lupines may provide a highly flexible N source alternative to frequently mulched grass-clover swards. Further attention should be directed towards the utilization of cut forage legume herbage as mobile N source.