Mineralization of lupine seed meal and seedlings used as N fertilizer in organic vegetable production

Summary

Seeds of grain legumes are currently discussed as N fertilizer in organic vegetable production. They can be produced by the farmers themselves and applied in well controlled amounts flexibly in time and space. Most research investigating the N mineralization of grain legume seeds was carried out using coarsely milled seeds. We hypothesized that seed germination alters the chemical composition leading to a higher N release compared to seed meal. In a pot experiment the C:N ratio of lupine seeds was shown to decrease noticeably within the first two weeks after sowing. After an incubation period of 1300°Cd net N mineralization was significantly higher for the lupine seedlings compared to the seed meal and close relationships between N mineralization and C:N ratio were found. In field experiments with white cabbage, carried out in 2005 and 2006, similar relationships were found but sowing followed by an early incorporation of seedlings after 12 and 13 days showed an N supply similar to the seed meal treatment only. Strong priming effects, mainly caused by the lupine seed meal, are discussed to be a possible reason. Late incorporation after 42 and 37 days resulted in significantly lower N supply and cabbage yield.