Mobilization of labelled N in soybean leaves during early podfill

Soybeans accumulate N in vegetative tissues up to pod initiation after which total vegetative N may remain constant during early phases of pod development. Eventually much of the vegetative N is mobilized to the pods. The mobilization of N from vegetative tissue to pods during the first few days of pod development is poorly understood but is important to an overall understanding of soybean N nutrition. The vegetative tissues of field grown soybeans were labelled with ¹⁵N and sampled weekly during the reproductive phase of plant growth. Three foliar applications of (¹⁵NH₄)₂SO₄ were made prior to pod initiation at a combined rate of 3.3 kg N/ha. To immobilize soil N and to increase soybean dependance on N₂‐fixation, sawdust was applied at a rate of 52 t ha^‐1 . Irrigation was required almost weekly because of a shallow soil profile and below normal summer precipitation. Mobilization of vegetative N began immediately upon pod initiation and continued at a linear rate through pod development. It appeared that N₂‐fixation was able to provide approximately half of the N in pods during early podfill. Nitrogen content of vegetative tissue declined as soon as pods began developing.