EFFECTS OF SOIL NITRATE LEVEL ON NITROGEN DISTRIBUTION AND REMOBILIZATION IN FIELD-GROWN SOYBEANS (Glycine max (L.) Merr.)
Soil tests, nodule weights, acetylene reduction rates and nitrate reductase measurements indicated that considerable differences in mode of N-nutrition of soybeans (Glycine max (L.) Merr.) were achieved by treating field plots with ground-up corn cobs or with high levels of NH4NO3. This had no effect on the final N content or dry weight of the seed or whole plant. Treatment differences at 78–85 days in N content, apparent mobilization (transfer) from non-seed parts of the plant, N content and mobilization of N in lower leaves suggest that the route to this common end product differed between the two soil treatments. Movement of N out of the leaves was estimated by weekly sampling of individual leaves at all nodes on the plant. Lower leaves began transferring N and dry matter to other parts of the plant before pods had begun to form. The leaves provided the same amounts of N (287–293 mg/plant) in both soil treatments, but pods, stems and roots in the +cobs treatment provided 234 mg N but only 166 mg N in the +NO3 treatment.Key words: Soybean, nitrate, N2-fixation, re-mobilization, nitrate reductase