Nitrogen Nutrition of Field‐grown Soybean Plants: I. Seasonal Variations in Soil Nitrogen and Nitrogen Composition of Stem Exudate1

Abstract

AbstractThis study was undertaken to document the quantities of N compounds present in soil and in stem exudate from soybeans throughout most of the life cycle of the plant. The results provide new information on the relative importance of various N compounds involved in translocation and the relation of these compounds to soluble N of the soil.Soybean plants (Glycine max L. Merr., ‘Harosoy 63’) were grown under field conditions in Wooster silt loam with four replications. Nodules formed on the roots early in the season, and plants were well nodulated throughout the growth period. Plots were sampled at 13 times during the growing season for the determination of soil moisture, nitrate and ammonium content of the soil, and nitrate, nitrite, and amino acid content of stem exudate.Exudate flow rate appeared to be related to soil moisture content early in the season but the relationship was not maintained after the flowering stage. Nitrite was not detected in stem exudate. Nitrate content of the soil and the exudate was highest during the growing stages preceding flowering. During growth stages following flowering the amino acid concentration in the exudate was many times the nitrate concentration, which did not vary with fluctuations in nitrate content of the soil. Asparagine was the predominant amino acid in the stem exudate at all harvest dates; asparagine N as per cent of total amino acid‐N was 61% over the entire experiment. The grand mean for amino acid‐N concentration in exudate was double the grand mean for nitrate‐N concentration.