Seasonal Nitrogen Accumulation and Fixation by Soybeans Grown at Different Densities1

Intra‐row spacing is expected to affect N accumulation and fixation because of the gross morphological changes that occur for plants grown at different densities. Enhancement of N fixation may lead to increases in soil N for nonleguminous crops and increase efficiency of soybean [Glydne max (L.) Merr.] production. Nodulating and non‐nodulating isolines of Lee cultivar soybeans were grown in a field experiment on Ships clay (very fine, mixed thermic, Udertic Haplustolls) at densities of 48,500, 97,000 and 194,000 plants ha−1 to determine the effect of plant density on seasonal patterns of N fixation and N distribution within plant tops. The distance between rows was 70 cm. The tops of the nodulating isoline plants were separated into leaflets, stems plus petioles, and pods. The rate of acetylene reduction and nodule mass was determined on the roots. Root density determinations were made using mini‐rhizotrons. The rate of dry matter accumulation per hectare and distribution of N within the plants was not greatly affected by planting density. No significant differences in root densities were found between isolines or among plant densities. The highest planting density of the nodulating isoline accumulated more than 300 kgNha−1 but total plant N in the non‐nodulating isoline was approximately 75 kgNha−1. During the first 17 days of the pod filling period, N accumulation in the pod sink was greatest and averaged approximately 7.5 kgNha−1 day−1 without a net loss of N from the leaflets. This indicates that N fixation can meet the N needs of soybeans without mobilization of leaf N.