Insufficient nitrogen supply from symbiotic fixation reduces seasonal crop growth and nitrogen mobilization to seed in highly productive soybean crops.

: Nitrogen (N) supply can limit the yields of soybean [Glycine max (L.) Merr.] in highly productive environments. To explore the physiological mechanisms underlying this limitation, seasonal changes in N dynamics, aboveground dry matter (ADM) accumulation, leaf area index (LAI), and fraction of absorbed radiation (fAPAR) were compared in crops relying only on biological N fixation and available soil N (zero N treatment) versus crops receiving N fertilizer (full N treatment). Experiments were conducted in seven high-yielding environments without water limitation, where crops received optimal management. In the zero N treatment, biological N fixation was not sufficient to meet the N demand of the growing crop from early in the season up to beginning of seed fill. As a result, crop LAI, growth, N accumulation, radiation-use efficiency, and fAPAR were consistently higher in the full versus zero N treatments, leading to improved seed set and yield. Similarly, plants in the full N treatment had heavier seeds with higher N concentration because of greater N mobilization from vegetative organs to seeds. Future yield gains in high-yield soybean production systems will require an increase in biological N fixation, greater supply of N from soil or fertilizer, or alleviation of the trade-off between these two mechanisms in order to meet the plant N demand. This article is protected by copyright. All rights reserved.