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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 ( f APAR) were compared in crops relying only on biological N 2 fixation and available soil N (zero‐N treatment) versus crops receiving N fertilizer (full‐N treatment). Experiments were conducted in seven high‐yield environments without water limitation, where crops received optimal management. In the zero‐N treatment, biological N 2 fixation was not sufficient to meet the N demand of the growing crop from early in the season up to beginning of seed filling. As a result, crop LAI, growth, N accumulation, radiation‐use efficiency and f APAR were consistently higher in the full‐N than in the zero‐N treatment, 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 2 fixation, greater supply of N from soil or fertilizer, or alleviation of the trade‐off between these two sources of N in order to meet the plant demand.

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