Post-Flowering Phosphorus Deficiency Negatively Impacts Biological Fixation but not Nitrogen Utilization Efficiency of Soybean

Understanding how phosphorus (P) deficiency during the reproductive phase of soybean [Glycine max (L.) Merril] affects nitrogen (N) acquisition via biological N fixation (BNF), and seed yield per unit of the accumulated nutrient remains incomplete. Soybean plants were fertigated with a sufficient concentration of P in the nutrient solution (500 µmol L^-1 P) until flowering. Subsequently, plants were maintained under this condition or subjected to nutrient deficiencies (20 or 100 µmol L^-1 P), resulting in three regimes of P supply during the reproductive phase. At the onset of maximum grain-filling rate and physiological harvest, various parameters were assessed, including nodulation traits, plant nutritional status and biomass production, accumulation, partitioning, and utilization efficiency of P and N. P deficiency after flowering negatively impacted soybean yield and dry mass production, as well as the concentration of P and N in plant organs, their total shoot content, and partitioning to grains. The poor BNF performance was associated with a reduction in the number and dry mass of nodules, triggered by a decrease in plant’s N demand. Nevertheless, low-P stress did not affect seed yield per unit of acquired nutrient, which was related to the fact that the decline in N partitioning to grains was accompanied by a proportional decreasing in their N concentration. The down-regulation of BNF, rather than an impaired N utilization efficiency, contributes to explaining reduced yield of soybean plants facing post-flowering P deficiency. Therefore, the development of precise P fertilization management approaches to maximize BNF and crop yield should prioritize strategies that ensure adequate P supply across the reproductive phase of soybean.