Temporal variation of soil N supply defines N fixation in soybeans

Soybean [Glycine max (L.) Merr.] plays a critical role in global food security and sustainability, acquiring most of the nitrogen (N) required for growth and high-protein seeds from symbiotic N fixation (SNF). However, there is scarce information on how soil N supply interacts with N fixation (kg ha-1) throughout the growing season. This study aims to (1) evaluate the allometric relationships between the total N uptake and dry matter by N source (soil and fixation), (2) describe seasonal changes on soil-plant N dynamics among genotypes with contrasting genetic background (released between 1981 and 2019), and (3) assess the influence of soil NO3 and NH4 on the temporal SNF changes during the growing season. Four genotypes were field grown during 2019 and 2020. Bayesian non-linear models described seasonal change on dry matter, N uptake, soil NO3, and NH4 (60 cm depth). The allometric relationship between N uptake and dry matter indicated that total N status is more sensitive to variations in N uptake from the soil than from fixation. Seasonal exposure to NH4 (given by the area under the curve) and a delay on maximum NO3 availability from early to mid-season suppressed end-season N fixation by two-fold the benefit on soil N uptake (kg ha-1). Both the magnitude and timing of soil N sources affected N uptake in all tested genotypes. Future research must explore broader SNF levels for understanding to what extent soil N can affect the N uptake sources, especially SNF, without decreasing seed yield in soybeans.

Automated summary

Soybean plays a critical role in global food security and sustainability, but the interaction between soil nitrogen supply and nitrogen fixation throughout the growing season is poorly understood. This study evaluated the relationship between nitrogen uptake and dry matter, described seasonal changes in soil-plant nitrogen dynamics among genotypes with different genetic backgrounds, and assessed the influence of soil NO3 and NH4 on temporal nitrogen fixation changes. The findings showed that nitrogen uptake from the soil had a greater impact on total nitrogen status than fixation. NH4 exposure and delayed NO3 availability suppressed end-season nitrogen fixation, highlighting the importance of soil nitrogen sources in soybean growth.