Abstract
Biological nitrogen (N) fixation is the most relevant process in soybeans (Glycine max L.) to satisfy plant N demand and sustain seed protein formation. Past studies describing N fixation for field-grown soybeans mainly focused on a single point time measurement (mainly toward the end of the season) and on the partial N budget (fixed-N minus seed N removal), overlooking the seasonal pattern of this process. Therefore, this study synthesized field datasets involving multiple temporal measurements during the crop growing season to characterize N fixation dynamics using both fixed-N (kg ha−1) and N derived from the atmosphere [Ndfa (%)] to define: (i) time to the maximum rate of N fixation (β2), (ii) time to the maximum Ndfa (α2), and (iii) the cumulative fixed-N. The main outcomes of this study are that (1) the maximum rate of N fixation was around the beginning of pod formation (R3 stage), (2) time to the maximum Ndfa (%) was after full pod formation (R4), and (3) cumulative fixation was positively associated with the seasonal vapor-pressure deficit (VPD) and growth cycle length but negatively associated with soil clay content, and (4) time to the maximum N fixation rate (β2) was positively impacted by season length and negatively impacted by high temperatures during vegetative growth (but positively for VPD, during the same period). Overall, variation in the timing of the maximum rate of N fixation occurred within a much narrower range of growth stages (R3) than the timing of the maximum Ndfa (%), which varied broadly from flowering (R1) to seed filing (R5–R6) depending on the evaluated studies. From a phenotyping standpoint, N fixation determinations after the R4 growth stage would most likely permit capturing both maximum fixed-N rate and maximum Ndfa (%). Further investigations that more closely screen the interplay between N fixation with soil-plant-environment factors should be pursued.
Highlights
IntroductionL.) is the most widely cultivated legume due to its importance as a source of protein and oil (FAOSTAT, 2015)
For the Ndfa (%), the time for the maximum was achieved around full pod formation (R4 growth stage, 1.15, ranging from 0.6 to 1.8 relative phenology) and remained nearly constant toward the end of the seed filling (Figure 1B)
This study provides new insights based on the synthesis from 15 recent site-year-management combinations of seasonal variation in N fixation dynamics for soybeans
Summary
L.) is the most widely cultivated legume due to its importance as a source of protein and oil (FAOSTAT, 2015). As soybean yields increase over time in the last decades (Rincker et al, 2014), the need to maintain high seed protein and its energetic cost increases the reliance of plant N demand on the N fixation process. In many scenarios, increasing the overall N-gap (plant N minus fixed N) has become a growing concern at global scale (Ciampitti and Salvagiotti, 2018). The emphasis on this N gap concept gains more relevance under the current estimates for an increase by 55% on soybean production by 2050 (Ray et al, 2013), which will outpace the rate of the expected rise in demand by ∼100%
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