Interplay between nitrogen fertilizer and biological nitrogen fixation in soybean: Implications on seed yield and biomass allocation

Authors: TAMAGNO, S - Kansas State University; SADRAS, V - South Australian Research And Development Institute; HAEGELE, J - Winfield Solutions; Armstrong, Paul; CIAMPITTE, I - Kansas State University

Submitted to: Nature Scientific Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/7/2018
Publication Date: 11/30/2018
Citation: Tamagno, S., Sadras, V.O., Haegele, J.W., Armstrong, P.R., Ciampitte, I.A. 2018. Interplay between nitrogen fertilizer and biological nitrogen fixation in soybean: Implications on seed yield and biomass allocation. Nature Scientific Reports. 8:17502. https://doi.org/10.1038/s41598-018-35672-1.
DOI: https://doi.org/10.1038/s41598-018-35672-1

Interpretive Summary: Soybean is an important crop to meet the rising global demand for protein and oil. The crop relies on mineral nitrogen from soil and biological nitrogen fixation (BNF), the latter often declines with increasing soil N. There is still much unknown about high yielding soybean and the balance between soil nitrogen and BNF effects on plants for the different maturity groups and varieties. This work explored the impact of N application on the seasonal dynamics of BNF and its effect on yield with emphasis on growth and shoots. Seed yield was shown to decline by about 12 lbs/acre for each percentage increase in BNF when the effect of growing conditions and variety was eliminated. Total plant biomass and seed protein concentration were not affected. Plots unfertilized with nitrogen produced the highest BNF.

Technical Abstract: Grain legumes, particularly soybean (Glycine max L.), are important to meet the rising global demand for protein. The crop relies on mineral nitrogen (N) from soil and biological N fixation (BNF), which often declines with increasing soil N. The interplay between the two sources of N is biologically interesting and agronomically relevant as the crop can accommodate the metabolic cost of fixing nitrogen by five non-mutually exclusive mechanisms, whereby nitrogen fixation: (a) reduces shoot growth and yield, or maintains shoot growth and yield, by (b) enhanced photosynthesis, or (c) reduced root:shoot ratio, or maintains shoot growth but reduces yield by (d) reducing harvest index, or (e) reducing concentration of oil and protein in seed. The objective of this work was to explore the impact of N application on the seasonal dynamics of BNF and its consequences for seed yield with emphasis on growth and shoot allocation mechanisms. Soil N was manipulated with four treatments: (1) an unfertilized control and fertilized crops with 112 kg N ha-1 applied at (2) sowing, (3) V4 (fourth-leaf), and (4) R2-R3 (full flowering to beginning of pod formation). Trials were established in 23 locations across the US Midwest, where soybean varieties of maturity groups from 0 to IV were used to account for latitudinal variation between 34° and 48°. Yield ranged from 3296 to 6326 kg ha-1, biomass from 6093 to 11376 kg ha-1, harvest index from 0.37 to 0.56, seed oil concentration from 31.9 to 41.8, and seed protein concentration from 16.7 to 23.9 g 100 g-1. A beta growth function described the seasonal dynamics of BNF (measured with the ureide method), which peaked around R6 (931 to 1117 °Cd after V2). Unfertilized controls returned the highest BNF; fertilizer reduced the peak of BNF with larger effects (up to 16%) recorded for applications at R2. Phenology was the largest source of variation in yield, which increased sharply with thermal time to R6 up to 1176 °Cd, and declined slightly after this threshold. After removing the effect of phenology, yield declined at 13 kg ha-1 for each percentage increase in BNF. While total biomass and seed protein concentration were not affected, HI accounted for the decline in yield with increasing N fixation. This indicates the cost of N fixation was met by a relative change in dry matter allocation against the energetically rich seed, and in favor of energetically cheaper vegetative tissue.