Time Course of Nitrogen Fixation in Field‐Grown Soybean Using Nitrogen‐15 Methodology¹

Abstract

The use of 15N techniques allows for the quantitative evaluation of N2 fixation and distribution and their impact on the N balance in various soil‐plant systems. The A‐value approach was used in this investigation to assess N2 fixed at various growth stages in fieldgrown soybean [Glycine max (L.) Merrill] cv. Chippewa in a Typic Eutrocrepts soil. At physiological maturity (R7), the amount of N derived from fixation (Ndfa) was 102 kg/ha, equivalent to 47% of total N assimilated, while the contributions from soil (Ndfs) and 15N‐labelled fertilizer (Ndff) accounted for 50 and 3%, respectively. Up to growth stage V6, which occupied half of the total duration of growth, Ndfa was less than 5% of N2 fixed by physiological maturity. A rapid increase in Ndfa occurred from R1 onwards, and during the reproductive stages (R1‐R4), which spanned less than one‐third of the total duration of growth, this represented about 45% of total Ndfa. An almost equal portion of N (approximately 43%) was fixed from pod‐filling (R5) to physiological maturity (R7), a period slightly more than one‐fifth of the total duration of growth. Therefore, substantial N2 fixation occurred during periods of active sink development and contributed more than 65% of the plant's N accrued during pod fill (R3‐R7). Nitrogen assimilated between R3 and R7 (when N2 fixation was high) seemed to be the predominant source of N for pod development. Thus there was a greater contribution from fixed N (55%) than soil N (43%) in pods and seeds at the end of R7. After grain removal, it was estimated that the growth of cv. Chippewa in this soil led to a net soil depletion of 54 kg N/ha.

Soybean response to nitrogen application across the United States: A synthesis-analysis

“…Comparatively, greater contribution from BNF of near 90% (to seed N content) and 97% (to total plant N uptake) was reported by Mastrodomenico and Purcell (2012) and George et al (1988), respectively. Zapata et al (1987) concluded that maximum BNF occurs between the R1 and R4 stages, and biologically-fixed N (55%) contributes more to N assimilation in pods and seeds by the R7 growth stage compared to soil N (43%).…”

N2 fixation by soybeans grown with different levels of mineral nitrogen, and the fertilizer replacement value for a following crop

“…While soybean is a relatively sensitive crop, both mineral and fixed N seem to be essential for maximum yield and N content. This is presumably because N 2 fixation begins only after nodule formation early in the vegetative stage, and nodule senescence together with competition from seeds for plant assimilates limit fixation during the reproductive stage (Harper 1974;Warembourg and Fernandez 1985;Zapata et al 1987). Nonetheless, the presence of a high level of mineral N adversely affects N 2 fixation.…”

“…The seeds of soybean can contain as much as 75% of the total plant N (Guffy et al 1989;Vasilas et al 1995;Koutroubas et al 1998). An intense transfer of N from vegetative parts to seeds occurs during the reproductive phase (Zapata et al 1987;Vasilas et al 1995;Koutroubas et al 1998). By harvest about 40-80 kg N ha -1 are left in the stover (Zapata et al 1987;Jefing et al 1992;Koutroubas et al 1998), and 20-40 kg N ha -1 in the roots, assuming that roots contain 11% of total plant N (United Co-Operatives of Ontario 1981).…”

“…There is usually a net depletion of soil N after a soybean crop. Depending on whether or not the stover is removed, and on the amount of N fertilizer applied, the balance of soil N between sowing and harvest can vary from +9 to -133 kg N ha -1 , excluding the root system (Zapata et al 1987;George and Singleton 1992;Jefing et al 1992). However, even if the crop fails to provide a clear net benefit in terms of total N in the soil, the N balance is usually assumed to be more positive than with a non-legume crop.…”

Nitrogen Sources and Rates Affect Soybean Seed Composition in Mississippi

“…On average, the BNF satisfies only 50-60% of soybean N demand [1] which is enough to achieve only 80-90% of soybean yield possible with N fertilization [2][3][4]. Any gap between the soybean N demand and N supply by BNF needs to be supplied by N uptake from other sources [5]. In an effort to provide soybeans with adequate N for maximum seed yield, producers have considered applying N fertilizer to their soybean crop.…”