Nitrogen fixation in high yielding soybean (Glycine max., L. Merr)

Under the hypothesis that grain yield is limited by nitrogen (N) supply in late reproductive stages when soybean is grown in high-yielding environments, a 2-year study was conducted in order to evaluate whether N fertilization increases grain yield and N uptake, and to what extent N2 fixation is complemented when fertilizer N was applied both on surface as well as below the nodules. Four N fertilization treatments were established in the soybean main plots of each year's soybean/maize rotation in a long-term experiment. Treatments were: a non-fertilized plot (N1) and 180 kg N ha-1 applied as (i) polymer-coated controlled release urea placed 20 cm below the surface before planting (N2, late N fertilization strategy); (ii) ammonium nitrate on the surface before planting and at V6 (N3, early N fertilization strategy) and, (iii) ammonium nitrate on the surface at R5 (N4, late N fertilization strategy). All treatments were superimposed on two contrasting fertilizer management histories for maize (M1=recommended nutrient management; M2=intensive nutrient management). Grain yield in N1 averaged 4850 kg ha-1, and a 5% increase was observed in response to N fertilization. This response was accompanied by a 9% increase in N uptake. Biological N2 fixation accounted for 50% of N uptake in N1, but only 32 and 38% in N3 and N4, respectively. All N fertilization strategies succeeded in providing the additional N needed to attain the apparent maximum grain yield, but the amount of fertilizer N needed to achieve grain yield response was larger with ammonium nitrate applied on the surface N. This was because a proportion of total N derived from surface applied ammonium nitrate depressed N2 fixation. In contrast, deep placement of slow-release urea did not depress biological N2 fixation, and thus, showed the advantage of providing the additional N needed to reach grain yield potential with minimal trade-off between fixed N and fertilizer N. The greater photosynthesis response to light observed in fertilized soybean suggests that N fertilization in high-yielding soybean should simultaneously take into account management practices intended to maximize canopy light capture.