Growth and nitrogen fixation in high-yielding soybean: Impact of nitrogen fertilization
In high-yielding soybean [Glycine max (L.) Merr.] environments, N uptake during seed-filling may be constrained when the late-season decline in biological nitrogen fixation (BNF) is coupled with insufficient soil N. Three N-fertilization strategies were compared with a control (N0) on soybeans in 2006 and 2007 in a high-yield soybean-maize (Zea mays L.) rotation experiment established in 1999. Our objective was to identify strategies for overcoming a late-season N-supply limitation to maximal seed yield coupled with N fertilizer-induced reduction in BNF. In the N0 treatment, no N was applied, but in each N fertilization strategy, 180 kg N ha–1 were applied as (i) polymer-coated controlled-release urea placed 20 cm below the soil surface before planting (NLu); (ii) ammonium nitrate split-applied to the surface before planting and at V6 (early application, NEa), or (iii) ammonium nitrate applied to the surface at R5 (NLa). The ureide method was used to estimate BNF. The N0 yield was 4850 kg ha–1, but the mean of the three N-applied treatments had 228 kg ha–1 (5%) higher yields. The N0 N uptake was 411 kg ha–1, but it was 36 kg ha–1 (9%) greater in the mean of the three N treatments. Nitrogen uptake attributable to BNF in the N0, NLu, NEa, and NLa treatments was 50, 46, 32, and 38% of total N accumulation, respectively. Yield maximization and mitigation of N fertilizer-induced reduction of BNF were both achievable when soybean plants acquired N from a slow-release urea applied before planting in a deep band located below the root nodulation zone.