Effects of nitrogen topdressing fertilization on yield and quality in soybeans

Soybean [Glycine max (L.)] has higher nitrogen requirements than other crops. We investigated the effects on soybean yield and quality of topdressing with nitrogen fertilizer. Nitrogen fertilizer was applied as a topdressing to soybeans at 0, 20, 30, and 40 kg ha−1 (N0, N20, N30, and N40 treatments, respectively); half of the total topdressing treatment was applied at the pre-flowering (R 1) stage and the other half at the post-flowering (R 2) stage. Yield was highest in the N20 treatment and decreased with larger quantities of topdressing. The protein and total amino acid content were highest in the N20 treatment but tended to decrease with a greater quantity of topdressing. contents of most individual amino acids peaked in the N20 or N30 treatments and decreased as topdressing quantity increased, although proline and arginine contents increased with quantity of topdressing. Isoflavone content tended to be highest in either the N30 or N20 treatment. The agronomic efficiency of nitrogen (AEN) was highest in the N20 treatment. There was a positive correlation between AEN and yield, protein, isoflavone, and amino acid content. Topdressing with 20 kg ha−1 N produced the highest yield, protein, and amino acid content. Topdressing with greater quantities of nitrogen fertilizer decreased the yield and quality of soybeans.