Wheat responses to ammonium and nitrate N applied at different sown and input times

Two kinds of field experiments were conducted in Yongshou, Shaanxi Province, and Louyang, Henan Province, to study wheat responses to nitrate and ammonium N applied at different times with three treatments: control, nitrate and ammonium N at a rate of 100 kg N ha−1 as basal fertilizer. During the plant growing period, pH, ammonium and nitrate N were determined in rhizosphere and bulk soil, and nitrate N, ammonium N and amide N in plants at three wheat growing stages. Results showed that in Yongshou, sown at 23 Sep. wheat seed yield increase by nitrate N was 1661 and 565 kg ha−1 in the two fields while 1022 and 336 kg ha−1 by ammonium N. In contrast, sown at 15 Oct. the respective increase was 378 and 399 kg ha−1 for nitrate N and 157 and 145 kg ha−1 for ammonium N, the increased ratio of nitrate to ammonium N being much higher for the second sowing time than for the first. Similarly, when N fertilizer was applied 15 days before wheat sowing, the yield difference between the two N forms was not large or even not significant at 0.05 level while at late time, the differences was remarkably significant. Such different results were caused by the velocity of the ammonium N nitrification: earlier sowing with N fertilization or earlier N fertilization before wheat sowing, higher soil temperature rapidly promoted ammonium N nitrified into nitrate N while the reverses were the case for N input later. Soil pH in rhizosphere increased with nitrate while decreased with ammonium N. Compared to ammonium N, nitrate N not only increased nitrate N concentrations and uptake amounts in plants, but ammonium N and amide N as well by rapid transformation of nitrate N. As a direct result, fertilization of nitrate N ensured a beneficial N nutrition environment for wheat growth.