To investigate the effect of applying 15N labelled ammonium sulphate with or without a nitrification inhibitor 3, 4-dimethylpyrazole phosphate (DMPP) on fertilizer use efficiency and crop productivity of different wheat genotypes, a field trial was conducted at the Nuclear Agricultural Department's farm of Iran in 2013-2014. The treatments included five wheat genotypes with different 13C isotope discrimination and three fertilizer treatments, an unfertilized control, 15N labeled ammonium sulphate and 15N labeled ammonium sulphate with DMPP in three replications. Soil samples were taken after two, four and six weeks after sowing and also at harvest time. Results from 15N experiment showed that DMPP delayed nitrification of ammonium for 42 days. Genotypes with lower discrimination index had greater uptake of ammonium ions which led to increase crop yield and nitrogen fertilizer use efficiency. The results also suggested that the use of DMPP may not be beneficial in some fast growing wheat genotypes.

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