Nitrogen use efficiency of wheat as affected by preceding crop, application rate of nitrogen and crop residues

This study was carried out under temperate climate condition of Khorasan province, Iran, during 2006-2008 growing seasons to determine the suitable crop rotation for enhancing nitrogen use efficiency of wheat. A three-factor experiment was laid out as split-split plot arrangement (RCBD) with three replications. Main plots were five preceding crops for wheat practices viz. wheat, potato, silage corn, clover and sugar beet. Sub plots were N fertilizer rates in preceding crop including no N (control), 50% lower than recommended N rate, recommended N rate and 50% more than recommended rate. The sub-sub plots were preceding crop residue return including no residue return (control) and 50% return. Results showed that nitrogen use efficiency (NUE), nitrogen uptake efficiency (NUpE), nitrogen utilization efficiency (NUtE), nitrogen harvest index (NHI) and grain protein content (GPC) in wheat were significantly affected by preceding crop. Fertilizer N rate applied on preceding crops, significantly influenced NUE, NUpE and grain protein content of wheat. NUE, NUpE, NUtE and NHI of wheat grown after potato recorded the highest values. While, the lowest NUE indices and grain protein content of wheat were found for wheat-wheat rotation. The higher NUE of wheat in the potato-wheat rotation was due to the higher NUpE in this rotation. NUE and NUpE in all crop rotations reduced with increasing N fertilizer rate. Return of crop residue had no significant effect on NUE indices expect NHI.