Nitrogen Agronomic Efficiency in Wheat-Based Double Cropping Systems under Different Rate of Nitrogen and Return of Crop Residue

This study was conducted under cold temperate climate condition in Khorasan during 2006-2008 growing seasons to determine the suitable wheat-based double cropping systems for increase nitrogen agronomic efficiency. A randomized complete block design in split-split plot arrangement with three replicates was used. Main plots were crop rotation with five different rotations including wheat:wheat, potato:wheat, silage corn:wheat, clover:wheat and sugar beet:wheat. 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 N rate. The sub-sub plots were preceding crop residue return with two levels including no residue return (Control) and 50% return of crop residue. Results showed that total yield, total N uptake, nitrogen agronomic efficiency, nitrogen physiological efficiency, nitrogen recovery efficiency and nitrogen harvest index were significantly affected by crop rotation and N fertilizer rates. Corn:wheat rotation recorded the highest NAE, NPE, NRE and NHI While the lowest NAE, NPE and NHI were found for sugar beet:wheat rotation. The highly NAE in the corn: wheat rotation is due to the higher NPE in this rotation. NAE, NPE and NRE in crop all rotations reduced with increasing N fertilizer rate. Return of crop residue had no significant effect on NAE and NRE, while decreased NPE and NHI