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.