evaluation of soil fertility in wheat-based double cropping systems under different rates of nitrogen and return of crop residue

abstract this study was conducted under cold climate condition in khorasan during 2006-2008 growing seasons to evaluation of soil fertility in wheat-based double cropping systems under different rate of nitrogen and return of crop residues. a randomized complete block design with split-split plot arrangement and three replicates was used. main plots were five crop rotations namely: wheat-wheat, potato-wheat, silage corn-wheat, clover-wheat and sugar beet-wheat. four 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 two sub-sub plots were preceding crop residue return including: no residue return (control) and 50% residue return. results showed that soil nitrogen content was not affected by crop rotation, nitrogen rate and return of crop residues. soil phosphorus content at 30-cm depth was significantly affected by preceding crop of wheat. although, nitrogen rate and crop residue return were not influenced on soil phosphorus. our results indicated that soil potassium content observed for the clover and wheat, respectively. there was a significantly interaction between preceding crop and return of crop residue for soil organic carbon in the 30 to 60 cm depth. but, soil organic carbon was not affected by preceding crop and nitrogen rate in the first year of experiment.