effects of nitrogen fertilizer levels on agronomic efficiency (ea), physiological efficiency (ep), and recovery efficiency (er), in rice (oryza sativa l.) cv. khazar was investigated in a rcbd of three replications in a sandy soil in guilan province, iran in 2003. this study compared the effects of six n rates, namely: control (no n fertilizer), 40 (basal), 40+40 (basal + midtillering), 40+20+2...