effects of nitrogen levels, nitrogen sources and zinc rates on the growth and mineral composition of lowland rice

nitrogen use efficiency (nue) is usually lower in paddy rice (oryza sativa l.) than in upland crops. for this reason, any attempt to improve nue through the use of different nitrogen (n) carriers, different rice cultivars, properly timed n application, the use of ni-trification and urease inhibitorsis of prime interest. moreover, zinc (zn) next to n seems to be the most important soil factor affecting rice growth. although the response of rice n and zn fertilization has been reported elsewhere, the authors are not aware of any such information for the calcareous paddy rice soils of iran. therefore, the present experiment was conducted to study the effects of varying sources and levels of n and zn rate on the growth and n and zn concentrations and uptake by rice. treatments consisted of five n levels (0, 50, 100, 200, and 400 mg n kg-1soil), four n sources [urea (u), sulfur–coated urea (scu), ammonium sulfate (as), and ammonium chloride (ac)] and three zn rates (0, 5, and 10 mg zn kg-1 soil as zinc sulfate). the experiment was factorially arranged in a com-pletely randomized design with three replicates. the results showed that the highest top dry weight was obtained with scu, and followed by as, ac and u. application of n up to 200 mg kg-1 as u, as, and ac increased rice growth significantly. however, there was a consistent significant increase in shoot growth with the addition of scu. in the present study, zn fertilization had no significant effect on rice growth. the uptake and concentra-tion of n and zn were increased by application of n and zn. however, the highest zn concentration and uptake were obtained with ac and scu, respectively. nitrogen and zn addition generally increased leaf area and the maximum leaf area was obtained with scu. nitrogen yield efficiency (nye) was reduced with increasing n rates (as u, as, and ac) and increased up to 200 mg n kg-1 as scu. apparent n recovery (anr) increased with increasing n up to 200 mg kg-1 as u, as, and ac and declined thereafter. however, there was an increase in anr with an increasing n level as scu. on the other hand, n physiological efficiency (npe) decreased with increasing n rates regardless of n sources. from the results reported here, it appears that 200 mg n kg-1 is the most appropriate n level and scu appears to be the most efficient n source for lowland rice.