Nitrogen supplying capacity of Iranian rice soils.

Anaerobic incubation study at 35 deg C during 4 weeks revealed that N-release pattern showed an initial flush of released-N within one week followed by slow release rates. The high significant correlation obtained between N-released during one week anaerobic incubation and grain yield or plant N-uptake in field indicated the importance of the initial N-release and probable positive relationship between the initial and later N-supplying capacity. Fractionation of soil organic-N were performed by acid hydrolysis. Organic-N fractions showed significant correlation with mineralizable-N and the order of correlation coefficients were as follows: nonhydrolysable-N = hydrolyzable ammonium-N>amino acid-N>amino sugar-N>unidentified-N. The ratio of polysaccaride-C to amino acid-N had higher correlation with mineralizable N than the ratio of total C to total-N, although correlation value for the former was still small and not reliable. Field experiments at 13 sites with PK and NPK treatments indicated that total-N, Organic-C, N-released during one week incubation, acid dichromate-N and alkali permanganate extractable-N could significantly account for variation in yield, yield response and plant N-uptake. N-response curve study at 7 fields reliably validated the N-availability indices for adjusting N-rates application ensuring maximum yield production and lower environmental pollution. At location having very low N-supplying capacity (Nt percent = 0.12, 0C percent - 1.07), 92 kg N/ha was necessary to obtain 4.5 tons/ha of grain yield of the test cultivar (Tarom), while 23 kg N/ha was sufficient to the same yield level at location having a high N-supplying capacity (Nt percent = 0.28, 0.C percent = 2.11). The optimum soil C content above which yield response to applied N-rates decreased was 2.11 percent. Agronomic N-use efficiencies were the same across all N-responsive location with relevant optimum rates of fertilizer application. Recovery efficiency from applied optimum N-rate was inversely related to the soil-N supplying capacity.