Estimation of Fertilizer Nitrogen Requirement for Average Rice Yield in Japanese Paddy Fields

概要

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The fertilizer N requirement (Nf_500) for average rice yield (500 g m^-2) in the paddy fields of Japan was estimated. The Nf_500 of a paddy field is defined as Nf_500 = N_f(500 - Y_0)/(Y - Y_0), where Y is the grain yield of the NPK plot, Y_0 is the grain yield within the PK plot, and N_f is the rate of N application in the NPK plot. Nf_500 was estimated from 86 sets of NPK experiments. The mean Nf_500 for all the paddy fields (n = 86) was 9.0 g N m^-2, that for single-cropped paddy fields (n = 49) was 7.3 g N m^-2, and that for double-cropped (rice-uplandcrop) fields (n = 37) was 11.2 g N m^-2. The higher Nf_500 in the double-cropped paddy fields could be attributed to the greater loss of fertilizer N. Multiple regression analysis showed that Nf_500 = - 1.79 N_0 - 0.12RE + 25.9 (r^2 = 0.62), where N_0 is the N uptake of the PK plot and RE is the recovery efficiency of applied N. Because the coefficients in the analysis were negative, higher values of N_0 and RE resulted in lower Nf_500 values. Rice straw application (approximately 500 g m^-2) decreased by 0.67 g N m^-2 (7.8%) of the amount of fertilizer N required to produce a grain yield of 500 g m^-2 in paddy fields in Japan. Nf_500 provides a useful measure to evaluate the productivity of paddy fields in terms of nitrogen economy.