Estimation of fertilizer nitrogen requirement for average rice yield in Japanese paddy fields

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 Nf 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-upland crop) 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 No 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.