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

Abstract

The fertilizer N requirement (Nf₅₀₀) for average rice yield (500 g m⁻²) in the paddy fields of Japan was estimated. The Nf₅₀₀ of a paddy field is defined as Nf₅₀₀ = N_f (500 - Y _n0) / (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. Nf₅₀₀, was estimated from 86 sets of NPK experiments. The mean Nf₅₀₀ 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 Nf₅₀₀, in the double-cropped paddy fields could be attributed to the greater loss of fertilizer N. Multiple regression analysis showed that Nf₅₀₀ = − 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 Nf₅₀₀ 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. Nf₅₀₀ provides a useful measure to evaluate the productivity of paddy fields in terms of nitrogen economy.