Effects of Compost and Nitrogen Fertilizer on Wheat Nitrogen Use in Japanese Soils

Wheat ( Triticum aestivum L.) grain yield (GY) and grain protein concentration (GPr) are influenced by N availability and supply. This study aimed to investigate wheat (cv. Ayahikari) response to compost and N fertilizer. A 3‐yr field experiment was conducted on four Japanese soils varying in N mineralization potential with or without annual compost application (≈220 kg N ha −1 yr −1 ). Four N fertilizer treatments including a zero‐N control were established each year, and equal amounts of N were applied at preplanting and stem elongation. A significant quadratic relationship of increasing GY with greater N uptake, and increasing GPr with greater N factor (aboveground N uptake, N up , per unit of GY) occurred for the pooled data. From these relationships, the optimum N up for appropriate GPr (105 g kg −1 ) for Japanese ‘Udon’ noodle was estimated to be 139 kg ha −1 and GY could be >5000 kg ha −1 . Fertilizer N rate for optimum N uptake in each soil–compost regime was estimated from a significant linear or quadratic relationship between N uptake and fertilizer N rate (N f ). The agronomic efficiency (yield increase per unit of fertilizer N) and apparent fertilizer N recovery at a given rate of fertilizer N tended to be lower in soils with annual compost than without. However, the fertilizer N requirement for an equivalent yield decreased, thus the fertilizer N surplus (N f − N up ) at optimum N uptake was lower in soils with compost application than without.