Development of Regional Nitrogen Rate Guidelines for Intensive Cropping Systems in China.

The intensification of agricultural production has been accompanied by an overuse of N fertilization and has accelerated global N consumption in the last 20 yr. We undertook 269 on-farm demonstration trials to evaluate variations in the optimal N rate (ONR) and developed regional N rate guidelines for intensive wheat (Triticum aestivum L.)-maize (Zea mays L.) rotation systems in the North China Plain. Large variations in the ONR (coefficient of variation = 38~39%) were observed across the 269 fields, resulting from large variations in the initial soil NO₃-N supply before sowing. A negative linear relationship between the ONR and initial soil NO₃-N suggested that 1 kg soil NO₃-N ha^-1 was equivalent to applications of 0.54 and 0.71 kg fertilizer N ha^-1 for wheat and maize, respectively. The current cropping system, with excessive N inputs and large variations in residual NO₃-N in the top 90-cm soil depth (averaging 188 kg N ha^-1 season^-1 and ranging 20-987 kg N ha^-1 season^-1), could be transformed with optimal N management to a steady state, in which the residual NO₃-N content is maintained at about 90 to 100 kg N ha^-1. Regional N rate guidelines should be 154 to 159 and 164 to 171 kg N ha^-1 for yields of 6 and 9 Mg ha^-1 for wheat and maize, respectively. This regional N rate guideline may not be completely accurate but is a robust and simple guideline acceptable to the 100 million smallholder farmers in China. [ABSTRACT FROM AUTHOR]