Characterizing N uptake and use efficiency in rice as influenced by environments

To compare N uptake and use efficiency of rice among different environments and quantify the contributions of indigenous soil and applied N to N uptake and use efficiency, field experiments were conducted in five sites in five provinces of China in 2012 and 2013. Four cultivars were grown under three N treatments in each site. Average total N uptake was 10–12 g m^?2 in Huaiji, Binyang, and Haikou, 20 g m^?2 in Changsha, and 23 g m^?2 in Xingyi. Rice crops took up 54.6–61.7% of total plant N from soil in Huaiji, Binyang, and Haikou, 64.3% in Changsha, and 63.5% in Xingyi. Partial factor productivity of applied N and recovery efficiency of applied N in Changsha were higher than in Huaiji, Binyang, and Haikou, but were lower than in Xingyi. Physiological efficiency of soil N and fertilizer N were lower in Changsha than in Huaiji, Binyang, and Haikou, while the difference in them between Changsha and Xingyi were small or inconsistent. Average grain yields were 6.5–7.5 t ha^?1 (medium yield) in Huaiji, Binyang, and Haikou, 9.0 t ha^?1 (high yield) in Changsha, and 12.0 t ha^?1 (super high yield) in Xingyi. Our results suggest that both indigenous soil and applied N were key factors for improving rice yield from medium to high level, while a further improvement to super high yield indigenous soil N was more important than fertilizer N, and a simultaneous increasing grain yield and N use efficiency can be achieved using SPAD-based practice in rice production.