The Contributions of Cultivar and Fertilization Improvements to Single Rice Nitrogen Use Efficiency Trends Across China

Nitrogen use efficiency (NUE) serves as a useful performance indicator to benchmark agricultural nitrogen management. It is determined by genotype-environment-management interaction, making the quantification of the key drivers and the mechanism behind spatiotemporal trends of NUE challenging. In this study, the soil Water Heat Carbon Nitrogen Simulator (WHCNS) model, integrated with multi-scale datasets, was applied to clarify the key drivers of the NUE trends for single rice in China from 1978 to 2019. The national NUE for single rice was 0.31 over the study period and showed a slight increase before 2005 and afterwards a rapid increase of 0.048/10 years, primarily controlled by fertilization management and cultivar shifts. The N application rate determined the total N input, influenced the N uptake and utilization efficiency, and therefore the NUE, while the effect of cultivar shifts on NUE is mainly determined by the increase in the proportion of photosynthetic products in the rice grain and thus the harvest index. The benefits of cultivar improvement (0.034/10 years) which were almost entirely offset by the excessive use of N fertilizers (-0.029/10 years) before 2005, became lower (0.021/10 years) and unstable thereafter in many subregions, indicating the challenge of cultivar selection under low N inputs. To reach the ambitious NUE target of 0.6 by 2050 for rice, improving NUE through better management alone will not be enough. Improvements in rice breeding are therefore urgently needed to meet future NUE challenges in rice production under climate change and population growth.