Climate Change Increases Nitrogen Concentration In Rice With Low Nitrogen Use Efficiency

Growing evidence indicates that climate change has significant impacts on crop production. However, the spatial variability and extent of the impacts on N concentrations in crops have yet to be explicitly described, although N concentration is a critical element of both the nutritional value of food and the planning of appropriate field management procedures. We present multi-tiered evidence showing that warming increases N concentration in rice and reduces internal N use efficiency (iNUE), thereby increasing N uptake in crops and encouraging greater N fertilizer use. First, we mapped current N concentrations and iNUE for single-season irrigation paddy rice production in 975 counties across China using data from 1,637 on-farm experiments and Random Forest regression modeling. Current N concentrations in rice grain and straw have increased with increasing temperatures from northern to southern China. The global meta-analysis provided further direct evidence that warming increases N concentration in rice. Future warming projections indicate that N concentrations in grain will increase by 5%-9.6% and that iNUE will decrease by 9%-14% under scenarios RCP4.5 and RCP 8.5 although higher CO2 and other factors may also affect N concentrations. Overall, these data demonstrate that N uptake by rice will need to increase by 7.8%-13.6% to maintain current grain yields across China based on warming alone. Our findings indicate that warming associated with increase in N concentration and reduction in iNUE would be expected to encourage N application, raise the urgency of measures to prevent excessive N application, and stronger counteracting measures need to be enacted.