Figure 3. Crop mix as an important driver for PUE. a) Global NUE and PUE of 11 crop groups (Table S12) between 1961-2014. Point 1 represents the average of 1961-1970. Point 2 represents the average of 2005-2014. PUE values of other single years are not shown. PUE values higher than one are plotted at one. ‘OtherCG’ means other cereal crops. “OtherOS” means other oil seeds. ‘OtherCrs’ means other crop types. b) China and U.S. PUE across crops between 1961-2014. The purple line shows China’s PUE. The black line shows U.S. PUE. The dashed purple line represents the resulted PUE in China if China improved its PUE to U.S. level for each crop type. The gap between the black line and the dashed purple line shows the difference in PUE determined by the difference in crop mix.
Improving the PUE of low-efficiency crops and crop mix can both improve national PUE. China has increased harvested area and P fertilizer input for fruit and vegetable, due to increased demand and high profits. Those crops’ harvested area and P fertilizer input have increased from around 5% and 12% in 1961 to 21% and 36% in 2010 (average of 2005-2014). This shift of low PUE crops reduced China’s PUE. If China improved the PUE of each crop type to the U.S. level (Fig. 3a), China’s PUE would be significantly improved (Fig. 3b). The remaining difference in PUE (61% lower in 2014) is caused by the difference in crop mix (Fig. 3b).