High PUE does not necessarily correspond to better P management, and it could be associated with a very low input rate and yield. African countries, FSU, and the U.S. all had relatively high PUE in 2010 (Table 3), but for very different reasons and with different implications. High PUE in the U.S. was accompanied by higher P inputs (1.8 Tg P yr-1, Table 3) and higher P in harvested crops (2.2 Tg P yr-1, Table 3), which benefited from the improvement in fertilizer management practices. In contrast, high PUE in African countries and FSU was the result of low P in harvested crops (1.0 Tg P yr-1 for both, Table 3) and lower P inputs (0.8 and 0.6 Tg P yr-1, respectively, Table 3). Meeting the goals of increasing food demand and managing pollution from agriculture requires increasing production and PUE, and limiting P surplus.
3.2 Key Drivers for PUE
We tested the EKC hypothesis to study the relationship between economic development and PUE. For other drivers, we used Pearson’s correlation coefficient to examine the relationship in each country and regression models across 113 countries. Our analysis results suggest drivers with a significant and consistent relationship with PUE in the global and regional models are economic development, fertilizer to crop price ratio, crop mix, NUE, farm size, and P yield (Table 4). All models mentioned in Table 4 consider time and country effects and have PUE as the dependent variable. Compared to the results of constant intercept models and fixed effects models not considering the time and country effects, the fixed effects models considering these effects behave better with higher adjusted R2 and lower AIC (Table S10). The fixed effects model in Table 4 is the final chosen model with a high adjusted R2 (96%) and the lowest AIC, using stepwise regression. More details of calculation, model setting, and comparison can be found in SI.
Table 4 Regression results of three methods studying the relationship between PUE and drivers from 1961 to 2014 in individual countries and across 113 countries. The grey box indicates the driver is not included in the model. *** suggests p < 0.001, **suggests p < 0.01, and * suggests p < 0.05.