3.3.2 Phosphorus depletion in 2050
The demand for P fertilizer is likely to increase with P inputs if we assumed that all countries would maintain their current P fertilizer to total P inputs ratio (average of 2005-2014) after 2014 (scenarios 1B-3B, Table 2). With this assumption, we projected that global P fertilizer input would increase from 16.4 Tg P yr-1 in 2010 to 25.4 Tg P yr-1 in 2050 in the 1B scenario (Table S3). With PUE improvement, P fertilizer input would decrease to 18.5 (2B scenario) and 16.1 Tg P yr-1 (3B scenario) in 2050 (Table S3). Similarly, P manure input would increase from 6.0 Tg P yr-1 in 2010 to 10.4 Tg P yr-1 in 2050 in the 1B scenario. Improvement in PUE would reduce this input to 7.6 (2B) and 6.6 Tg P yr-1 (3B) in 2050 (Table S3).
The global phosphate rock reserves would be sufficient for crop production from 2017 to 2050. In all nine scenarios, the fertilizer demand to supply ratio would be at around 0.1% (Table S16), meaning that crop production would consume only about 10% of the global phosphate rock reserves. On the regional scale, the ratio of P demand to supply ratio would decrease as PUE improves (Fig. 5). However, non-major P producing countries would still face the P depletion challenge by 2050 or even earlier (Fig. 5, Table S16). Even in the most ambitious scenario and most ideal case (Fig. 5i), India (demand to supply ratio 3.7), Mexico (demand to supply ratio 1.7), and non-major P producing countries would still need to rely on phosphate rock or P fertilizer import (Fig. 5, Table S16) to support their food production. This ratio in the U.S. and China would be between 0.2-0.8 (Table S16), as they had relatively high P storage (Table S18). However, to secure food production, these two countries have adopted increasingly tight policies on phosphate rock export (Sattari et al. 2014, van de Wiel et al. 2015). P depletion challenge is severe on a regional scale.