Figure 5. P depletion challenge (represented by P fertilizer demand to supply ratio) in major phosphate rock producing countries in nine scenarios (Table 2). (a) 1A scenario. (b) 2A scenario. (c) 3A scenario. (d) 1B scenario. (e) 2B scenario. (f) 3B scenario. (g) 1C scenario. (h) 2C scenario. (i) 3C scenario. Orange area: fertilizer demand to supply ratio larger than one, indicating P depletion by 2050. Blue area: fertilizer demand to supply ratio smaller than one, indicating P sufficiency by 2050. Grey area: non-major P producing countries with no phosphate rock production data.
4 Discussion
4.1 Improving PUE to Address Phosphorus Challenges
Addressing P pollution and depletion depends on the improvement of PUE, especially the PUE of low-efficiency countries and crops. These low PUE countries include India and China, and low PUE crops include rice, fruit, and vegetable.
Our study also identified key drivers for PUE improvement. We found a positive relationship between PUE and NUE, fertilizer to crop price ratio, and average farm size; and a negative relationship between PUE and the percentage of the harvested area of cereals, fruit, and vegetable. The relationship between PUE and economic development was close to an EKC curve on a regional scale, and it is synergy on a global scale.
P-efficient technologies and management practices are critical for PUE improvement. Some of these technologies and practices are available and have become more affordable. However, farmers’ decision making is affected by many other socio-economic drivers, such as access to information and financial capacity (Baumgart-Getz et al. 2012, Davidson et al. 2015, Liu et al. 2018). To promote these technologies and strategies, we need more inter- and transdisciplinary research on farmers’ decision making and further collaboration of academic researchers, government, farmers, private crop advisors, fertilizer producers, and consumers (Davidson et al. 2015).
4.2 Global Targets and Regional Priorities
The priorities on the global and national scales are different. On a global scale, food security and P pollution challenges are the most urgent. The global P fertilizer demand to supply ratio was approximately 10%, meaning sufficient P supply for global crop production.
To meet the moderate food security goal (FAO 2018), we would need to increase the global harvested P from the current 13.6 Tg P yr-1 to 20.5 Tg P yr-1 by 2050 (Table 3). To reduce the global P surplus from the current level (9.1 Tg P yr-1) to the planetary boundary (4.5-9 Tg P yr-1), we would need to improve the current cropland PUE from 60% to 69%-82% by 2050.
Given their different socio-economic and agronomic conditions, different regions have unique challenges in P use to address (Fig. 6). In the BAU scenario, India, Pakistan, and a few other countries have all three priorities: increasing yield, reducing P surplus, and enhancing P sufficiency. While in countries such as China and Brazil, the priorities are to increase yield and reduce P surplus. The U.S., Russia, and Morocco would only need to focus on increasing yield. In most other countries, especially African countries, the priorities are to increase yield and enhance P sufficiency (Fig. 6a).
As PUE improves, most countries would face fewer challenges. In the MPA scenario, reducing the P surplus would not be the priority in most countries, except for China, Japan, and a few other countries (Fig. 6b). In the most ambitious HPA scenario, the challenges in most countries would be to increase yield from 2010 levels and enhance P sufficiency (Fig. 6c).