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).