Figure 6. Regional priorities in the three scenarios. (a) Regional priorities in the BAU scenario. (b) Regional priorities in the MPA scenario. (c) Regional priorities in the HPA scenario. Yellow area: regions with the priority to increase P yield (i.e., estimated 2010 average P yield is lower than the projected 2050 P yield). Blue area: regions with the priority to enhance P sufficiency (i.e., P fertilizer demand to supply ratio is greater than one if assuming P fertilizer to total input ratio remains at the 2010 level). Orange area: regions with the priorities to increase P yield and reduce P surplus (i.e., P surplus rate in 2050 is larger than the upper planetary boundary, 6.9 kg P ha-1 yr-1). Green area: regions with the priorities to increase P yield and enhance P sufficiency. Pink area: regions with all three priorities.
For countries facing the pressing risk of depleted reserves (e.g., India and Mexico, Fig. 6a-c), their priorities should also include investing in technologies and infrastructures that can recover and recycle P from sources such as manure and waste. Establishing stable trade relationships with countries with rich reserves could be another strategy to address this challenge.
Policymakers should also recognize the heterogeneity of P pollution within a country. Some regions have environmental problems more acute than those in other regions. In the U.S., even though the P surplus was negative as a country (Table 3), many regions in the U.S. (e.g., the Great Lakes region and the Gulf of Mexico) are still suffering from eutrophication caused by high N and P loads to the water (Van Meter et al. 2018, Le Moal et al. 2019).
4.3 Potential Sources of Uncertainties
4.3.1 Phosphorus Inputs
In this study, we only consider fertilizer and manure as P inputs. Other inputs’ data by country and crop type are not available, and they only accounted for a small amount of total P input in the previous studies. Between 2002-2010, P deposition, crop residues, seed, and sludge were around 2%, 13%, 1%, and 5% of total P inputs to global cropland, respectively (Lun et al. 2018).
4.3.2 Phosphorus Content
Another source of uncertainty in P budget work is the P content of each crop. Previous studies usually used nutrient content data from published work with different estimates (Zhang et al. 2020). Besides, these studies assumed that P content by crop type would not change over time and that the P contents were consistent in all spatial units (Bouwman et al. 2017, Lun et al. 2018). We made the same assumption in this study. Our results are comparable to previous studies on P budget analysis on the global scale (Table S5 and S6).
By assuming the same P content in all countries and years, the impact of this uncertainty on the analysis of P budget historical trends becomes smaller (Bouwman et al. 2017). To quantify this impact, we conducted a Monte Carlo simulation (1,000 iterations) for major countries and crop types, testing both normal and uniform distributions (SI Section Text S6). We found that this uncertainty did not affect PUE significantly. However, further studies on whether and how each crop’s P content varies by country and time are of great value.
4.3.3 Phosphorus Planetary Boundary
Estimates of P planetary boundary and methodologies for calculation remain uncertain. Steffen et al. (2015) estimated the regional planetary boundary range for fertilizer P going to erodible soil as 6.2-11.2 Tg P yr-1, while Springmann et al. (2018) suggest the planetary boundary range for global P fertilizer input at 6-12, or 8-17 Tg P yr-1, depending on the recycling rate of P. Here we use P surplus rather than P fertilizer input to evaluate P pollution because 1) P surplus measures the amount of applied P that is subject to being accumulated in soil or lost to the environment; 2) it has more direct environmental impacts than P fertilizer input; 3) a similar indicator, N surplus, have been proposed for tracking progress towards reductions in nutrient pollution caused by food production on farm to regional scales (Eagle et al. 2018).
Note that the P surplus does not reflect the actual environmental impacts. Thus, the estimated boundary should only be used as a reference point to provide a general direction for P management improvement. It warrants more research to understand: 1) whether the concept of a planetary boundary for P is useful for informing policy; 2) which indicators, such as global and regional P surplus, could be used for setting such a planetary boundary target; and 3) how to interpret the implication of the global target on a local to regional scale.
4.3.4 Phosphorus Residual
Partitioning P surplus into residual P retained in the soil and P loss is also challenging to assess on national to global scales. Sophisticated biogeochemistry modeling is necessary, but not yet well developed on the scale we were working on. There are at least two ways to estimate P loss. One way is to develop a dynamic soil model to evaluate annual change considering varying soil characteristics and P budget (Zhang et al. 2017). Another simpler way is to assume a constant fraction of P in inputs or surplus, leaving the cropland system. Sattari et al. (2012) assumed that P loss accounts for around 10% of the sum of fertilizer and manure inputs. Bouwman et al. (2013) and Lun et al. (2018), on the other hand, assumed that P loss is about 12.5% of total P inputs. Even more conservatively, Springmann et al. (2018) assumed 20% of P stored in the sediment was lost.
In this study, P loss data during the historical period (i.e., 1961-2014) are from the IMAGE-Global Nutrient Model (Bouwman et al. 2017). To project P loss after 2014, we assumed that 12.5% of P inputs would leave the cropland system (Bouwman et al. 2013, Lun et al. 2018) as P loss, to ensure that P loss had non-negative values. Note that the uncertainties in P loss estimation do not affect the calculation of P surplus and PUE, but they do affect the calculation of P residual. The estimation of P residual will be improved when more soil data before 1961 are available, and more mature soil models are developed.
4.3.5 Projection of Phosphorus Budget
The projection of future P budget was based on certain assumptions and can introduce uncertainties. We adopted the projection data of the middle pathway scenario developed by FAO (FAO 2018). This scenario assumed that moderate food security improvement would be achieved by 2050 (FAO 2018). This assumption means that the 2050 harvested P may have been underestimated if a more ambitious food security goal will be achieved. Also, given that most African countries are still at the early stage of agricultural intensification, the PUE in these countries in the three scenarios could have been overestimated, and their P inputs could have been underestimated. However, if PUE on the national scale will be significantly improved (such as in the HPA scenario) and more alternative P sources will be available, P pollution and depletion problems will still be properly addressed.
To project future P fertilizer input, we assumed that each country would maintain its current fertilizer to manure use ratio (average of 2005-2014) from 2015 to 2050. To estimate the uncertainties of this assumption, we have developed two other extreme cases to discuss how P input sources will affect P depletion (see Section 3.3.2). The results show that these uncertainties do not change our conclusions.
5. Conclusion
The world faces P depletion and pollution challenges, which can be measured by P fertilizer demand to supply ratio and P surplus. Improving PUE in crop production is one key pathway to address these two challenges and achieve synergies between agricultural productivity, sustainability, and resilience. To keep the global P surplus within safe limits, we would need to improve the global PUE in crop production from the current 60% to 69%-82%. On a regional scale, priorities and PUE improvement levels vary by local conditions. To achieve PUE improvement goals, we need strategies targeting key socio-economic and agronomic drivers for PUE. These drivers include economic development, crop mix, NUE, fertilizer to crop price ratio, and average farm size.
In this resource-limited and developed world, addressing P challenges also requires efforts beyond improving cropland PUE. In regions with limited phosphate rock reserves, addressing P depletion also depends on P import and alternative sources. While in regions where agriculture is not the only source of P pollution, managing non-agricultural P loads should be part of the solution. If we can effectively reduce P loads from non-agricultural sources (e.g., industrial and domestic wastes (Chen and Graedel 2016, Mekonnen and Hoekstra 2018)), the boundary for P surplus from cropland could be relieved. Methods to reduce non-agricultural P loss include, for example, reducing P loss along its supply chain (Nedelciu et al. 2020) and recovering P from wastewater treatment plants (Chrispim et al. 2019).
Acknowledgments
We thank the OCP Research LLC for providing financial support and valuable feedback.
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