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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