Increased phosphorus (P) fertilizer use and livestock production has fundamentally altered the global P cycle. We calculated spatially explicit P balances for cropland soils at 0.5° resolution based on the principal agronomic P inputs and outputs associated with production of 123 crops globally for the year 2000. Although agronomic inputs of P fertilizer (14.2 Tg of P·y −1 ) and manure (9.6 Tg of P·y −1 ) collectively exceeded P removal by harvested crops (12.3 Tg of P·y −1 ) at the global scale, P deficits covered almost 30% of the global cropland area. There was massive variation in the magnitudes of these P imbalances across most regions, particularly Europe and South America. High P fertilizer application relative to crop P use resulted in a greater proportion of the intense P surpluses (>13 kg of P·ha −1 ·y −1 ) globally than manure P application. High P fertilizer application was also typically associated with areas of relatively low P-use efficiency. Although manure was an important driver of P surpluses in some locations with high livestock densities, P deficits were common in areas producing forage crops used as livestock feed. Resolving agronomic P imbalances may be possible with more efficient use of P fertilizers and more effective recycling of manure P. Such reforms are needed to increase global agricultural productivity while maintaining or improving freshwater quality.

Disparities between the nutrients applied to agricultural soils via fertilizer or manure and the nutrients removed by harvested crops result in nutrient imbalances that can influence environmental quality and productivity of agricultural systems (1). Growing consumption of inorganic phosphorus (P) fertilizers derived from mining of nonrenewable phosphate rock (2) has contributed to major increases in crop yields since the 1950s (3). Concurrent growth in fertilizer use and livestock production has more than tripled global P flows to the biosphere over preindustrial levels (4), resulting in P accumulation in some agricultural soils that acts as a driver of eutrophication in freshwater and coastal systems (5–7). At the same time, limited availability of P fertilizers in other regions has contributed to prolonged P deficits that can deplete soil P and limit crop yields (8–10). Although agricultural P surpluses and deficits have been documented for several regions (e.g., refs. 11 and 12), there is still limited understanding of the spatial patterns of P imbalances at the global scale.

Patterns of nutrient imbalances across agricultural systems may reflect contrasting agricultural practices, economic development, and broader agricultural policies (1, 13). Understanding agricultural P use is key to managing global phosphate rock reserves (14) and mitigating the risk for potentially irreversible eutrophication of lakes (15). Despite considerable advances in the development of spatially explicit global nitrogen balances (e.g., ref. 16), most previous global P balance studies have relied on globally or regionally aggregated data (4, 5, 17, 18), limiting our ability to infer spatial patterns of surpluses and deficits. The only spatially explicit global P balance study that we are aware of used estimates of inputs and outputs based primarily on regional or national agricultural statistics distributed over four aggregated cropping systems by using the IMAGE model (19). Here, we use empirical data to calculate P balances for croplands circa the year 2000 at 0.5° resolution in latitude and longitude (∼50 × 50 km) to examine patterns of agronomic P imbalances globally. These P balances were calculated by using spatial estimates of the principal agronomic P inputs (P fertilizer and manure applications) and outputs (P in harvested crops) for cropland soils based on spatially explicit global maps of >100 crops.

Results

Spatial Patterns of Agronomic P Imbalances. We classified P surpluses and deficits by quartiles to compare P imbalances across all regions globally (Fig. 1). In total, 29% of the global cropland area had overall P deficits and 71% of the cropland area had overall P surpluses. A sizeable fraction of the global cropland area (∼31%) had only small negative or positive imbalances (within ±2 kg of P·ha−1·y−1 from zero), corresponding to the lowest two quartiles for deficits and the lowest quartile for surpluses (Fig. 2). These minor imbalances occurred in every region but were most prevalent in Africa and Oceania. Fig. 1. Global map of agronomic P imbalances for the year 2000 expressed per unit of cropland area in each 0.5° grid cell. The P surpluses and deficits are each classified according to quartiles globally (0–25th, 25–50th, 50–75th, and 75–100th percentiles). Fig. 2. Distributions of P surpluses and deficits by quartiles shown as percent of total cropland area in each continent and as percent of global cropland area. Moderate P imbalances [lower-middle and upper-middle quartiles for surpluses (3–13 kg of P·ha−1·y−1) and upper-middle quartile for deficits (−2 to −3 kg of P·ha−1·y−1)] were characteristic of croplands in every region except Africa, occurring in 47% of croplands globally. The largest share of moderate surpluses occurred in South Asia (India, Pakistan, and Thailand) and North and Central America (United States, Canada, and Mexico). Moderate deficits occurred in only 8% of the global cropland area, largely in Eastern Europe (Russia and Ukraine) and West Africa, as well as smaller tracts of other regions, such as southeastern Australia. The largest imbalances of agronomic P, corresponding to the top quartiles of both deficits and surpluses (−3 to −39 kg of P·ha−1·y−1 and 13–840 kg of P·ha−1·y−1, respectively), were spatially concentrated in certain areas. Just 10% of the global cropland area with the largest P deficits contributed 65% of the cumulative global P deficit (Fig. 3). The most widespread large deficits were in South America (particularly Argentina and Paraguay), the northern United States, and Eastern Europe. Similarly, 10% of the cropland area with the largest surpluses contributed 45% of the cumulative global P surplus. These large surpluses (which had a median value of 26 kg of P·ha−1·y−1) covered most of East Asia, as well as sizeable tracts of Western and Southern Europe, the coastal United States, and southern Brazil, but <2% of the cropland in Africa (Fig. 2). A more detailed breakdown of the P balance quartile ranges and variations by continent highlights the particularly large intraregional variation in agronomic P imbalances in Europe and South America (Fig. S1). Fig. 3. Cumulative distributions of global cropland P imbalances (surpluses or deficits, sorted from largest to smallest) in relation to cumulative global cropland area.

Global Agronomic P Flows. Fertilizer application to croplands in the year 2000 totalled 14.2 Tg of P·y−1, of which more than half was applied to cereal crops. The largest P fertilizer application rates occurred predominantly in East Asia, Western Europe, the midwestern United States, and southern Brazil [Fig. S2 and Potter et al. (20)]. Approximately 9.6 Tg of P·y−1, or 40% of total manure P excreted by livestock in 2000 (20), was used for cropland application based on estimates of recoverable manure for 12 regions (21) and for US states (22). Recoverable manure P shows much greater spatial variation than P fertilizer applications (Fig. S2), with clusters of more intense manure P applications occurring in many countries (such as the United States and Brazil) and more widespread high manure P applications in East Asia and Western Europe. The production of 123 crops in the year 2000 (23) removed 12.3 Tg of P·y−1 from cropland soils. The greatest crop P removal occurred in the northern United States, Western Europe, East Asia, South America (particularly southern Brazil and Argentina), and Australia, largely reflecting crop yields. Cereal crops accounted for approximately half and by far the largest share of P removal, most of which was attributable to harvest of wheat, maize, and rice. Our global estimate of total P inputs to cropland soils exceeds P removed by harvested crops, resulting in a global agronomic surplus of 11.5 Tg of P·y−1. We also calculated P balances based on contrasting crop residue management scenarios by using plausible high and low residue recycling and removal estimates from Smil (24) that reflect broad differences in residue management between developed and developing countries (detailed in SI Methods). The high residue removal scenario resulted in a slight decrease in our global P balance estimate (to 11.2 Tg of P·y−1), whereas the low residue removal scenario resulted in a considerable increase (to 12.5 Tg of P·y−1) due to the influence of residue P recycling inputs. These crop residue scenarios had minimal influence on the spatial patterns of P surpluses and deficits (Fig. S3).

Agronomic Drivers of Cropland P Imbalances. We found considerable spatial variation in the main drivers of P surpluses based on the magnitudes of fertilizer and manure inputs relative to crop P use (using crop P removal as a proxy for crop use) (Fig. 4A). Fertilizer alone exclusive of manure inputs exceeded crop P use in the largest fraction of P-surplus cropland in all continents except Africa (Fig. 4B), and particularly in intensive agricultural regions of Asia and North America (40% of the cropland area in each continent). The combination of fertilizer and manure was the primary driver of P surpluses in ∼30% of the global cropland area with P surpluses; manure and fertilizer each individually exceeded crop use in half of this area, particularly in southern China and eastern Brazil, whereas the sum of fertilizer plus manure exceeded crop use in the remaining half. Manure P alone exclusive of fertilizer P exceeded crop use in only 11% of croplands globally, particularly in areas with high livestock densities but relatively limited cropland areas (e.g., parts of the United States) or in regions with relatively low P fertilizer application and low P surpluses (e.g., across central Africa). Fig. 4. Agronomic drivers of P surpluses based on the magnitude of fertilizer or manure P applied relative to crop P use in different locations (A) and summarized according to percent of cropland area by continent and globally (B). Each category is mutually exclusive based on locations where either fertilizer alone or manure alone exceeded crop P use, where fertilizer and manure each individually exceeded crop P use, or where only the sum of fertilizer and manure exceeded crop P use. Half of the cropland area with the largest P surpluses (>13 kg of P·ha−1·y−1) globally corresponded to locations where P fertilizer and manure applications each individually exceeded crop P use, but P fertilizer application alone was particularly influential in some regions. When summed across both categories (Fig. 4B), P fertilizer applications exceeding crop P use coincided with a greater proportion (87%) of the global cropland area that had large P surpluses compared with manure P applications in excess of crop P use (62%). In particular, a much greater proportion of the large P surpluses in Asia and South America corresponded to locations where P fertilizer application alone exceeded crop P use compared with areas where manure P alone exceeded crop P use. Roughly the same proportion of cropland areas with large P surpluses in Europe and North America corresponded to locations where either fertilizer or manure P, or both, exceeded crop P use. The types of crops grown contributed substantially to the locations of deficits. Forage crops, and particularly grasses, were associated with large P deficits in several regions. These crops received ∼5% of the total global P fertilizer application in countries with crop-specific fertilizer data in 2000 (which collectively represent 95% of global P fertilizer inputs), yet they accounted for >20% of global crop P removal. Approximately 13% of crop P removal globally was attributable to mixed leguminous grasses and alfalfa, which may receive manure applications but are only fertilized in a few countries (25). Nonforage croplands in several areas had small or moderate P surpluses (e.g., throughout the United States and Australia) (Fig. S4), confirming that some P deficits (Fig. 1) were linked to harvest of forage crops. Forage crops were less influential for P deficits in other locations (e.g., Argentina and Nigeria) (Fig. S4; see SI Methods for further explanation). For example, the concentration of top quartile deficits in South America was primarily related to soybean harvest in Argentina and, to a lesser extent, harvest of grasses and wheat. Soybean received on average 2.5 kg of P·ha−1·y−1 of P fertilizer in Argentina circa 2000 (10% of the reported P fertilizer rate for soybean in neighboring Brazil; ref. 25), which was a small fraction of the P removal rate for soybean (15 kg of P·ha−1·y−1).