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Biogeochemistry: The fate of phosphorus
- Nature Geoscience
- 9,
- 343–344
- doi:10.1038/ngeo2702
- Published online
Phosphorus is essential for food production, but it is also a key cause of eutrophication. Estimates of phosphorus flux for the past 40–70 years reveal that large river basins can experience phases of phosphorus accumulation and depletion.
Subject terms:
Phosphorus is essential for the production of food, feed, fibre and biofuels. Unlike nitrogen fertilizers, which can be produced industrially through the Haber–Bosch process, phosphorus is mined from finite global reserves1, so there is good reason to use it efficiently. In addition, increased phosphorus concentrations in soils and wastewaters have contributed to widespread eutrophication and the formation of dead zones in coastal regions, with both environmental and economic consequences2 (Fig. 1). Understanding the long-term fate of the phosphorus in terrestrial and aquatic ecosystems can provide a broader perspective on the implications of the use of this important nutrient. Writing in Nature Geoscience, Powers and colleagues3 report past records of phosphorus inputs and outputs in three large world river basins. In two western basins, phosphorus accumulated rapidly when large amounts of fertilizer were applied, and then declined when phosphorus exports became larger than inputs. A third basin in China appears to still be in a phase of rapid phosphorus accumulation.
Phosphorus is a key nutrient for agricultural production, but it can cause eutrophication of inland and coastal waters. Powers and colleagues3 analysed decades of phosphorus fluxes in three river basins. In basins in the USA and UK, phosphorus accumulated until the 1990s, followed by a shift towards a moderate depletion phase. In the Yangtze River Basin in China, phosphorus accumulation started later and has yet to shift to a depletion phase.
© GEOGPHOTOS / ALAMY STOCK PHOTO
Population growth is deeply disrupting the phosphorus cycle on the surface of the Earth. In agricultural systems, the soil phosphorus that is taken up and removed in harvested biomass needs to be replaced in order to sustain production. In the mid-nineteenth century, the discovery of a commercially viable process for producing phosphatic fertilizer from natural phosphorus-rich rocks increased the availability of phosphorus fertilizer and proved essential one century later for feeding the growing global population1. But in the second half of the twentieth century, phosphorus levels in surface waters and sediments increased substantially due not only to diffuse sources of phosphorus from agriculture, but also due to domestic and industrial point sources related to the use of phosphorus-based detergents and the lack of adequate wastewater treatment2. Phosphates have been eliminated from detergents in the United States and Europe, but many countries in Asia lack policies for phasing out phosphates. Regarding diffuse sources, farmers in watersheds draining to the Gulf of Mexico or Chesapeake Bay are, for example, required to adopt nutrient management plans to limit phosphorus export to surface waters. In contrast, fertilizer subsidy programmes in China can act to reduce incentives to limit fertilizer use4.
Part of the soluble mineral phosphorus supplied as fertilizer and phosphorus in wastewater is rapidly removed from solution when it becomes bound to soils or sediments. A large proportion of the resulting phosphorus-bound particles may be deposited on the floodplains, in reservoirs and in deltaic zones5. Moreover, they can also become trapped in dammed rivers6. This accumulation of phosphorus in soils and sediments creates pools of increasing size that are known as legacy phosphorus. The accumulation of phosphorus in soils and rivers raises questions of how to manage such legacy phosphorus stocks. Understanding its accumulation and release will be particularly important in Asia and South America, where the expansion of agriculture and new large dam projects are still promoted.
In order to understand how anthropogenic activity affects phosphorus dynamics on the scale of river basins, Powers and colleagues3 compiled data on phosphorus fluxes over the past 40 to 70 years in the largely rural Maumee Basin, USA and the mixed agricultural–urban basins of the Thames River, UK and the Yangtze River, China. They focused on phosphorus inputs from fertilizer and sewage, and used demographic, economic and physiographic data to estimate annual phosphorus inputs into the basins. Phosphorus removal from each basin consisted of losses in river water or in harvested food and feed. Although other approaches have evaluated phosphorus inputs and outputs on this scale5, 7, the approach by Powers and colleagues3allowed them to identify decadal trajectories of phosphorus dynamics, and to identify patterns of phosphorus accumulation and depletion in the basin. By focusing on basins in the USA, UK and China, they were able to contrast basins with markedly different development trajectories.
Powers and colleagues confirm that in all basins, anthropogenic phosphorus fluxes — additions in the form of fertilizer inputs and exports in the form of food and feed — largely dominate over the phosphorus exported in rivers. They highlight a distinct contrast between the Yangtze Basin, located in a newly industrialized country, and the Maumee and Thames Basins, which were industrialized much earlier. For the latter two basins, phosphorus accumulation occurred up until the 1990s, when a reverse trend led first to a balance between inputs and exports, and then to periods when exports exceeded inputs. This transition is likely due to a decline in fertilizer use and improved infrastructure that contributed to reduced phosphorus inputs in wastewater. In contrast, the Yangtze Basin is still in the accumulation phase, as a result of increasing fertilizer inputs and limited sewage evacuation to surface waters. The remarkable phosphorus accumulation rate of the Yangtze Basin could amount to 8% of the phosphorus mined globally every year, and 17% of the 10 Tg annual increase in soil phosphorus around the world.
Although the study did not quantify the amount of phosphorus in different pools — instead considering only inputs and exports — Powers and colleagues argue that storage of phosphorus in both agricultural soils and in the river network explain its accumulation. Identifying areas of phosphorus accumulation might help stakeholders to promote best management practices. For example, reductions in phosphorus mineral fertilizer applications, improved manure management and limiting erosion would all be beneficial for targeting high phosphorus legacies.
The global picture of phosphorus accumulation reveals different problems in different regions. In regions where phosphorus has accumulated, losses through erosion could represent a long-term threat to river quality8. In poor countries, particularly in sub-Saharan Africa, fertilizer use is limited and phosphorus-deficient soils are unable to support adequate food production for local populations9. From a global perspective, the inherently finite nature of phosphorus reserves raises concerns about the long-term outlook for fertilizer availability and food production around the world. Policies will be needed to address these contrasting phosphorus challenges. For example, the accumulation of large amounts of phosphorus from domestic effluents in urban areas could be repurposed as fertilizer and returned to phosphorus-deficient rural areas.
The approach used by Powers and colleagues3 is particularly useful for addressing some of our societal phosphorus challenges as it provides insights into long-term phosphorus dynamics at the scale of large regional river basins. River basins represent key sources for the eutrophication of coastal and inland waters and are an attractive scale for policy development because they provide the basis for integrated measures of phosphorus dynamics across landscapes. The accumulation–depletion sequence in these basins provides a framework for managing the long-term effects of anthropogenic phosphorus inputs, even after the inputs themselves decline.
References
- References•
- Author information
- Elser, J. & Bennett, E. Nature 478, 29–31 (2011).
- Seitzinger, S. P. et al. Glob. Biogeochem. Cycles 24, GB0A08 (2010).
- Powers, S. M. et al. Nature Geosci. http://dx.doi.org.ezp.sub.su.se/10.1038/ngeo2693(2016).
- Smith, L. E. D. & Siciliano, G. Agr. Ecosyst. Environ. 209, 15–25 (2015).
- Garnier, J. et al. Glob. Biogeochem. Cycles 29, 1348–1368 (2015).
- Maavara, T. et al. Proc. Natl Acad. Sci. USA 112, 15603–15608 (2015).
- Russell, M. J., Weller, D. E., Jordan, T. E., Sigwart, K. J. & Sullivan, K. J. Biogeochem. 88,285–304 (2008).
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- MacDonald, G. K., Bennett, E. M, Potter, P. A. & Ramankutty, N. Proc. Natl Acad. Sci. USA108, 3086–3091 (2011).
Author information
- References•
- Author information
Affiliations
Julien Némery is in the LTHE UMR 5564 Laboratory, University Grenoble Alpes, Grenoble 38000, France
Josette Garnier is in the METIS UMR 7619 Laboratory, University Pierre and Marie Curie, Paris 75005, France