Knowledge for better food systems

Paper: Residual soil phosphorus as the missing piece in the global phosphorus crisis puzzle

FCRN mailing list member Ken Giller is one of the authors of this paper on phosphorous which finds that average global phosphorous needs in 2050 will be less than previous estimates assume due to the presence of residual phosphorous in soils (from historical applications). The implication of this is that the global phosphate supply will last longer than hitherto expected.

Abstract

Phosphorus (P) is a finite and dwindling resource. Debate focuses on current production and use of phosphate rock rather than on the amounts of P required in the future to feed the world. We applied a two-pool soil P model to reproduce historical continental crop P uptake as a function of P inputs from fertilizer and manure and to estimate P requirements for crop production in 2050. The key feature is the consideration of the role of residual soil P in crop production. Model simulations closely fit historical P uptake for all continents. Cumulative inputs of P fertilizer and manure for the period 1965–2007 in Europe (1,115 kg⋅ha−1 of cropland) grossly exceeded the cumulative P uptake by crops (360 kg⋅ha−1). Since the 1980s in much of Europe, P application rates have been reduced, and uptake continues to increase due to the supply of plant-available P from residual soil P pool. We estimate that between 2008 and 2050 a global cumulative P application of 700–790 kg⋅ha−1 of cropland (in total 1,070–1,200 teragrams P) is required to achieve crop production according to the various Millennium Ecosystem Assessment scenarios [Alcamo J, Van Vuuren D, Cramer W (2006) Ecosystems and Human Well-Being: Scenarios, Vol 2, pp 279–354]. We estimate that average global P fertilizer use must change from the current 17.8 to 16.8–20.8 teragrams per year in 2050, which is up to 50% less than other estimates in the literature that ignore the role of residual soil P.

The authors write in the discussion section: “Our results show that accounting for the role of residual soil P leads to lower projections of P demand for the period 2008–2050. Global P application rates per hectare will increase, but less than proportional to the required increases in production and yield. However, the situation differs among continents. In Europe, Asia, Latin America, and Oceania, crop production can benefit from the residual P accumulated due to past P fertilizer and manure use and P uptake can increase even with a reduction in P application rates between 2008 and 2050. In contrast, target P uptake rates can be achieved in North America with a slight increase in P application rates. Due to the minimal P application rates in the past decades, in Africa more than a fivefold increase from 4 kg·ha−1 in 2007 to ∼23 kg·ha−1 in 2050 is needed to achieve the target P uptake …Our estimate of the global P input required in global cropland of 1,200 million tons P for the 2008–2050 period includes P from both mineral fertilizer and animal manure. The total amount of P use in 2050 is smaller than recent projections in all regions except Africa. Accounting for the relative contribution of P from animal manure in global croplands under the GO scenario (32%) [ NB the Global Orchestration scenario of the Millennium Ecosystem Assessment http://www.maweb.org/documents/document.332.aspx.pdf ] , global inorganic P fertilizer use must be 20.8 Tg in  2050. This is 10–40% less than estimated in other studies. The demand is even less for the other scenarios. However, a large part of the P in animal manure that is recycled in cropland originates in grasslands. This transfer of P from grasslands to cropland is particularly important in developing countries, and contributes to the build-up of residual soil P in cropland. Given the increasing future demand for grass, additional fertilizer P will be required to maintain soil fertility in the world’s soils under grassland. Our results suggest that residual soil P can contribute to crop production with a considerable lag time. The model results provide important information on where and how much P is needed to achieve food security in 2050.”

 

Reference

Sattari S Z, Bouwman A F, Giller K E and van Ittersum M K (2012).  Residual soil phosphorus as the missing piece in the global phosphorus crisis puzzle, PNAS April 17, 2012 vol. 109 no. 16 6348-6353

You can download the paper here.

The Wuppertal Institute covers the study here.

 

Commentary

There is also a commentary on the study as follows: 

The commentators write: “Sattari et al.’s analysis creates optimism that present and future crops can recoup some benefits from the mistakes of our past. Yet while information on accumulated soil P ought to inform decisions about fertilizer needs, there are several reasons to temper our optimism. History also teaches us that, in addition to crop requirements, a web of biophysical and socioeconomic factors drives agricultural decisions and outcomes.”  For example, farmers often apply too much fertiliser as a form of insurance, and soil erosion ratesare high, this means the phosphorous surpluses flow into water sources, causing eutrophication.  Extreme climate events can make matters worse.  Greater understanding of the interactions between C, N and P cycles is needed, through the development of models that could help in understanding feedbacks that play out at multiple time scales.

Reference of commentary

Townsend AR, Porder S. Agricultural legacies, food production and its environmental consequences. Proceedings of the National Academy of Sciences. 2012 Apr. 17;109(16):5917–5918.

You can download the commentary here.

 

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