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Phosphorus: a limited resource essential to agriculture in the 21st century

Modelling work conducted at planetary scale has demonstrated that, regardless of climate change models used, the availability of phosphorus determines the level of biomass production and, consequently, of carbon storage in ecosystems responding to these changes. Contrary to expectations, the work revealed that phosphorous limitations are less strong in tropical ecosystems than in the rest of the world.  

Applying manure to crop cover. © INRA, FORMISANO Sophie
Updated on 06/09/2017
Published on 04/21/2015

Phosphorus (P) lies at the intersection of food security and climate change mitigation concerns. Two articles recently published by INRA researchers shed light, nationally and globally, on these issues.

The need to measure agriculture’s dependence on synthetic fertilisers

Steadily increasing demand for agricultural biomass for food and non-food uses, and growing scarcity of the resources needed to produce fertilisers (natural gas and energy for nitrogen, phosphate rocks for P) raise the question of agriculture’s dependence on synthetic fertilisers. Dependence on nitrogen is usually calculated on an annual basis to estimate the ratio between the amount of nitrogen applied through synthetic fertilisers and the nitrogen content of the harvested products. In this way it was demonstrated that, on a planetary scale, 40% of protein production is dependent on synthetic nitrogen fertilisers. Similar statistics do not exist for P, even though the growing scarcity of phosphate rock resources – found in geological deposits and necessary to make P fertiliser – is a threat to global food security in the medium term. Calculating agriculture’s dependence on synthetic P fertilisers is more complex because, unlike nitrogen, P accumulates in soil, leading to false estimates when calculations are made on an annual basis.

Measuring the effect of phosphorus availability on carbon sequestration

In ecosystems with less human activity, responses to global changes linked to the increase of CO2 in the atmosphere and of atmospheric nitrogen deposition are limited by stoichiometric imbalances that influence other nutrients, P in particular. Consequently, carbon sequestration and climate change mitigation capacities are lower than predicted by global models that do not account for the low availability of P in the soil of a number of ecosystems. Measuring this effect is a key challenge for predicting future climates and for developing mitigation scenarios. 

Applying a modelling approach may lead to national and international solutions to these challenges.  

Agricultural production and soil fertility depend strongly on phosphate fertiliser accumulating in the soil over decades

The model put forward to measure how much P fertility in farm soils depends on synthetic P fertilisers made it possible to reconstruct changes in soil P fertility in France since the Second World War. Since 1948, P stocks used in plant nutrition, which is broken down into two compartments, labile P and stable P, have increased significantly. Stock levels tended to stabilise in the 1970s, in connection with the declining use of fertiliser. The model demonstrated that, at present, approximately 82% (ranging from 68% to 91%) of soil P is from human activity, meaning it came either directly (through applied synthetic fertilisers) or indirectly (through applied organic fertilisers containing P originally from synthetic fertilisers) from geological deposits used to make P fertilisers.

Likewise, approximately 84% of the phosphorus in agricultural products is human induced.

If these figures had been calculated on an annual basis, as is the case for nitrogen, they would lead to an underestimation of human-induced phosphorus: 51% and 60% respectively. This reflects the advantage of the model in calculating more accurately our dependence on P fertility and agriculture’s dependence on synthetic P fertilisers. These findings show that, despite considerable declines in the use of synthetic P fertilisers since the 1970s due to the introduction of more sustainable fertilisation practices, agricultural production in France remains very strongly reliant on the legacy of P fertility it has inherited from decades of using phosphorus extracted from geological deposits.

Adapting the model to global scale

The fertilisation-related P flows model is currently being used in a research project seeking to identify the determining factors affecting phosphorus levels in farm soils worldwide. The model will be applied to a range of countries to assess their dependence on soil P fertility and the dependence of their agriculture on human-induced P from past agricultural practices (particularly the length of time they have been using synthetic fertilisers). The aim is to have a global picture of soil P fertility dependent on synthetic fertiliser use.

A second research project is studying the role of P availability in regulating ecosystem responses to global changes. The aim is to refine predictions for various world regions and for different types of biomes. Estimating soil P availability at global scale represents a considerable challenge to planetary models.

Phosphorus experts agree that it is necessary to develop an international framework to govern phosphorus resources. This will first require significant communication and education measures to raise collective awareness of and a sense of responsibility towards this global problem. Efforts to this end are underway in Europe through the work of the European Sustainable Phosphorus Platform.

References

- Penuelas, J., Poulter, B., Sardans, J., Ciais, P., van der Velde, M., Bopp, L., Boucher, O., Godderis, Y., Hinsinger, P., Llusia, J., Nardin, E., Vicca, S., Obersteiner, M., Janssens, I. A. (2013) Human-induced nitrogen–phosphorus imbalances alter natural and managed ecosystems across the globe. Nature Communications, 4. DOI: 10.1038/ncomms3934.
- Ringeval, B., Nowak B., Nesme T., Delmas M., and Pellerin S. (2014) Contribution of anthropogenic phosphorus to agricultural soil fertility and food production. Global Biogeochem. Cycles, 28, DOI: 10.1002/2014GB004842.
- Special issues of Plant and Soil and Nutrient Cycling in Agroecosystems are currently being prepared, as is special research topic in Frontiers in Nutrition and Environmental Sustainability.

Contact(s)
Scientific contact(s):

Associated Division(s):
Environment and Agronomy, Forest, Grassland and Freshwater Ecology
Associated Centre(s):
Nouvelle-Aquitaine-Bordeaux, Occitanie-Montpellier

Phosphorus in numbers

- In France

  • French agriculture’s dependence on synthetic phosphate fertilisers: 84% (meaning 84% of the phosphorus in agricultural products comes from synthetic fertiliser accumulated in the soil)
  • Dependence of soil fertility on synthetic phosphate fertilisers: 82% (meaning 82% of the phosphorus in soil comes from accumulated synthetic fertiliser)
  • Phosphorus losses : 52% of phosphorus from synthetic phosphate fertiliser is lost each year through runoff.
  • Certain areas have surplus soil phosphorus (Brittany, through use of livestock manure) while others are deficient (Centre region)

- Internationally

  • Stocks of phosphate rock will run out within 100 to 300 years (used to make mineral fertilisers)
  • Morocco, China, and the United States control 85% of phosphate mines

Raising awareness

This research was presented at the Phosphorus in Soils and Plants Fifth International Symposium (PSP5) held, for the first time in Europe, on 26–29 August 2014 in Montpellier, France. The focus of the symposium was “Facing Phosphorus Scarcity”. It was attended by 276 participants from some 40 countries.

This was followed by the fourth Sustainable Phosphorus Summit (SPS4), held on 1–3 September 2014, with 208 participants from 40 different countries. It was organised by the Global Phosphorus Research Initiative (GPRI), which wanted to increase awareness in the international scientific community, and in society at large, of the importance of phosphorus issues.  Between these two events, a Junior Researchers’ Seminar was held on 31 August 2014 with the support of the AGRO Laboratory of Excellence (LABEX). It brought together 42 scientists from the global North and South, who prepared a presentation that was delivered at SPS4 on the major research issues and obstacles.

These events together constituted Phosphorus Week 2014, organised by INRA and CIRAD. The week was an opportunity for researchers from across a wide range of fields to meet, including not only researchers studying agriculture and the environment, but also process science, social sciences, and politics, spanning the entire phosphorus cycle and its applications extending beyond the agricultural industry alone. Political and economic stakeholders in fields such as the agricultural supply industry, biotechnology, and water purification also participated.

 
The next Sustainable Phosphorus Summit (SPS5) will be held in China in 2016, and aims to work in cooperation with the Global Partnership for Nutrient Management (GPNM). The next Phosphorus in Soils and Plants symposium (PSP6) will be held in 2018 in Belgium and will seek, as the Montpellier symposium did, to increase the involvement of researchers and stakeholders from the South.