Human activity emits large amounts of nitrogen and phosphorus into the environment: over the last 30 years, nitrogen emissions have increased steadily, while phosphorus emissions have remained more or less constant. This has resulted in a global imbalance between these two nutrients in land and water, putting species and ecosystems at risk. This is one of the main conclusions of our article 'The global nitrogen-phosphorus imbalance' (1), published in January 2022 in the journal Science based on our recent research data. In the text, we present the state of the issue and its scope to the international scientific community, as well as alternatives and solutions aimed at political decision-makers. We propose solutions such as precision farming, biotechnology, regulations to recycle phosphorus, and reducing livestock production.

In recent decades, humans have enriched the biosphere with nitrogen through over-fertilisation and thus altered its relationship with phosphorus. This is why we claim that human action is altering the ratio between these two elements essential for life, which affect the growth rate of micro-organisms, plants and animals. The balance between nitrogen and phosphorus is key to photosynthesis and the creation of structures by plant species, for example. Furthermore, for optimal growth, adequate amounts and proportions of these two elements are needed. Different species can have optimal functioning under different proportions of nitrogen and phosphorus, and therefore we change the balances of nature when altering these proportions in the environment.

We therefore believe that the time has come for national and international environmental agencies and policy makers to recognise the risks posed to the biosphere and humanity by the imbalance between nitrogen and phosphorus. We strongly advocate that international environmental agencies should address this situation through a coordinated international policy.

JOSEP PEÑUELAS REIXACH^1,2, JORDI SARDANS GALOBART²

¹CSIC | Spain

²CREAF researcher | Spain

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Among the possible alternatives, we recommend increasing the efficiency of nitrogen and phosphorus use and cycling through precision farming, which avoids disproportionate fertiliser application. We also advocate the implementation of both management methods and innovative biotechnology to increase the efficiency of plants in capturing nutrients and benefiting from phosphorus sources. Other necessary policies include stimulating phosphorus recycling through international, national and regional regulations, subsidies or legislation, as well as reducing livestock production. In fact, some of these measures are already in the early stages of implementation in some countries. 

Humans are over-fertilising the biosphere with nitrogen through nitrogen oxides emitted when we burn fossil fuels, when we plant nitrogen-fixing crops, and when we use excessive fertilisers that leach into waterways. Although human activities have also increased the amount of phosphorus in soils and waters - for example, applying phosphorus-rich fertilisers and detergents - the overall increase in phosphorus in the soil is still smaller and more stable than the ever-increasing nitrogen.

In fact, these are two synergistic problems. On the one hand, the presence of nutrients in the soil has increased disproportionately, and on the other hand, the balance between nitrogen and phosphorus has been disturbed. When there are too many nutrients in the environment, it becomes eutrophic: the increase of nutrients in freshwater causes algae and phytoplankton to grow out of control, until the ecosystem collapses. As a result, some countries have developed water treatment strategies aimed at reducing the concentration of chemical compounds. However, the technology used by water treatment plants retains more phosphorus than nitrogen, further encouraging an imbalance between the two nutrients.

The global imbalance between nitrogen and phosphorus may be even greater at the local and regional level, because the inputs of these two compounds are not evenly distributed around the world. Phosphorus is less soluble in water and does not volatilise, is often absorbed and precipitates in the soil in mineral form, and accumulates in soils and sediments in chemical forms that are often unavailable to living organisms such as microbes and plants. Therefore, it tends to remain close to the source of emission. In contrast, nitrogen is much more soluble in water and much more volatile, which makes it easier for it to be dispersed over a larger radius with respect to its emission source.

The biological impacts of the growing imbalance between the two nutrients have been observed in inland water bodies, on the structure and function of soil living communities, as well as on the species composition of plant communities. The lack of stability will have an increasing impact as the imbalance continues to shift in the same direction.

Food security and agricultural production are also impacted by this imbalance, which has a direct impact on natural ecosystems, crops and people. Fertilisers containing nitrogen have an unlimited source - the atmosphere - from which this nutrient can be extracted through the Haber-Bosh reaction. This innovation has allowed its production to increase steadily, as well as its use as a fertiliser since the 1950s. However, sources of phosphorus have been largely limited to mines and are concentrated in a few countries, such as Morocco.

In this sense, phosphorus could become economically inaccessible to low-income and food-deficit countries as these sources become depleted or unavailable due to geopolitical and economic issues. In the future, phosphorus-producing countries are likely to manage their reserves to maximise profits for domestic mining and agricultural industries, making phosphorus-based fertilisers increasingly unaffordable for farmers in poorer countries and further exacerbating the imbalance between the two nutrients in regions where the problem is most acute. It would be a crisis that would make the economic gap between rich and poor countries even more evident.

The lack of balance between these two elements in the soil modifies the chemical composition of crops and can affect the health of people who consume products grown on these soils, thus causing a public health problem. For example, in regions where there is excessive use of inorganic and organic phosphorus fertilisers, phosphorus accumulates in soils and water bodies. Food produced in these environments can lead to over-consumption of phosphorus by the local population, which can have negative implications for their health. Nutrient imbalances are also known to cause infectious and non-infectious human diseases clearly associated with diet, such as coeliac disease. In 2021 we already warned that excessive nitrogen fertilisation of wheat crops could explain the high prevalence of coeliac disease, from the article 'Could Global Intensification of Nitrogen Fertilisation Increase Immunogenic Proteins and Favour the Spread of Coeliac Pathology?' (2) published in the journal Foods.

When the relationship between nitrogen and phosphorus is destabilised, human activities also generate imbalances between other elements. For example, changes have been observed in plant tissues in the ratio of carbon to nitrogen, in relation to iron, zinc, calcium and potassium, among others. This indirectly leads to the fact that organisms, communities and ecosystems on planet earth are seeing changes in their elementome, i.e. the combination of chemical elements of each species. According to the article 'Empirical support for the biogeochemical niche hypothesis in forest trees' (3), published in January 2021 in Nature, Ecology and Evolution and of which Jordi Sardans is first author, it is confirmed with robust data that each species has a unique atomic identity that, like the genome, varies from one species to another according to its evolutionary proximity and environmental conditions.

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