20 November 2023

The phosphorus problem started thousands of years ago


A Swiss study found that humans began altering the phosphorus cycle many millennia ago. Before industrialization and the spread of fertilizers accelerated the process

by Matteo Cavallito


Alteration of the phosphorus cycle at the hands of humans would have begun several thousand years ago. This is supported by a research from the University of Bern, which tried to estimate the amount of the element removed from the soil to end up in lake basins, with all the known negative consequences.

“It is well known that phosphorous from agricultural fertilisation and wastewater is harmful to lakes,” says a statement from the same Swiss university, which calls into question consequences such as eutrophication, fish die-offs, the spread of toxic algae and loss of biodiversity. “But the entry of phosphorous into water is also contributing to the scarcity of this element, which is essential for agriculture.”

The study

Working with colleagues from the United Kingdom and China, the researchers analyzed sediments from 108 lakes around the world. The layers in the core samples they analyzed were up to 12,000 years old. “TThese data enabled us to make a global projection of phosphorous entry into lakes for the first time,” explained Martin Grosjean, director of the Oeschger Center for Climate Change Research at the University of Bern and lead author of the study.

The analysis revealed that phosphorus input into central European lakes increased significantly as early as the beginning of the Bronze Age, about 4,000 years ago. At that time, the area was reportedly subject to heavy deforestation and intensification of land use. This is reported to lead to increased soil erosion and phosphorus runoff into lakes.

The total outflow? 2.7 billion tons

“Human activity has fundamentally altered the global phosphorus (P) cycle,” the study states. “Yet our understanding of when and how humans influenced the P cycle has been limited by the scarcity of long-term P sequestration records, particularly outside Europe and North America” In this context, however, “lake sediments provide a unique archive of past P burial rates and allow the human-mediated disruption of the global P cycle to be examined.”

Through this type of analysis, the researchers have found similar dynamics in other regions of the Northern Hemisphere albeit at more recent times: about 2,000 years ago in China and about 400 years ago in North America.

The numbers, they explain, are the result of the later onset of population growth and intensification of land use in these regions. The real turning point, however, comes with industrialization. From the pre-industrial period to the present, annual phosphorus input into lake sediments has increased 6-fold from 240,000 to 1.5 million tons per year. The authors estimate that 2.7 billion tons of phosphorus have accumulated in lake sediments worldwide over the past 12,000 years.

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Global agriculture too dependent on phosphorus

Fostering the acceleration experienced since the 19th century has not only been industrialization. Crucial, in fact, has also been the spread of chemical fertilizers in agriculture. Today, argued a study earlier this year by INRAE, the Institut national de la recherche agronomique, a French government agency, and Bordeaux Sciences Agro, a training school at the local university, about 47 percent of the phosphorus in the soil is of human origin, that is, attributable to the use of mineral fertilizers.

The figure reflects a strong dependence of global agriculture on the use of these substances. This phenomenon is accompanied by soil erosion, which, according to a study by the University of Basel, is responsible for more than 50 percent of the global loss of the element in agriculture. Hence the need to apply more sustainable strategies such as improving resource recycling and using mixed cropping systems to reduce phosphorus use in fields.