The world’s total human population has jumped to over 7.4 billion just this year. Feeding the human species takes a tremendous toll on natural resources including water, soil and phosphorus — a chemical element in fertilizer essential for food production.
In modern agriculture, fertilizer often leaks into waterways such as rivers, lakes and oceans. The phosphorus (P) in the runoff stimulates algae blooms and then, when algae die and decompose, dead zones develop and fish die off. But much of the “lost” phosphorus doesn't end up in water bodies — large amounts of P also accumulate in the landscape. Until now, scientists have not fully understood the magnitude of this accumulation.
The study by Washington State University, International Plant Nutrition Institute, Universities of Arkansas, Bristol, Durham, Lancaster, Arizona State University, Centre for Ecology and Hydrology, China Agricultural University and Minnesota Department of Agriculture is published in Nature Geoscience.
The researchers studied three river basins where food and water security are directly linked to phosphorus. The analysis included the UK’s Thames River basin, the Maumee River Basin in the mid-western section of the US and the Yangtze River Basin in China.
The study areas ranged in size from approximately 5,000 to 700,000 square miles. Historical records dating back 70 years were used to measure the human impact on the flows of phosphorus into and out of each catchment through trade, food waste, human waste and agricultural runoff, comparing these flows to losses of P from each river's discharge. The results showed that massive amounts of phosphorus have accumulated in the landscape — a form of “legacy P” that may affect aquatic ecosystems for decades or even centuries.
Of the three sites, only one showed clear improvement over several decades — the UK’s Thames River. By following the UK’s lead, the researchers suggest other countries might improve their ability to manage phosphorus.
Dr Nicholas Howden, Senior Lecturer in Water in the University of Bristol’s Department of Civil Engineering, said: “This work demonstrates that long-term monitoring of data is important. It enables us to identify the lasting impacts of increased food production on river basin processes over many decades and to see how we may be able to solve problems that emerge.”
Professor Phil Haygarth of Lancaster University added: “This study was very important because for the first time we have long-term data from these big rivers from around the world and we can see the scale of the problem and it is quite compelling. The information from the Thames shows there is hope, but we can’t be complacent. Phosphorus is vitally important to farming – and it is a finite resource - but it is ending up in our rivers and estuaries where it is causing serious problems.”
The study’s novel analyses illustrate the challenges researchers face in figuring how to manage the storage, exploitation and reactivation of phosphorus that is already present in our environment.
Dr Stephen Powers, postdoctoral researcher with Washington State University and lead author of the paper, said: “Somewhat of a surprise is that in populated landscapes, there is a huge amount of phosphorus in food waste, such as animal bones, and in sewage sludge removed during wastewater treatment.
“Until recently these waste flows have been largely ignored in catchment studies that involve phosphorus.”
Dr James Elser, research scientist with the Arizona State University School of Life Sciences and School of Sustainability, and co-author of the study, added: “After we understand how human activity affects the accumulation of phosphorus in the environment, we can then focus our research efforts on reducing its long-term impact, even on figuring out how to recycle it. This will help secure food and water supplies for future generations.”
Drs Elser and Powers said the next step is to develop strategies that will reduce the impact of this “legacy P”. The pair added that it is important to create new technologies and policies that recycle P for re-use as fertilizer, rather than allowing it to escape and build up in the landscape.
Paper
Long-term accumulation and transport of anthropogenic phosphorus in three river basins by Stephen M. Powers et al in Nature Geoscience