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Overuse of phosphorus requires more conservative global limits

Threatens a key biophysical process that regulates stability of the Earth “system”, says international team of researchers.


Four of nine key biophysical processes that regulate the stability of the Earth “system” are in jeopardy because of human activity, according to the results of studies recently published in the journal Science. The international team of 18 scientists who co-authored the article has been fine-tuning a set of planetary and regional boundaries that define “a safe operating space” for humanity.

“The human enterprise has grown so dramatically since the mid-20th century that the relatively stable, 11,700-year long Holocene epoch, the only state of the planet that we know for certain can support contemporary human societies, is now being destabilized.” they wrote. “A continuing trajectory away from the Holocene could lead, with an uncomfortably high probability, to a very different state of the Earth System, one that is likely to be much less hospitable to the development of human societies.”

One of the violated boundaries is the amount of phosphorus and nitrogen, key ingredients of farm fertilizers, that can safely cycle through soil to bodies of fresh water and finally into the oceans. The research reported for these flows was carried out by Elena Bennett, associate professor in McGill University’s department of natural resource sciences and in the school of environment, and Stephen Carpenter, director of the University of Wisconsin’s Center for Limnology.

In an interview, Dr. Bennett noted that earlier work on the planetary boundary for phosphorus only took into account data on the impact on marine environments. It didn’t look at another aspect vital to human well-being, namely how phosphorus flows affect fresh water bodies such as lakes, rivers and reservoirs. The research by Dr. Bennett and Dr. Carpenter confirmed their belief that including fresh water systems in the analysis would significantly alter the phosphorus boundary, making it more conservative.

Dr. Bennett has examined phosphorus data for areas of Quebec with lakes that have excessive nutrient levels, so-called eutrophic lakes. As a share of the total amount of phosphorus in a region, including what people import as fertilizer, food and detergent, the amount that actually makes its way into such lakes is only about five percent. But even that proportion is far too high, explained Dr. Bennett, because lakes, with so much less volume than oceans, are highly sensitive to excessive nutrients.

“When you cross these planetary or regional boundaries,” she said, “you become increasingly more likely to experience the kind of event you really don’t want to experience. There’s increased risk of surprises.” One of these surprises was the toxic blooms of blue-green algae in lakes, linked to high levels of phosphorus. This problem led to the closing of more than 75 recreational lakes in Quebec in 2007.

“It’s like you’re stepping off the curb into traffic. While you’re up on the sidewalk [within the boundary], it’s pretty unlikely you’re going to get hit by a car. Stepping off the curb doesn’t necessarily mean you’ll be hit, but it sure increases the likelihood.”

Excessive buildup of phosphorus in farm soils in industrialized countries and its dispersal through erosion and other mechanisms into fresh water bodies differs from the situation in many developing countries. In Africa, especially, little fertilizer is applied to farm fields, and soils are typically deficient in this nutrient which is so vital to plant health. At the same time, the world supply of mined phosphorus for agriculture is limited and concentrated in just a few countries, notably Morocco.

Regional differences in phosphorus use and flows – too much in some areas and not enough in others – suggest that intentional redistribution and conservation of this earth element could lessen the environmental pressure on the planet, and improve food security at the same time. This could be done, says Dr. Bennett, through reduced consumption and recycling. For example, no-till farming methods and more precise timing of fertilizer applications could save a lot of wasted phosphorus. Moreover, reducing the amount of spoiled and wasted food, typically 30 to 40 percent of production, could save one million tonnes of phosphorus each year.

Besides excessive flows of phosphorus and nitrogen, the Science article reported that planetary boundaries related to three other key processes have been transgressed: climate change, loss of biosphere integrity (biodiversity loss and species extinction), and changes in land systems. In the case of global warming, the internationally agreed upper climate limit of two degrees lies beyond the climate change boundary. It is, the authors said, a risky target for humanity, and an absolute minimum for the global climate negotiations.

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