Published online 29 September 2010 | Nature | doi:10.1038/news.2010.498

News

Overuse of the fertilizer has wasted a valuable natural resource and caused serious pollution.

This dead fish, in Lake Tai, eastern China, is likely a victim of excess phosphorus, which causes algal blooms in lakes and rivers. Color China Photo/AP

Researchers are warning that inappropriate management of phosphate fertilizer and animal manure in China has resulted in serious water pollution and substantial waste of phosphorus, a non-renewable inorganic chemical.

Soils in many parts of the world are deficient in the chemical, which is required for plant growth. In its phosphate form, phosphorus is a vital part of the cell's genetic material, and is also found in adenosine triphosphate (ATP), the main energy carrier in cells.

As demand for food production rises, so does the global demand for synthetic fertilizer, in which phosphorus is a key ingredient, says Luc Maene, director-general of the International Fertilizer Industry Association. Last week, as the non-profit research organization organization IFDC (formerly the International Fertilizer Development Centre), headquartered in Muscle Shoals, Alabama, announced its latest estimate of the global reserve of phosphorus, Chinese researchers presented their findings on China's phosphate use at the 4th International Symposium on Phosphorus Dynamics in the Plant-Soil Continuum in Beijing. They called for a rethink of current fertilization practices.

Phosphate, phosphate everywhere

Zhang Fusuo, a plant nutritionist at the China Agricultural University in Beijing, says China has 9% of the world's arable land and has to feed 21% of the world's population, yet its soils are not particularly fertile. An unpublished study by Zhang and colleagues, presented at the symposium, traced how the country has steadily boosted its use of phosphate over three decades.

In 1980, Zhang says, close to 80% of cropland in China contained less than 10 milligrams of phosphate available to plants per kilogram of soil — indicating a phosphate deficiency. Since then, the Chinese government has created a series of policies to encourage the production and use of phosphate fertilizer. But with phosphate use increasing at a rate of 5% per year, 85 million tonnes has accumulated in the soil. "The average phosphate content in the soil has nearly tripled and only a quarter of cropland is deficient in this nutrient now," says Zhang. "This has greatly increased crop production."

Such heavy fertilizer use has made China one of the biggest consumers of phosphate fertilizer. Last year, it used 11 million tonnes, or about 35% of global consumption, according to Zhang. With ever-increasing food demand, there is no sign that phosphate use in China will dwindle.

"The question is, do we really need to use this much?" says Zhang. "Many guidelines for fertilizer use are inappropriate and the recommended doses can be many times higher than the necessary amount."

Monitor and maintain

Zhang says that there should be a national network for monitoring the build-up or deficiency of phosphate in soils, so that farmers can tailor amounts of fertilizer accordingly.

"The point is to build up and maintain the soil phosphate content at a level that is optimal for crop yields," Zhang says. This 'build-up and maintenance' strategy alone could reduce current levels of phosphate fertilizer use by more than 20%, he estimates.

In addition, technologies are needed to improve the efficiency of phosphate use by plants, researchers at the symposium said; in most cases, they estimate, plants take up less than 15% of phosphate in the soil. Zhang and his colleagues are trying to crank that up by manipulating the chemistry and biology of the rhizosphere, the narrow layer of soil surrounding roots.

Many researchers also called for better livestock management. Livestock rearing is increasingly hugely in China, and phosphate concentrations in animal feed in the country are much higher than Western standards, says Zhang. With a limited labour force but ample subsidized chemical fertilizer available in most of rural China, dumping this phosphate-rich animal manure into waterways has become an easier and cheaper option than using it to fertilize cropland.

Pollution problem

A pollution census conducted by China's government earlier this year earlier this year (see 'China takes stock of environment') found that livestock is the largest contributor to run-off pollution from the land into waterways, and, through manure, is responsible for 56% of phosphorus discharges — with cropland contributing to another 11.4%. Excess phosphorus in many of China's lakes, coastal waters and rivers has caused repeated occurrences of harmful algal blooms — known as eutrophication. The blooms consume much of the oxygen dissolved in the water, killing fish and other plant life, and can release toxins that are poisonous to humans and animals.

A study led by Liang Tao, an ecologist at the Beijing-based Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, examined phosphate run-off into Lake Tai, which has become a notorious example of China's eutrophication problems. It found that the area of land from which water drains into the lake exports more than 6 kilograms of phosphate per hectare - most of which came from manure.

"This is an amazing amount of phosphate run-off," says Peter Kleinman, a soil scientist at Pennsylvania State University in University Park, who collaborates with Liang and presented their results at the symposium. Most developed countries would be alarmed if the export rate exceeded a sixth of that, he adds.

According to Zhang, 3.3 million tonnes of phosphate fertilizer — nearly a third of China's total consumption — could be saved if all the animal manure is used to fertilize cropland. "There is a lot that can be done to save both the environment and the natural resource," he says.

As China deliberates its next five-year budget plan, Zhang is in talks with the agriculture ministry and fertilizer industry about how to optimize fertilizer use. "There seems to be a consensus now on the detrimental effects of overuse of phosphorus fertilizer," says Zhang. "Let's hope that this will be reflected in the government policies."