Excessive use of nitrogen fertilizers in China in the past few decades has polluted its groundwater, given rise to acid rain, soil acidification and increased greenhouse gas emissions, Chinese experts have said.

In their article, published in the Proceedings of the National Academy of Sciences, they urged farmers in the country to reduce their use of synthetic nitrogen fertilizers and change their farming practices.

"Integrated management packages... include efficient recycling of manures and crop residues, the use of legume crops in rotation to increase internal nitrogen cycling and further reduction in the reliance on synthetic nitrogen fertilizers," they wrote.

Overuse of synthetic nitrogen not only happens in China but in many other Asian countries, which are under pressure to feed large and growing populations.

The study showed that while annual grain production increased to 484 million tons in 2005 from 283 million tons in 1977 in China, its use of synthetic nitrogen fertilizers ballooned to 26.21 million tons from 7.07 million tons over the same period.

In China, scientists and the government strongly encouraged the use of nitrogen fertilizers until the 1990s, when they realized the serious damage it caused the environment, but they have been unable to reverse the practice.

"Persuading farmers to limit fertilizer inputs is difficult because many of them still hold to traditional opinions that higher crop yield will be obtained with more fertilizers," wrote the team, led by Ju Xiaotang at the College of Resources and Environmental Sciences in Beijing.

However, they stressed: "Only by reducing fertilizer nitrogen inputs can degraded environments be gradually restored, enhanced and protected."

They also called for the introduction of certain legislative controls in China, similar to the ones in the European Union, including removing government subsidies, introducing a nitrogen fertilizer tax, educating farmers on environmental awareness and employing practices that avoid serious environmental degradation.

(Shanghai Daily February 18, 2009)