Tropes

Nature 00122.txt

Nitrogen fertilizer warning for China: Nature Newsresearchers warn that the overuse of nitrogen fertilizer in China is poisoning air, soil and water and say farmers could cut their use of the fertilizers without compromising crop yields. Until recently, the use of fertilizer in China was promoted actively by scientists, government and the'extension staff'who promote new agricultural practices to farmers. As a result, the average grain production per unit area of farmed land doubled between 1977 and 2005. The cost for that increase was a significant surge in fertilizer use from 7 million tonnes in 1977 to 26.2 million tonnes in 2005. It's unclear where fertilizer nitrogen goes once applied or whether the current practice is cost-effective, says Zhang Fusuo at the China Agricultural University in Beijing, who led the study. As demand for food and energy rises, such questions are more pressing than ever, he adds. To address these issues, Zhang and his colleagues studied two typical farming systems based on double cropping (in which a second crop is planted in the same field after the first crop has been harvested): the wheat-rice system in a region near the Tai Lake in eastern China and the wheat-maize system in the North China Plain in the northeast of the country. Reporting today in the Proceedings of the National Academy of the United State of America1, the researchers show that, as more fertilizer is applied, plants become less efficient at taking up nitrogen and more nitrogen is lost into the environment. The team tracked the fate of fertilizer nitrogen and found that 20-50%of it leaked into air and groundwater although the main pathway through which it was lost varied from crop to crop. For example wheat and maize farmlands in the North China Plain lose 19%and 25%of applied nitrogen, respectively, as a result of ammonia evaporation or'volatilisation'.'By contrast, in the Tai Lake region, 36%of fertilizer nitrogen is lost from rice fields and 44%from wheat fields through a process called denitrification, in which nitrate is converted to nitrogen gas. Nitrates also leached into ground and surface water in both regions, causing serious pollution problems, explains Zhang. Fertilizer nitrogen was lost also as nitrous oxide, albeit at a much lower rate than through ammonia volatilisation, denitrification and leaching. The researchers then compared the conventional practice with an optimal level of fertilizer application calculated by taking into account both nitrogen input from soil irrigation water and air and nitrogen loss. Their field experiments show that this optimal level, which was a third of the average amount applied by Chinese farmers, could maintain crop yields when properly managed. Most farmers believe that higher crop yields will be achieved with more fertilizer, whereas our study shows that sometimes less is more, says Zhang. Other crops will have to be tested to determine the level of fertilizer that is optimal for them, he adds. Timing is another crucial factor, says Zhang. Farmers apply 50 80%of the total amount of fertilizer around the time of planting. But the experiments show that optimal yields were achieved when the majority of fertilizer was applied later to seedlings, when growth is fastest. Each year, Chinese farmers apply around 600 kilograms of fertilizer per hectare, which is amazingly high according to Pamela Matson, an agricultural ecologist at Stanford university in California who was not involved with the study. In the United states, farmers use 100 kilograms per hectare on wheat farmland. But China's figure is compared also high to other developing countries with intensive farming practices: Mexico's farmers use 250 kilograms per hectare. The calibre of the research is superb, says Matson, whose interdisciplinary approach to fertilizer management2 was adapted by Zhang and his colleagues. It's a very broadly based, multi-year, multi-site study of an important problem probably the most comprehensive analysis to date. It's a very important wake-up call for China, says Matson. The current level of fertilizer use in the country has serious environmental consequences at both regional and global levels. The study points to a better, more scientific approach to managing the application of fertilizer, says Zhang. It's much cheaper and more environmentally friendly, he adds. And we are not compromising crop yields.

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