Excess nitrogen fertilizer increasing warming in China

Halving the amount of nitrogen fertilizer used in certain areas of China would substantially decrease greenhouse gas emissions without affecting crop productivity and the area's natural carbon sink according to research led by a CHANS-Net member.

The study, Food benefit and climate warming potential of nitrogen fertilizer uses in China, published Nov. 1, 2012 in Environmental Research Letters, showed that a 60 percent reduction in fertilizer use would significantly reduce emissions from areas that are over fertilized, such as the North China Plain and middle and lower reaches of the Yangtze River Basin.

China is currently tasked with meeting the food demands of 22 percent of the world's population; however, its overreliance on nitrogen-based fertiliser has dramatically increased its emissions of nitrous oxide -- the most potent greenhouse gas.

According to the study, since 2002, the warming effect caused by nitrous oxide emissions has been significantly greater than the cooling effects from the croplands storing carbon dioxide.

Looking at the past six decades, the researchers found that between 1949 and 1990 nitrogen fertilizer increased the rates of crop production and the storage of soil carbon. But from 1990 onward, they found that the rate of soil carbon storage stopped increasing and the rate of crop production slowed.

In the 1990s, nitrogen fertilizer was contributing to 53 percent of crop production but since then has contributed to 49 percent, even though more of it was being used, suggesting it had become less effective.

Nitrogen fertilizer can be beneficial to the climate, providing crops with essential nutrients so they can grow and create a larger natural carbon sink in soils, taking in excess carbon dioxide that is released into the atmosphere. As a result of this balance, the warming effects of nitrous oxide may be lessened.

When applied to crops and plants, nitrogen enters the soil and reacts with bacteria, which gives off nitrous oxode as a waste product. Nitrous oxide is the third highest contributor to climate change behind carbon dioxide and methane, but is the most potent of the greenhouse gases because it absorbs infrared radiation more readily.

According to the researchers, the warming effects of nitrogen fertilizer-induced nitrous oxide emissions in China are now much greater than the cooling effects of nitrogen-stimulated carbon dioxide uptake, resulting in overall warming.

CHANS-Net member and co-author Hanqin Tian, of Auburn University, said, "Nitrogen fertilizer has become less efficient in recent years as the nitrogen input has surpassed nitrogen demands of plants and microbes. Excess nitrogen is not stimulating plant growth but leaving the system through leaching and nitrous gas emissions.

"We need to advance education programs to inform Chinese farmers of both the economic and environmental costs of excessive nitrogen fertilizer use. Effective management practices such as compound fertilizer use and optimised irrigation and tillage should be advanced to increase nitrogen use efficiency."