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Excess nitrogen fertiliser increasing warming in China

30 October 2012 Institute of Physics (IOP)

Halving the amount of nitrogen fertiliser used in certain areas of China
would substantially decrease greenhouse gas emissions without affecting
crop productivity and the area?s natural carbon sink.

This is according to a new study, published today, 1 November, in IOP
Publishing's Environmental Research Letters, which showed that a 60 per
cent reduction in fertiliser use would significantly reduce emissions from
areas that are, anyway, "over-fertilised", 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 per cent of
the world?s population; however, its over-reliance on nitrogen-based
fertiliser has dramatically increased its emissions of nitrous oxide (N2
O) the most potent greenhouse gas.

According to the study, since 2002, the warming effect caused by N2O
emissions has been significantly greater than the cooling effects from the
croplands storing carbon dioxide (CO2).

Looking at the past six decades, the researchers found that between 1949
and 1990 nitrogen fertiliser increased the rates of crop production and
the storage of soil carbon; however, from 1990 onwards, they found that
the rate of soil carbon storage stopped and the rate of crop production
slowed.

In the 1990s, nitrogen fertiliser was contributing to 53 per cent of the
crop production but since then has contributed to 49 per cent, even though
more of it was being used, suggesting it had become less effective.
Nitrogen fertiliser 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 (CO2) that is released into
the atmosphere. As a result of this balance, the warming effects of N2O
can be less impactful.

When applied to crops and plants, nitrogen enters the soil and reacts with
bacteria, which gives off N2O as a waste product. N2O is the third highest
contributor to climate change behind CO2 and methane but is the most
potent of the greenhouse gases as it absorbs infrared radiation more
readily.

According to the researchers, the warming effects of nitrogen
fertiliser-induced N2O emissions in China are now much greater than the
cooling effects of nitrogen-stimulated CO2 uptake, resulting in overall
warming.

Co-author of the study, Dr Hanqin Tian, said: "Nitrogen fertiliser 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 fertiliser use.
Effective management practices such as compound fertiliser use and
optimised irrigation and tillage should be advanced to increase nitrogen
use efficiency."

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