"It took nearly 50 years to achieve food sufficiency in China," Xiao-Tang Ju of China Agricultural University and the Ministry of Education told environmentalresearchweb. "An unanticipated cost has been that massive fertilizer inputs have led to significant environmental degradation. China consumes more than 30% of the world’s nitrogen fertilizer with 9% of global arable land since the 1980s."

With that in mind, researchers at China Agricultural University, the Chinese Ministry of Education, the Chinese Academy of Sciences, Hebei Agricultural Univeristy, China, and the Agri-Food and Biosciences Institute in the UK have found that using 30-60% less nitrogen kept crop yields the same but halved nitrogen loss to the environment. The team studied two double-cropping systems – waterlogged rice/upland wheat in the Taihu region of east China and irrigated wheat/rainfed maize on the North China Plain.

All four crops showed different nitrogen behaviour, "depending on climatic, soil and management practices". The team calculated that the rice/wheat system had an annual nitrogen surplus of 87 kg per hectare. This system had large losses by denitrification, which the scientists reckoned could be reduced by improving carbon management and controlling the water regime.

The wheat/maize system had a 212 kg per hectare nitrogen surplus and large nitrogen losses from ammonia volatilization. The researchers say that changing nitrogen application techniques and using deep placement of urea or ammonium bicarbonate could substantially reduce ammonia volatilization losses from calcareous soils. Current agricultural practices use 550-600 kg of nitrogen per hectare.

"A better nitrogen balance can be achieved without sacrificing crop yields but significantly reducing environmental risk by adopting optimum nitrogen fertilization techniques, controlling the primary nitrogen loss pathways and improving the performance of the agricultural Extension [advisory] Service," said Ju. "The over-application of nitrogen also represents an unnecessary economic expenditure for farmers."

According to the researchers, the new recommendations for fertilization levels should take into account the nitrogen supplying capacity of the soil, and nitrogen deposited from air and irrigation water. Since the 1980s heavy pollution has more than doubled the amount of nitrogen deposited from the atmosphere and in irrigation water in both regions of China studied.

"Only by reducing fertilizer nitrogen inputs can degraded environments be gradually restored, enhanced and protected," said Ju. "Although several environmental standards have been set in the past, there are still no legislative controls in China equivalent to those in the European Union. China would benefit from adopting and enforcing relevant agricultural regulations. All these goals could be achieved by removing government subsidies, introducing a nitrogen fertilizer tax, improving local Extension Services, and educating farmers for environmental awareness."

But the team says that persuading farmers to limit fertilizer inputs is difficult because many of them still hold to now traditional opinions that higher crop yield will be obtained with more fertilizer.

Now the researchers, who reported their work in PNAS, plan to investigate how they could increase crop yields by 20% by proper management of nitrogen, at the same time reducing environmental risk.