"A variety of studies have proven the detrimental effects of excess nitrogen fertilizer on atmospheric and aquatic environments," Hanqin Tian of Auburn University, US, told environmentalresearchweb. "However, few of them paid attention to agronomic nitrogen-input effects from a climate perspective. For the first time, we simultaneously estimated changes in crop yield, soil organic carbon storage, carbon-dioxide uptake and nitrous-oxide emission in response to chemical-nitrogen fertilizer use in China's croplands."

Along with colleagues from the Marine Biological Laboratory, US, Chinese Academy of Sciences and Massachusetts Institute of Technology, US, Tian used a coupled biogeochemical model dubbed the Dynamic Land Ecosystem Model (DLEM) to simulate carbon and nitrogen cycling in agricultural land from 1949 to 2008. The researchers estimated changes in the yields of maize, rice, spring wheat, winter wheat, soybean, barley and other crops, as well as the net fluxes of carbon dioxide and nitrous oxide induced by nitrogen fertilization.

Nitrogen-fertilizer-induced crop yield peaked in the early 2000s and soil-carbon sequestration reached a maximum in the 1990s, while nitrous-oxide emission has continued to rise as more nitrogen is added, they found.

"Over the recent decade, stimulation of global climate warming by excess nitrogen fertilizer use – through nitrous-oxide emission – was estimated to outweigh the climate benefit [of fertilization] in increasing carbon-dioxide uptake, showing a turning point in the early 2000s in terms of global-warming potential," said Tian. "If we reduced the current nitrogen-fertilizer level by 60% in 'over-fertilized' areas, nitrous-oxide emission would substantially decrease without significantly influencing crop yield and soil-carbon sequestration."

Intensively managed regions such as the North China Plain and middle and lower reaches of the Yangtze River Basin are over-fertilized, the team discovered. Here nitrous-oxide emissions completely counteracted the carbon-sink effect of increased vegetation growth; in some cases they exceeded it by more than double. Only a small portion of China's cropland still acts to mitigate climate.

The relative contribution of chemical-nitrogen fertilizer to crop yield has decreased from 53% to 49% over the past two decades, said the researchers. They believe this is due to overuse and increased contributions from other factors such as the genetic improvement.

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

The researchers believe that enhancing nitrogen use efficiency would be more effective than increasing nitrogen inputs for sustaining China's food security and diminishing the climate warming and water pollution aggravated by anthropogenic nitrogen enrichment. "We need to advance education programs to inform Chinese farmers of both the economic and environmental costs of excessive nitrogen fertilizer use," said Tian. "Effective management practices such as compound fertilizer use and optimized irrigation and tillage should be advanced."

The scientists' future research will take the full nitrogen cycle into account. "Indirect nitrous-oxide emissions derived from re-deposition of volatilized nitrogen and nitrogen leached to downstream aquatic ecosystems will be considered," said Tian. "We will quantify nitrogen export from land ecosystems to coastal areas as another environmental cost. And in combination with detailed local conditions, we will provide more scientific suggestions for farmer-based management and strategy determination within this multi-factor world."

The team reported the results in Environmental Research Letters.