Fusuo Zhang
Fusuo Zhang

The chief culprit behind the pH decrease appears to be application of nitrogen fertilizer, although base cation uptake and acid rain also play a role.

"Serious soil acidification will threaten food security and environmental safety worldwide," Fusuo Zhang of China Agricultural University told environmentalresearchweb. "Our work has shown that soil quality or soil health should be paid more attention in intensive agricultural production systems receiving high nitrogen and other resource inputs."

Zhang and colleagues from China Agricultural University; the Agri-Food and Biosciences Institute, UK; Rothamsted Research, UK; and Stanford University, US, recommend that optimal nutrient-management strategies can significantly reduce nitrogen fertilizer rates, and hence slow soil acidification, without decreasing crop yields.

"Fertilization based on comprehensive, knowledge-based nitrogen management practices has become one of the most urgent requirements for sustainable agriculture in China and in other rapidly developing regions worldwide," the researchers write in their paper in Sciencexpress. Excessive application of fertilizer can also lead to other environmental problems, such as eutrophication and dead zones.

Chinese agriculture has intensified greatly since the early 1980s. Grain production reached 502 million tons in 2007, an increase of 54% over 1981 levels. Over the same time period, nitrogen fertilizer use soared by 191%, reaching 32.6 million tons in 2007. The rates of fertilizer applied in some regions of China are very high compared with rates in North America and Europe.

The wheat–maize, rice–wheat and rice–rice double-cropping systems in China usually receive more than 500 kg of nitrogen fertilizer per hectare per year, while nitrogen-use efficiencies are 30–50%, say the researchers. Greenhouse vegetable systems, which are the country's major cash crops, typically receive even more fertilizer; in Shandong province nitrogen fertilizer rates of more than 4000 kg per hectare are common, with nitrogen use efficiency below 10%.

In four widespread cropping systems in China, Zhang and colleagues found that nitrogen cycling processes released 20–221 kilomoles of hydrogen ions per hectare per year whereas base cations uptake by plants contributed 15–20 kilomoles of hydrogen ions per hectare per year. Acid deposition through rainfall lead to 0.4–2.0 kilomoles of hydrogen ions per hectare per year in strongly affected areas, meanwhile. The lower nitrogen fertilizer rates in other regions typically produce 1.4–11.5 kilomoles of hydrogen ions per hectare per year as a result of nitrogen cycling.

"We find that excessive use of nitrogen fertilizer is the main reason for the accelerated anthropogenic soil acidification," said Zhang. "Soil acidification in Chinese croplands is different from previous reported soil acidification in some forest soils which was induced mainly by acid deposition, such as sulphur and nitrogen deposition."

The team used a number of analyses to prepare its data. "Evidence from site-to-site comparison, long-term dynamics of soil pH under conventional farming practice, and mega-data analysis for pH change between the 1980s and the 2000s provide the strongest support for significant acidification in major Chinese croplands," said Zhang.

Now the researchers plan to concentrate on risk assessment, eco-environmental impacts and prevention strategies for soil acidification in Chinese croplands.