"What's even more worrying is that China's consumption of plastic mulch is still increasing and the trend will not change in the near future," Yan said, adding that the country's annual consumption of plastic mulch is expected to exceed 2 million tons in 10 years.

Yan has surveyed the yield of crops grown in farmland affected by plastic waste. He found that when the amount of the plastic residue in the soil reached 52.5 kg per hectare, the yield of wheat grown in it would reduce by 2-3 percent, corn by about 10 percent and cotton by 10-23 percent.

"The large amounts of plastic residue in our farmland will seriously reduce the sustainability of China's agriculture," Yan warned.

On top of the worries about decreasing yields, there are also environmental concerns. Plastic residue is light and usually mixed with earth, so it is difficult to collect and filter out to be recycled. "When there is a big enough pile of plastic waste, farmers generally burn it, polluting the environment even further," Jiang said.

Organic compounds that do not biodegrade are known as persistent organic pollutants (POPs), and 12 of the most harmful of these chemicals have been restricted or banned internationally by the 2004 Stockholm Convention on POPs. These compounds linger in the environment for a long time and can enter the human body through food or respiration, causing poisoning, cancer and even death.

"Burning plastic mulch results in the release of at least five of the 12 POPs listed by the Stockholm Convention," Jiang said.

Yan admitted that there are also many other consequences caused by residual plastic mulch that need to be evaluated. For example, it remains unclear whether such residues will affect underground water. He calls for a thorough investigation led by the central authorities so as to understand the general situation of plastic mulch pollution around the country.

"Otherwise, we cannot work out an effective plan to treat and prevent the problem," Yan said.

Seeking a solution

In recent years, some farmers in Xinjiang have started using specific machines to collect residual plastic mulch from their farmland, but their efforts have not received positive feedback from experts. They say it cannot solve the problem because many other farmers are reluctant to follow suit.

"A major problem is that the economic returns of collecting and recycling residual plastic mulch are too low. Both farmers and enterprises have no incentive to do so," Yan said.

Sun Wenchuan is a farmer in Xinjiang's Yuli County. According to him, it will take a person 20 days to clean up his 2.7 hectares of farmland. "According to current labor costs, I would have to pay 3,000 yuan ($500) for the work," Sun said. In contrast, the price for recovered plastic residue is only 1.2 yuan ($0.19) per kg.

Li Zhong, Director of the Agricultural Bureau of Yuli County, admitted that the lack of efficient recycling mechanisms has become a major hindrance for further promoting the treatment of plastic residues in the county.

In response, Yan suggested the government give subsidies to enterprises concerned, encouraging them to expand their networks for the purchasing of used plastic mulch.

Aside from the costs, another factor hindering proper recycling is that the amount of plastic mulch that does not meet standards is far too high.

In the 1990s, a national standard was established to regulate the quality of plastic mulch. It stipulates that the thickness of mulch should be around 0.008 mm, but a discrepancy either way of 0.003 mm is allowed. In order to save costs, many mulch manufacturers take advantage of the permitted discrepancy and have chosen to produce mulch only 0.005 mm thick. Farmers also prefer thinner mulch due to cost concerns. According to Xiao, he needs 47.5-60 kg of 0.008 mm mulch to cover 1 hectare of farmland, but only 30-37.5 kg of 0.005 mm mulch.

However, due to its lower density, thinner mulch breaks apart much more easily, creating difficulties for recycling.

"Stricter rules are needed to regulate the production and recycling of plastic mulch, or else the problem of plastic pollution in our farmland cannot be solved," said He Wenqing, another researcher with the CAAS' Institute of Environment and Sustainable Development in Agriculture.

A more effective method for alleviating plastic pollution in farmland is to use biodegradable alternatives, say experts.

Biodegradable mulch has been around for some time, but higher prices make it unpopular with farmers. "Generally, the prices for biodegradable mulch are 3-5 times that of non-biodegradable ones," said Chen Xuesi, a researcher with the CAS' Changchun Institute of Applied Chemistry in Jilin Province.

Chen is leading a research project on biodegradable mulch. He said that his team hopes to develop a kind of biodegradable mulch that can be manufactured cheaply in two years. "The price of the new product will be twice that of non-biodegradable mulch. We hope this lower price-point will help open the market for biodegradable mulch," Chen said.

Wang Xinyong, Director of the Institute of Soil and Fertilizer at the Xinjiang Academy of Agricultural Sciences, called for increased input to speed up the development of biodegradable mulch. "The government should also consider giving subsidies to manufacturers and farmers to encourage them to produce and use pollution-free products," Wang said.

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