As China lobbies world leaders to back its free trade plan at an Asia-Pacific summit this week, businesses are complaining about Beijing's use of non-tariff barriers from customs clearance to quality restrictions to curb raw material imports.
Amid a slowdown in economic growth, the world's top commodities buyer is facing a supply glut that has sent local prices tumbling and miners deep into the red. Inventories of iron ore, coal and cotton are bulging at ports across the country and state granaries are overflowing.
To help shield local markets, experts and traders said Beijing has employed a range of non-tariff barriers to curb imports of corn, cotton and distillers' dried grains.
After months of lobbying by the China National Coal Association to limit imports, Beijing also slapped a 3-6 percent tax on coal imports last month - hitting $8 billion worth of exports from Australian miners.
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"This is the first time in recent memory that we are seeing quite a lot of protectionism in China targeting commodities imports," said Simon Evenett, professor of international trade at Switzerland's University of St Gallen.
"This is a new dimension in Chinese intervention in commodities markets."
With global commodity prices already reeling, the rise of perceived protectionist measures in China poses a further threat by artificially restricting demand.
China initiated 89 food safety and animal and plant health measures in 2013, up from just 25 in the preceding year, according to World Trade Organisation data. The number of technical barriers to trade measures introduced also rose to 79 in 2013, from 27 in 2012.
The move comes as Beijing has increased its use of a 6-year-old anti-monopoly law to launch a flurry of investigations targeting foreign companies, including Microsoft and Volkswagen AG, sparking concerns it is seeking to support domestic firms.
China's Premier Li Keqiang has said that those investigations were conducted "transparently and fairly" and were not targeted at foreign companies.
Australia has played down the impact of China's coal tariffs and says it will win an exemption through a free trade agreement (FTA) that is widely expected to be signed next week.
But veteran coal traders point to Indonesia, warning that Beijing still has many ways to keep foreign supplies at bay.
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In recent months, Chinese customs have stepped up checks on Indonesian coal and held up shipments, traders said, even though China and the Association of Southeast Asian Nations already have an FTA.
"The customs office will find fault with how the coal is classified; whether it is lignite or steam coal, and will hold up shipments when they are deemed to be labelled incorrectly," said an executive at a large international trading house.
Importers need to provide fresh certification documents when such issues arise, leading to delays of more than a month.
"This is clearly a tactic to restrict imports and it works. Some of our clients have cut back on Indonesian coal and switched to local supplies instead because they think the small discount is not worth the hassle," said a Shanghai-based broker.
Imports of Indonesian lignite have been falling since the second quarter, with September arrivals down nearly a quarter from April.
China's customs office did not respond to a faxed request for comment. The government has said that controls on coal were necessary to tackle pollution.
Beijing's shift in attitude was first noticed in November last year, when authorities rejected a U.S. corn cargo after finding an unapproved strain of genetically modified (GMO) grain.
What was at first perceived as a one-off issue, because similar shipments had gone through in the past, has become a multi-billion dollar trade disruption that has caused over 1.25 million tonnes of U.S. corn to be sent back.
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China's agriculture ministry has said that the measures were necessary to prevent genetically modified strains from taking a toll on food and environmental safety.
As a list of commodities is confronted by increased import hurdles, a surge in China's exports of steel and aluminium has sparked complaints of unfair subsidies, many of which tend to come from provincial governments. China's steel industry has denied it is being supported by Beijing.
"Our main message to the Chinese government is please stop subsidising the unprofitable aluminium smelters," Vladislav Soloviev, first deputy chief executive at Russian aluminium giant Rusal, told Reuters.
"China is a member of the WTO and needs to comply with the rules."