Song Xinyuan is left frustrated while planting his rapeseed. As spring approaches, he is one of the many farmers across China planting crops.

"We used to have about seven mu (0.47 hectares) of farmland, but five mu was bought for industrial development," said the 52-year-old from the eastern province of Anhui. "What's more, we never expected the new petrochemical plant to pollute the underground water. This reduces crop output by over 30 percent. Crops often wither."

Song once filed a lawsuit against the Anhui provincial environmental protection bureau. He accused the agency of violating rules in approving the petrochemical plant. He lost the lawsuit.

Song is not alone.

According to the second national land survey results in December, 3.3 million hectares of land was not suitable for plantation due to varying degrees of pollution.

China has pledged to keep 1.8 billion mu (120 million hectares) of arable land, an amount considered as a "red line" minimum to ensure food security.

Authorities, however, face an uphill battle because of worsening soil and water pollution, urbanization, and land transfers for non-grain crops.

Some delegates said they were haunted by the situation, as they gathered for the National People's Congress (NPC) in Beijing for the annual session of China's top legislative body.

"According to my knowledge, the acreage of farmland is not declining. But the balance is coming from people developing barren land in deserted mining areas and other places with less sunshine and fresh water. This is not making up for the loss of fertile farmland," said NPC deputy Chen Wenfu.

"The actual decline in farmland quality poses a potential threat to the country's food security and we should be clear about the risk," said Chen, also an academician with the Chinese Academy of Engineering.

Illegal land seizures are another concern.

In a village in Shenyang, capital of the northeastern province of Liaoning, the illegal development of villas last year encroached on more than 60 hectares of farmland and left local villagers with less land to grow crops.

To curb land seizures, local authorities should pay a heavy price for allowing illegal grabs, said Xiao Xingzhi, a professor at the Dongbei University of Finance and Economics.

To protect food security, governments need to take tough measures, including punishing or excluding officials from job promotions and suspending approvals of development projects if they are found to be involved in illegal land grabs, Xiao said.

Meanwhile, as agricultural investors seek fatter profits, more land is being transferred for cash crops rather than grains. This poses a risk to grain security.

Over the past year, the Chinese central government has been promoting rural land transfers to encourage large-scale farming development. But in many regions, bigger farms are used for non-grain crops.

Some local governments even support non-grain projects as they try to gain more revenues through taxes, said NPC delegate Xu Congxiang. "The non-grain tendency could reduce the country's grain acreage and thus pose a threat to grain security," said Xu, also a village chief in Anhui Province.