More than 33 million hectares of farmland in China has been turned back into forest or grassland in the past two decades, making a significant contribution to the country's greening work.

Since the adoption of the Conversion of Cropland to Forest and Grassland Program in 1999, nearly half the country's increase in artificial forest area has been due to the reversion of farmland, the State Forestry and Grassland Administration revealed on Tuesday.

The nationwide ecological recovery program aims to minimize wide-scale soil erosion and degradation of vegetation, and improve water conservation.

The forest coverage rate in regions included in the program has increased by an average of 4 percentage points in the past 20 years, the administration said.

More than 500 billion yuan ($72.6 billion) has been spent on reversion work, it said, nearly triple the total investment on the Three Gorges Dam, the world's largest hydroelectric project.

An official evaluation report in 2016 said the annual value of the program's ecological contribution — including benefits to land nutrition, water conservation and air purification — is 1.38 trillion yuan.