[Change of farmland soil organic carbon content in typical region in Hunan Province].

Taking Yuanjiang City (112 degrees 16' - 112 degrees 56' E, 28 degrees 42' - 29 degrees 11' N) in Hunan Province as an example, the changes of soil organic carbon content and its distribution in farmland soils in Dongting Lake region were investigated, based on the dense sampling in typical plots and the investigation of historical materials. The results showed that soil organic carbon in different land uses and cropping systems, such as rice/rice, rice, paddy-upland rotation, increased stably, but soil organic carbon in dry land induced in some extent. Mean soil organic carbon content in farmland soils of the typical sampling region in 2004 was (26.66 +/- 4.93)g x kg(-1). Compared with that in 1979,soil organic carbon content increased by 22.64%. In 1979,soil organic carbon content in paddy soil mainly ranged from 20 g x kg(-1) to 25 g x kg(-1) (frequency is 62.5% in all sampling plots), and in 2004 increased to 20-35 g x kg(-1) (frequency is 88.0%). This indicated that soils under different types of land use in Dongting Lake region could all store carbon, and organic carbon pool in farmland soils would play "sink" role in the cycle of atmosphere CO2.