Variance analysis of soil carbon sequestration under three typical forest lands converted from farmland in a Loess Hilly Area

Quantifying soil carbon sequestration may be an important consideration under large scale afforestation because it has been counted in global carbon budgets according to the Kyoto Protocol.The conversion of cropland to forest as part of a huge ecological afforestation engineering scheme has played a very important role in reversing ecological destruction in the Loess Plateau and strongly affects the carbon cycle.This research was conducted to determine the changes in total soil organic carbon and its labile fraction in soil to 100 cm under three typical forested lands.These typical forest lands are Caragana,Buckthorn,and Robinia,which have been converted from farmland between 10 and 40 years ago in a Loess Hilly Area.The results showed that,compared with sloped farmland,the concentration of total organic carbon and labile organic carbon in soil at 100 cm was not higher in Caragana forest land 10 years after conversion from farmland.The carbon pool was significantly increased in Buckthorn and Robinia after the same period.Compared with 10 years since farmland conversion,the total organic carbon and labile carbon was further increased in all forest lands after 20 to 40 years of conversion from farmland.This increase followed the order Robinia Buckthorn Caragana,and the highest increase in total organic carbon reached 90.92,27.87,and 14.89 Mg/hm2,and for labile organic carbon was 30.28,10.51,and 9.67 Mg/hm2 respectively.The changes in soil organic carbon in different soil layers were also significantly different with time since farmland conversion.The soil organic carbon and its labile fraction was increased mainly in the 0—40 cm soil layer 10 years after farmland conversion in all forest lands.The soil organic carbon pools in the 40—100 cm layer were increased significantly 20 years after farmland conversion.As a result,all soil layers showed a contribution to soil organic carbon increase with long term conversion of cropland to forest.Forty years after farmland conversion 35.4%,27.9%,and 27.1% of the increased total organic carbon and 20.2%,45.1%,and 23.1% of the increased labile organic carbon in the soil to 100 cm was sequestrated in the 0—20 cm layer under Caragana,Buckthorn,Robinia forest land respectively.By contrast,the proportion of soil organic carbon sequestration showed inconsistent changes among the forest lands in the 20—100 cm soil layer,with an average of 17.4% of the increased total organic carbon and 17.6% of the increased labile organic carbon sequestrated in each 20 cm layer.Additionally,in comparison with sloped farmland,the ratio of labile organic carbon to total organic carbon was significantly different in each soil layer of all forest lands 40 years after farmland conversion.This was especially so in the 60—100 cm soil layer where the ratio was increased by 146.7%,76.9%,and 126.1% in Caragana,Buckthorn,and Robinia forest lands,respectively.The ratio in the 100 cm soil layer also increased by 63.7%,34.0%,and 47.0% in Caragana,Buckthorn,and Robinia,respectively,which indicated the activation of soil carbon pools had been enhanced,and the quality of soil was improved indirectly.Consequently,conversion of cropland to forest could sequestrate carbon in soil and Robinia is the better forest land to improve the soil organic carbon pool.Soil carbon sequestration following the afforestation of former arable land would be a powerful carbon sink for anthropogenic CO2 production in the Loess Plateau in the future.