Difference analysis of soil organic carbon pool in returning farmland to forest in loess hilly area

To determine the effects of different plantation on the quantity and quality of soil carbon pool, the fractions store and carbon management index (CMI) of soil organic carbon (SOC) in returning farmland in loess hilly region were investigated. After 12 years of returning farmland, the mass fraction of total organic carbon (TOC) of forest lands was increased mainly in 0-40 cm soil layer compared with the farmland, and the increasing rate presented a trend of Buckthorn>Robinia> Prunus armeniaca>Poplar>abandonment. The maximal growth of TOC was in the layer of 0-10 cm (71.1%-156.9%), and the minimal was in 20-40 cm (23.5%-68.9%). Thus, TOC in 0-100 cm soil layer improved significantly in each forest land. The labile organic carbon (LOC) density in 100 cm depth soil improved from 9.9% to 106.8% in the following order: abandonment, Robinia, Buckthorn, Prunus armeniaca and Poplar. Non-labile organic carbon (NLOC) was increased from 14.2% to 43.0% in the following order: abandonment, Robinia, Prunus armeniaca, Poplar and Buckthorn. The forest species exhibited different content and distribution of LOC and NLOC in each soil layer. Compared with the farmland, Prunus armeniaca, Buckthorn and Robinia improved CMI by 1.28 times in 0-20 cm soil, and Prunus armeniaca and Poplar improved CMI by 1.2-2.49 times in 20-100 cm soil. Consequently, returning farmland showed the potential of promoting soil carbon pool and quality. Buckthorn and Prunus armeniaca improved organic carbon content and the quality of carbon pool in soil respectively.