Effects of Returning Farmland to Tea on Soil Organic Carbon Pool of Hilly Region in the Western Sichuan

【Objective】Soil organic carbon pool can effectively characterize the sequestration capacity of soil carbon, study on dynamics of soil carbon pool in typical area of returning farmland to tea of hilly region in the western Sichuan is of great importance to understand the changes and effect evaluation during the process of ecological restoration. 【Method】The research regarded different ages of returning farmland to tea(2-3, 9-10 and 16-17 years old) located in Zhongfeng Township of Mingshan District as a study plot, and the cropland was used as contrasts. Several indexes were analyzed such as the concentration and density of soil total organic carbon(TOC), labile organic carbon(LOC), and non-labile organic carbon(NLOC) to understand the effect of returning farmland to tea, the soil carbon pool management index(CPMI) was calculated, and the relationship between soil organic carbon pool, CPMI and the de-farming years was analyzed. 【Result】The mass fraction of TOC firstly decreased and then increased after returning farmland to tea. In 0-10 cm, 10-20 cm and 20-40 cm soil layers, TOC concentration increased by 5.67%, 5.67% and 0.78 % compared with the control after 16 years of returning farmland to tea, TOC density also increased significantly in this period. The mass fraction of LOC after returning farmland to tea increased significantly, and the average increase was 23 times compared with TOC's. The LOC density in 40 cm soil layer improved from 132.26% to 182.26% in the following order: 2-3 years, 9-10 years, and 16-17 years of returning farmland to tea. The density of soil LOC in the 0-10 cm and 0-20 cm soil layers accounting for 0-40 cm soil depth were more than 30% and 50%, respectively. Mass fraction and density of NLOC were consistent with the trend of TOC after returning farmland. Compared with the control, 2-3 years of returning farmland to tea had the highest efficiency ratios of LOC, and the surface accumulation was relatively obvious but there was no significant difference which shows that the efficiency ratios of LOC were not sensitive to the change of soil depth. With the increase of de-farming year, Soil CPMI fell after the first increase, but was significantly higher than the control, and reached the maximum value in 16-17 years of returning farmland to tea. The CPMI in 0-10 cm and 20-40 cm soil layers were comparatively large, indicating that surface and deep carbon pools have a certain effect on carbon sequestration, and the impact by returning farmland to tea on the surface layer's carbon pool was particularly evident. 【Conclusion】Soil TOC, LOC and NLOC concentrations increased with the increasing period of retuning farmland to tea. After 16 years of returning farmland to tea, the density of TOC, LOC and NLOC were significantly higher than the control in 0-40 cm soil depth, the carbon accumulation in 0-10 cm soil layer was the best. The efficiency ratios of soil LOC significantly increased by returning farmland to tea, the CPMI in 0-10 cm soil layer of 16-17 years of returning farmland to tea was the highest, indicating that the engineering of returning farmland to tea enhanced soil carbon sink and improved soil quality.