Conversion of cropland to Chinese prickly ash orchard affects soil organic carbon dynamics in a karst region of southwest China

Knowledge about the effect of land use conversion on soil organic carbon (SOC) due to Grain-for-Green Program in karst regions is needed to assess carbon cycles in karst ecosystems. In order to evaluate the effects of land conversion, soil sampling was performed in a revegetation sequence down to 0-0.3 m, which included the conversion of cropland to Chinese prickly ash orchard (Zanthoxylum bungeanum Maxim.) for 7, 14, and 35 years, in a karst region of southwest China. The soil samples were fractioned into three aggregate-size classes: macroaggregate (250-2000 mu m), microaggregate (53-250 mu m), and silt + clay fraction (< 53 mu m). Results indicated that SOC concentrations and stocks increased after the conversion of cropland to Chinese prickly ash orchard. The highest SOC concentrations and stocks were all found during the 14 years land use conversion at both the 0-0.15 and 0.15-0.3 m depths. However, the SOC concentrations and stocks showed a decrease trend after 14 years land conversion at both soil depths. For all land use types, the macroaggregate-size class was the dominant fraction in storing SOC, followed by silt + clay fraction and microaggregate. In this study, the increases in macroaggregate-associated SOC stocks were due to increases in both macroaggregation and macroaggregate-associated SOC concentrations, especially dominant by increases in macroaggregate-associated SOC concentrations. However, the increases in microaggregate- and silt + clay-associated SOC stocks were only due to increases in amount of the microaggregates and silt + clay fractions, respectively, after conversion from cropland to Chinese prickly ash orchard. The results suggest that conversion of cropland to Chinese prickly ash orchard can play an important role in SOC sequestration. Nevertheless, for long-term Chinese prickly ash orchard plantation, some appropriate management practices should be implemented to prevent SOC losses in karst region of southwest China.