Afforestation with Caragana korshinskii enhances the quantity and stability of soil organic carbon in the northern Loess Plateau, China

The northern Loess Plateau, China is an eco-environment fragile zone with the most serious soil and water loss in the world. Hence, soil face the direct threat of texture coarsening, fertility decline and loss of microbial biodiversity, etc. Selecting rational afforestation species will be beneficial for the maintenance and improvement of soil structure and function in this area. Populus simoni (PS, tree), Caragana korshinskii (CK, shrub) and Salix psammophila (SP, shrub) are major perennial woody plants for afforestation in this region, although they play important roles in preventing soil and water loss, blocking wind and fixing sand and increasing farmers’ income, it is not clear how long-term plantation of these plants affect the quantity and stability of soil organic carbon (SOC) and whether the afforestation species are sustainable for soil resources. So, mature stands over 30 years per species were chosen, and SOC and its stability (the intra-microaggregate fine particulate OC and mineral-associated OC are protected C) whthin 0-10 cm and 10-20 cm were compared. Wet sieving method was used to separate water-stable aggregates, and physicochemcial methods were used to isolate different organic carbon fractions. The results indicated that: 1) SOC content of the bulk soil for CK stands increased by 46.4% and 75.8%, respectively at 0-10 cm, and 32.6% and 67.5%, respectively at 10-20 cm compared to PS and SP stands. 2) The <0.25 mm microaggregate content accounted for more than 74% for three stands, and CK stands had higher free microaggregate (0.053-0.25 mm) during both depths and higher small macroaggregate (0.25-2 mm) at 0-10 cm compared to PS and SP stands, the mean weight diameter at the 0-10 and 10-20 cm soil for CK stands were significantly increased compared with PS and SP treatments. 3) The protected C accounted for 74% ~85% of the total SOC. The protected carbon was significantly and positively correlated with the SOC across treatments. In conclusion, long-term C. korshinskii planation can increase the quantity and stability of SOC via increasing the content of protected C, suggesting C. korshinskii is the rational afforestation species to combat soil degradation under future climate change from the angle of C sequestration.