Effects of recovery models on organic carbon pathways: A method using 13C natural abundance

The rehabilitation of disaster-prone areas can enhance the stability of soil structure and is a common way to increase organic carbon storage. The response of soil carbon sequestration pathways to different recovery modes is not clear, especially in mountain soils. After 11 years of recovery, we evaluated soil organic carbon (SOC) sequestration pathways in plantations (dominated by Olea europaea ‘Leccino’), croplands [Zea mays (L.)] natural shrublands (Lycium chinense Mill), and natural grasslands [Setaria viridis (L.) Beauv]. The physical and chemical properties of the soil and the 13C natural abundance of each aggregate and its density components were studied. The results showed that, during the restoration process, the soil organic carbon content of natural grassland increased the most, while the δ13C value of soil of natural shrubs was the highest. The natural abundance of 13C was used to reveal the pathway of C flow in soil organic matter (SOM), as follows: free light fractions (ρ < 1.6 g cm−3)→mineral fractions (ρ > 2.0 g cm−3)→dense occluded fractions (ρ from 1.6 to 2.0 g cm−3) (in plantation, natural shrubland, and grassland). However, in cropland soil, C flowed as follows: mineral fractions→free light fractions→dense occluded fractions. Specifically, the SOC content decreased with aggregate particle size, and after entering the soil, plant litter was first stored in large aggregates and then decomposed into the free light fraction. The study revealed the mechanism of organic carbon sequestration in the restoration area, emphasizing that artificial restoration treatment can change the carbon conversion pathway, and reduced the sequestration of organic carbon.