Evaluating soil carbon stability by combining δ 13 C and soil aggregates after afforestation on agricultural land and thinning management

Background and aims Afforestation and thinning management are effective ways to mitigate global warming. Soil carbon reconstruction mechanisms can be effectively explored by linking soil aggregates and isotopic 13 C. Methods Soil samples were collected from agricultural land (AL) and larch plantations (established in 1965 and thinned in1995, UT: 2500 tree‧ha −1 , MT: 1867 tree‧ha −1 , and ST: 1283 tree‧ha −1 ). The soil was separated into three aggregate sizes (LMAC: > 2 mm, SMAC: 2–0.25 mm, MIC: 0.25–0.053), minerals associated with organic matter (MAOM: < 0.053 mm), and carbon fractions within macroaggregates. Results We found that afforestation on agricultural land significantly increased the mean weight diameter (MWD). However, intensifying thinning decreased MWD by increasing SMAC. Moreover, after afforestation, the carbon concentration in soil aggregates and MAOM significantly decreased, and the carbon stability of macroaggregates weakened but could be strengthened after thinning. Thinning decreased the C/N in the soil aggregates and MAOM when no change in carbon concentration. The effect of thinning intensity on C/N was obvious with decreasing of particle size. The δ 13 C, mainly controlled by soil aggregates, significantly decreased in each soil aggregate after afforestation but increased after thinning. Additionally, the carbon concentrations, C/N and δ 13 C of small-size particles (< 0.25 mm) and the distribution of SMAC are important for SOC, C/N, CO 2 fluxes and δ 13 C in bulk soil. Conclusion We conclude that soil aggregate distribution is conducive to soil carbon renewal, suggesting that increasing thinning intensity is beneficial for accumulating older carbon and acquiring nitrogen in more stable fractions.