Effect of Coniferous and Broadleaf Mixture Measures on Soil Organic Carbon Stability of Pinus massoniana Plantation in Red Soil Erosion Area

[Objective] To investigate the effects of coniferous and broadleaf mixture measures on the stability of soil carbon47 pools in Pinus Massoniana Plantation in red soil erosion areas. [Methods] In this study, we examined the impact of forest restoration on soil organic carbon fractions in the Pinus massoniana pure forest (CF) and Pinus massoniana and Schima superba mixed forest (MF) in Changting, Fujian Province. We analyzed soil profiles from different years (Y10, Y20, and Y41) by categorizing soil organic carbon into particulate organic carbon (POC) and mineral-associated organic carbon (MAOC). [Results] (1) Compared with CF, the contents of MAOC, SOC, and POC in the 0—10 cm soil layer in mixed forest significantly increased. The long-term stand mixing effectively avoided the depletion of MAOC in long-term restoration of pine forest. (2) POC/SOC in the 10—20 cm and 40—60 cm and 0—20 cm soil layers in Y20-MF and Y41-MF decreased significantly after coniferous and broadleaf mixture, but MAOC/SOC in the 20—60 cm and 0—10 cm soil layers in Y10-MF and Y41-MF increased significantly; (3) MAOC/SOC was observed in the 20—60 cm soil layer, while in Y41-MF, it was found in the 0—10 cm soil layer. Additionally, in mixed forests, MAOC/SOC in the 0—10 cm soil layer continued to increase with age, whereas POC/SOC decreased significantly. The opposite trend was observed in the masson pine forests. (4) Linear fitting revealed that POC and MAOC were both significantly and positively correlated with SOC, but the increase in SOC in mixed forest soils were more dependent on the increase in MAOC, whereas the masson pine forests soil was more dominated by the non-stable carbon component (POC); (5) Redundancy analysis indicated that DOC, TN, TP, and NH+4 collectively explained 66.2% of the variation in carbon fractions. This suggests that the enhancement of soil nutrient effectiveness following the forest conversion is crucial for MAOC accumulation in mixed forests. [Conclusion] This study demonstrates that forest conversion in subtropical red soil erosion areas could increase the accumulation of stabilized carbon pools by enhancing soil nutrient effectiveness. This process helps avoid the loss of soil carbon pools during long-term restoration efforts.