Close-to-nature transformation: enhancing the chemical stability of organic carbon in Pinus massoniana and Cunninghamia lanceolata forests

The close-to-nature forest management affects the soil carbon cycle process by changing the vegetation composition of the stand. However, the effects of close-to-nature t forest management on the chemical stability of soil organic carbon (SOC) have been poorly understood. We analyzed chemical composition and stability of organic carbon taken from three group (soil, litter and fine roots) of Pinus massoniana and Cunninghamia lanceolate close-to-nature forests (PCN and CCN) and their control pure forests (PCK and CCK). Compared with the control pure forests, the proportion of soil alkyl C, the alkyl C/O-alkyl C (A/O-A) ratio, and the evenness of distribution of the four main SOC chemical compositions were all higher in the close-to-nature transformation forests, especially in the PCN. The microbial biomass carbon (MBC) was significantly higher than that of control forests (CCK, PCK). However, the close-to-nature transformation did not significantly affect the soil bacterial richness and diversity. The Alkyl C and O-alkyl C in litter and fine roots were significantly correlated with the SOC chemical components, and there was also a positive correlation between the soil A/O-A ratio and the litter A/O-A ratio as well as the fine root A/O-A ratio. The RDA analysis showed that fine root alkyl C and fine root O-alkyl C were the two most critical factors affecting the chemical composition of SOC. Our findings highlight that the close-to-nature transformation exhibits a relatively high resistance to decomposition. Converting the pine and fir plantations into uneven-aged mixed forests through close-to-nature transformation is capable of enhancing the chemical stability of organic carbon in coniferous plantations.