Effects of environmental factors and tree species mixtures on the functional groups of soil organic carbon across subtropical plantations in southern China

Purpose There is a large area of coniferous plantations, such as Cunninghamia lanceolata and Pinus massoniana , in subtropics. Recent evidence has demonstrated that environmental factors and tree species mixtures have significant influences on soil organic carbon (SOC) storage in plantations. However, the diverse functional groups of SOC reveal different chemical stability. Therefore, the functional groups of SOC in subtropical plantations need to research. Methods The SOC functional groups analyzed by 13 C nuclear magnetic resonance, and their distribution were compared among subtropical plantations from northern, middle, and southern latitudes, and between monospecific and conifer-broadleaf mixed plantations. Results Compared to that of the northern and middle latitude subtropical plantations, the evenness of the four SOC functional groups was higher in the southern latitude subtropical plantations. The temperature and soil bacterial α-diversity were positively correlated, and the soil fungal α-diversity was negatively correlated with the evenness of the SOC functional groups. The proportion of the recalcitrant SOC functional group (alkyl C) was higher in the conifer-broadleaf mixed plantations than in the coniferous plantations, particularly in C. lanceolata plantations. The climatic factors, soil pH and soil microbial α-diversity, rather than the functional groups of leaf litter and fine root C, led to the differences in the SOC functional groups between the monospecific and mixed plantations. Conclusion Our findings highlight that there is a minor risk of C decomposition in the southern subtropical plantations, and converting coniferous plantations to mixed plantations, which could improve the chemical stability of SOC in subtropical regions.