Tree species mixing enhances rhizosphere soil organic carbon mineralization of conifers in subtropical plantations

Tree species mixing is a prevalent practice to improve soil quality in monoculture plantations and the rhizosphere soil organic carbon (SOC) dynamics of conifers will be affected by mixing the species with broad-leaved trees; however, the influence characteristics and underlying mechanism of mixing on rhizosphere SOC mineralization remain elusive. Herein, the rhizosphere soils of two conifers (Pinus massoniana and Pinus elliottii) from pure and mixed plantations were incubated for 50 days. Bacterial communities were investigated by high-throughput sequencing and qPCR of the 16S rRNA gene. The SOC quality represented by carbon functional groups was analyzed using a Fourier transform infrared spectrometer. A higher rhizosphere SOC quality of conifers, particularly the lower ratios of aromatic/aliphatic compounds and (aromatic + aliphatic compounds)/polysaccharide, was found in the mixed plantation compared to the pure plantation. Mixing increased the relative abundance of the conifer rhizosphere copiotrophic bacteria (Proteobacteria and Bacteroidetes) and led to an increase in the bacterial copiotroph:oligotroph ratio. The redundancy analysis showed that the aromatic/aliphatic compound ratio was the predominant factor that significantly affected the soil bacterial communities. The rhizosphere SOC mineralization rate increased after mixing and had a positive relationship with copiotrophic bacteria and the bacterial copiotroph:oligotroph ratio. Our analyses indicate that planting broad-leaved trees in coniferous plantations increases organic carbon mineralization from the rhizosphere soil of conifers and provide potential driving patterns of SOC mineralization related to SOC quality and bacterial life strategy.