Mixing with coniferous tree species alleviates rhizosphere soil phosphorus limitation of broad-leaved trees in subtropical plantations

Planting broad-leaved trees with coniferous species typically influences the rhizosphere soil microorganisms of broad-leaved trees. However, how tree species mixture affects microbial community compositions and microbial nutrient limitation represented by soil extracellular enzyme stoichiometry (EES) remains poorly understood. We investigated the microbial taxonomic composition and analyzed the extracellular enzyme activities (EEAs) related to carbon (C), nitrogen (N) and phosphorus (P) acquisition in the rhizosphere soil of Schima superba in pure and two mixed plantations (mixed with Pinus massoniana or P. elliottii). The rhizosphere soil organic matter quantity and quality were determined using Fourier transform infrared spectroscopy. Mixing with coniferous species significantly increased the rhizosphere soil fungal Shannon and ACE diversity values and led to the prevalence of microbial oligotrophs in the rhizosphere soil of S. superba. Mixing increased the relative abundances of bacteria belonging to Chloroflexi and fungi belonging to Basidiomycota but decreased the relative abundances of bacteria belonging to Proteobacteria and fungi belonging to Ascomycota in the rhizosphere soil of S. superba. The decreasing rhizosphere soil organic C and dissolved organic C concentrations and increasing ratio of aromatic/aliphatic compounds were responsible for the changes in the microbial community. The average natural logarithm of the C, N, and P enzyme ratio was 1:0.92:1.68, and the enzyme vector angles were more than 45°, indicating that soil microorganisms had a relatively high demand for P. Mixing increased the natural logarithm of the rhizosphere soil enzyme activity N:P ratio and decreased the vector angle, and the soil EES and vector angle were dependent on the relative abundance of microbial oligotrophs. Our results suggest that tree species mixture significantly changes the microbial community composition and alleviates microbial P limitation in the rhizosphere soil of broad-leaved trees.