Impact of broadleaf tree introduction on rhizosphere fungal communities and root phosphorus-cycling genes in conifers of near-natural transformed plantations

Phosphorus (P) is crucial for plant growth. However, its low availability in subtropical soils necessitates that trees rely on microorganisms for effective P acquisition. The introduction of broadleaf trees has been shown to facilitate P acquisition in coniferous plantations by altering the rhizosphere fungal communities. Despite this, functional shifts in these communities and the expression of root phosphorus cycling genes (PCGs) remain inadequately understood. This study investigated coniferous Pinus massoniana and Cunninghamia lanceolata plantations interplanted with broadleaf species associated with arbuscular mycorrhizal (AM) or ectomycorrhizal (ECM) fungi. Rhizosphere soil and fine roots from the conifers were analyzed to examine soil bioavailable P fractions, root mycorrhizal colonization, rhizosphere fungal community composition, enzyme function predictions, and root PCGs expression. We found that citric-P in rhizospheric soil of P. massoniana increased with the introduction of Quercus gilva (an ECM-associated tree species), whereas Bray-P content in the rhizosphere of C. lanceolata decreased upon the introduction of either Q. gilva or Phoebe zhennan (an ECM-associated tree species). Moreover, the relative abundance of saprophytic fungi (e.g., Mortierella) increased following the introduction of broadleaf trees. Specifically, the introduction of Q. gilva was associated with elevated levels of organic P mineralization genes (e.g., phoA) and enzymes (e.g., phytases and acid phosphatase (ACP)) in conifers. In contrast, the introduction of P. zhennan increased the expression of inorganic P solubilization genes (such as qppC in P. massoniana roots and ppa in C. lanceolata roots). Key contributors to P absorption in conifer roots included Cenococcum, Rhizopogon, and Glomus. This study advances our understanding of P cycling in coniferous rhizospheres and the dynamics of coexisting mycorrhizal tree systems, yielding valuable insights into sustainable management of plantation ecosystems.