Phosphorus availability influences potassium chemistries in the ectomycorrhizal fungi Pisolithus tinctorius and Paxillus ammoniavirescens.

Ectomycorrhizal (ECM) fungi play essential roles in tree nutrition and soil biogeochemical cycling by mediating the acquisition and storage of mineral nutrients. While phosphorus and nitrogen exchange between host plants and ECM fungi are well documented, potassium (K) dynamics remain poorly understood. Using synchrotron-based X-ray fluorescence (XRF) imaging and K-edge X-ray absorption near-edge structure (XANES) spectroscopy, we compared the spatial distribution and chemical speciation of K and P in two Boletales fungi - Pisolithus tinctorius and Paxillus ammoniavirescens - grown under P-sufficient and P-limited conditions. Both species exhibited reduced K and P abundance under low P, but P. ammoniavirescens maintained higher and more spatially variable concentrations of both elements. K XANES analyses revealed distinct species-specific chemical fingerprints: P. tinctorius displayed a reduced diversity of K species under P limitation, dominated by humic- and tartrate-bound forms, whereas P. ammoniavirescens preserved a broader suite of organic and inorganic K compounds, including persistent KH2PO4. These results indicate that ECM fungi employ divergent strategies for K and P management, reflecting their ecological specialization. P. tinctorius adopts a conservative nutrient-retention strategy, while P. ammoniavirescens exhibits greater physiological plasticity. Together, these findings provide new insight into the functional and evolutionary diversity of nutrient regulation among ECM symbionts.