An interpretable soil-pH suitability index for arbuscular mycorrhizal fungal genera

Soil pH is among the most consistent abiotic axes structuring arbuscular mycorrhizal (AM) fungal communities, yet the pH dependence of individual taxa has not been turned into an interpretable instrument that resolves preference at the genus level and maps globally. HESI (Hydrogenic Edaphic Suitability Index) is a fuzzy-set trapezoidal pH-membership function for twelve Glomeromycotina genera whose four parameters are read from binned presence-rate profiles in GlobalFungi by an explicit rule rather than fitted, then scored on an independent SoilGrids pH surface. Under spatial block cross-validation, which tests transfer to unsampled regions, the mean binned Boyce index was + 0.19; in-sample across the whole GlobalFungi pool, it was positive for eleven of twelve genera (mean + 0.52), eight with bootstrap intervals above zero and five distinguishable from a label-permutation null. The flexible pH spline matched or exceeded machine-learning models, most clearly at the species level, where the learners fell below zero while the unfitted function remained positive. The genus with a negative index and the one with no discriminating window mark the diagnostic limits of a single-axis model. Species-level pH optima estimated independently in GlobalAMFungi agreed in rank with those from GlobalFungi (ρ = +0.46, p = 0.030), and family-level optima with small-subunit virtual-taxon optima (ρ = +0.79 across eight families), so the niches track an independent marker; whether the scored surface transfers is unresolved at these sample sizes. Soil pH sorted which genera occur, a signal distinct from the temperature-dominated richness of published global maps. The per-genus surfaces are offered as spatially explicit, testable hypotheses.