Soil and host-tree associations with Tuber aestivum's ITS abundance in productive Istrian oak forests

Understanding the ecological drivers of Tuber aestivum productivity in natural oak forests remains limited despite the species’ broad ecological niche and rising economic value. We characterised 65 productive plots of T. aestivum associated with Quercus pubescens and Q. cerris in forest stands of the Istrian peninsula, Croatia. At each plot, we applied a paired sampling design consisting of two systematically collected soil samples: one taken beneath the harvested ascocarp and the other taken 5 m outside the visible brûlé or tree crown area, yielding a total of 130 samples. We quantified soil physical–chemical properties (pH and texture), macronutrients (C, Ca, K, N and P), and mycelial abundance by qPCR (ITS2 copies g⁻¹ soil). Across the study area, a positive relationship between clay content and mycelial abundance emerged as the most consistent, highlighting the importance of soil water retention in T. aestivum mycelial development. In contrast, higher N levels and imbalanced K: Ca ratios were associated with lower mycelial abundance, particularly in Q. pubescens stands, whereas P showed a marginal, host‑dependent positive trend detectable only in Q. pubescens . Trends of increasing and decreasing median pH and nutrient values were detected between paired samples, with these patterns differing between host tree species. Altogether, our results indicate that T. aestivum mycelial development is promoted by clay-rich soils in oak forests and demonstrate the value of paired sampling for disentangling host- and soil‑driven processes in wild T. aestivum ecosystems.