Spatial Patterns and Driving Mechanisms of Nematode-Trapping Fungi in a Small Mountain Watershed: Cross-Scale Implications for Microbial Spatial Patterns

Microbial spatial distribution patterns represent a fundamental question in microbial biogeography. However, existing evidence primarily stems from regional and global scales. This study focused on Nematode-Trapping Fungi (NTF) within the Lasha Mountain small watershed (21.09 km2) in the Hengduan Mountains. A total of 119 soil sampling points were established across the watershed, adhering to a principle of spatial uniformity. Species identification was performed by integrating morphological characteristics with ITS molecular sequencing, thereby obtaining spatial distribution data for NTF species. By analyzing species occurrence frequency and the variance-to-mean ratio (VMR), and incorporating 12 environmental factors—encompassing topography, vegetation, and soil physicochemical properties—the spatial heterogeneity of NTF and its potential local environmental associations were examined. The results indicated that the species richness of all NTF exhibited a significant non-random distribution among the sampling points (VMR = 1.4816, p = 0.0003). Kernel density analysis indicated that high species richness was primarily concentrated in the middle-lower reaches of the watershed and the main channel confluence area. Clear geographic differentiation was observed at the genus level, with Dactylellina predominantly distributed in the northern part of the watershed, Arthrobotrys concentrated in the southern part and the main channel confluence area, and the few detections of Drechslerella were exclusively observed in the eastern part of the watershed. Low-occupancy restricted species were the main contributors to the non-random spatial heterogeneity of the community, whereas the distribution patterns of widespread species approached randomness. Marginal tests from permutational multivariate analysis of variance (PERMANOVA) demonstrated that soil moisture (p = 0.012), pH (p = 0.022), total phosphorus (p = 0.036), and gap size (p = 0.039) showed significant marginal associations with NTF community differentiation. Multivariate analyses incorporating both environmental and spatial variables explained only 38.19% of the community variation. This study demonstrates that detectable non-random variation in species richness persists at a small scale, providing an empirical case for microbial biogeography research in the small mountain watersheds of the Hengduan Mountains.