Effects of Microhabitats Around Isolated Trees on Soil Microbial Community Composition in Karst Rocky Desertification Areas

Isolated trees in karst rocky desertification areas act as resource islands and create fine-scale microhabitat gradients, yet the response of soil microbial communities to these fine-scale gradients has not been measured in karst systems. We sampled surface soil around seven isolated trees (which included both naturally remnant and replanted individuals) at three canopy positions—under-canopy (IT1), canopy edge (IT2), and outside the canopy (IT3)—and analyzed bacterial, fungal, and Actinomycetota communities by 16S rRNA gene and ITS sequencing. Soil organic carbon and total nitrogen declined significantly from IT1 to IT3, whereas pH and the mineral elements (TP, TK, Ca, Mg) showed no significant positional differences. At the family level, microbial α-diversity did not differ among positions. The dominant taxa were broadly shared across the three positions, and the compositional differences that did occur were concentrated in a small number of families. Position effects on community composition were moderate for bacteria and Actinomycetota and weaker for fungi, while the measured soil properties explained only a small part of the family-level variation, leaving most of it unexplained. These results indicate that isolated trees generated a clear SOC and TN gradient but did not drive a comparable reorganization of the soil microbial community, and the dominant assemblages remained largely similar across the three positions. The data therefore do not support a single dominant environmental driver or pronounced niche differentiation. For vegetation restoration in karst rocky desertification areas, these findings suggest that retaining isolated trees can help maintain particular microhabitat-associated microbial taxa and preserve the compositional distinctiveness of canopy-edge and under-canopy microsites. Such retention alone, however, is unlikely to produce a broad shift in overall microbial diversity.