Recent studies demonstrated that urban environments, characterized by small-scale heterogeneity and intensive management practices, induce contingency in vegetation community assembly and shifts from niche-based to stochastic assembly processes. However, the extent to which this phenomenon affects the structural and functional relationships among different communities remains poorly understood. We hypothesized that trophic interactions may mediate the cascading of stochasticity to other communities. To this end, we studied the spatial variations in biodiversity of plants, fungi and springtails in an artificial urban lawn in the University of Salerno campus (southern Italy) across adjacent lime tree canopy and open lawn areas (transects of 12 plots each). In each plot, two soil cores for fungal analyses and two for springtail analyses were collected, whereas vegetation was surveyed within a 625 cm² phytosociological quadrat. Plant and springtail species were identified using dichotomous keys, whereas fungal taxa were identified through DNA metabarcoding. Functional traits were measured directly or obtained from the literature.

Results highlighted significant (P < 0.05) relationships between springtail and fungal diversities, as assessed by generalized linear models, and significant (P < 0.001) correlations between plant and fungal diversities, according to Mantel tests, suggesting interdependence among the three investigated communities. Given this interdependency, we supposed that vegetation contingency affects fungal community constraining in turn springtail biodiversity. Moreover, the high spatial variability in all the communities, coupled to the dependence of fungi and springtails upon environmental conditions, suggest that vegetation stochasticity cascades down to increase variability of the latter through environmental filtering at micro-scale. Such hypothesis is supported by co-occurrence network analyses that indicate higher intra-system heterogeneity and stochastic assembly processes.

Collectively, findings coherently indicate that contingency in vegetation controls the structural and functional diversity of fungal and springtail communities, resulting in high heterogeneity at micro-scale deriving from fine-scale environmental filtering mediated by trophic interactions.