Fungal-bacterial associations in urban allotment garden soils

The soil microbiome in urban agriculture has not received much attention to date despite its important role in soil functionality. In this work, we evaluated the composition and diversity of fungal and bacterial communities through DNA extraction and ITS/16S marker gene sequencing in 40 soil samples collected from 10 urban allotment garden areas in the city of Santiago de Compostela (northwestern Spain). Despite anthropogenic activities are expected to affect negatively the microbial diversity, the richness of both fungal and bacterial communities was comparable to that of soils from other urban land-use categories with lower anthropogenic influence, i.e., urban forests, urban grasslands, and urban agricultural fields. However, the deterministic effect of soil properties and geographical distances was almost negligible in the surveyed allotment gardens. Neutral community models confirmed that the fungal and bacterial communities followed a random distribution (pseudo-R²_{fungi|bacteria} = 0.653 | 0.898) and that they were more random (tNST_{fungi|bacteria} = 0.54 | 0.74) compared to the other abovementioned land-use categories (tNST_{fungi|bacteria} = 0.46 | 0.67, p ≪ 0.001). Network analysis showed that in contrast to natural soils with lower anthropogenic influence, microbial associations formed very small modules; and frequently, microbial units remained unconnected. Taken together, our findings provide evidence that the soil microbial communities in the studied urban allotment gardens comprised a random assortment of microbes and their interactions, thereby supporting potential implications of anthropogenic activity for soil health and ultimately ecosystem functionality.