Environmental and agronomic drivers of bacterial and fungal alpha diversity in crop rhizosphere: a multifactorial field study.

Interactions with biotic and abiotic factors significantly impact soil microbial diversity, shaping it with varying intensities. This study investigated the effects of crop type (barley and maize), locality (Slovakia and the Czech Republic), growing year (2021-2023), seed treatment variant (control; seeds treated with fungicide; seeds coated with a superabsorbent polymer (SAP); and seeds treated with both fungicide and SAP), and repetition on microbial biomass and the alpha diversity of bacterial and fungal communities in the rhizosphere. Microbial community structure was assessed using automated ribosomal intergenic spacer analysis. The highest microbial biomass, measured as total extracted double-stranded DNA (dsDNA), was recorded in the rhizosphere of barley grown in the Czech Republic, reaching values above 40 µg dsDNA/g of dry rhizosphere soil in 2023. In contrast, the lowest microbial biomass was found in the rhizosphere of maize from the same region and year, with values as low as 5.6 µg dsDNA/g of dry rhizosphere soil. Alpha diversity analysis revealed higher richness in fungal communities than in bacterial communities, with up to 69 fungal operational taxonomic units (OTUs) and 44 bacterial OTUs detected per profile. Growing year and locality were the dominant drivers of microbial biomass and diversity, while crop type had a moderate effect. Seed treatments and experimental repetitions did not independently affect microbial diversity; however, several significant interactions among factors were detected, indicating context-dependent microbial responses. These findings highlight the complexity of rhizosphere microbial communities and emphasize the importance of temporal and spatial variability when evaluating management practices in agricultural systems.