Biogeoclimatic Regions and Land-Use Structure Bacterial Biodiversity and Inferred Nitrogen-Cycling Potential in Headwater Stream Sediments

Abstract

Abstract Microbial communities in freshwater ecosystems are structured by both natural environmental variation and anthropogenic pressures, yet how these drivers interact to shape biodiversity and ecosystem functioning remains poorly understood. Here, we assessed how biogeoclimatic regions and land-use pressures jointly structure bacterial diversity and inferred nitrogen-cycling potential in stream sediments along a broad environmental gradient across the Iberian Peninsula. We used 16S rRNA metabarcoding and functional inference across samples spanning three biogeoclimatic regions and four land-use types. Bacterial community composition and diversity showed strong regional differentiation, with pronounced taxonomic turnover and distinct alpha-diversity patterns, whereas differences among land-use types were weaker, less spatially coherent, and primarily reflected shifts in specific community components. Despite this variation, a consistent core microbiome was observed across samples, suggesting the presence of taxa shared across contrasting conditions. Inferred nitrogen-cycling functional potential partially mirrored taxonomic patterns, with Mediterranean lowlands enriched in predicted nitrification and denitrification genes, Sierra Nevada in predicted nitrogen fixation and DNRA genes, and the Cantabrian Mountains showing a more even functional profile. Land-use effects on inferred functional potential were subtle and nested within regional patterns. These results suggest that bacterial diversity and inferred nitrogen-cycling potential are more strongly shaped by biogeoclimatic conditions than by land-use, consistent with a hierarchical organization of environmental drivers. The correspondence between taxonomic composition and inferred functional potential was more evident at the regional than at the local scale. Our findings highlight the importance of incorporating regional baselines and integrating taxonomic and functional approaches into freshwater biomonitoring.