Soil bacterial community composition rather than diversity exhibits edge effects in a farming-pastoral ecotone

Edge effects have significant implications for ecological conservation and biodiversity maintenance. However, whether soil bacteria exhibit edge effects in large-scale ecotones remains unclear. In this study, we investigated soil bacterial communities in the farming-pastoral ecotone of northern China (FPEN), one of the world's largest ecotones, and compared them with those in adjacent grassland and cropland areas. Paired samples from both land types within the mosaic distribution of the ecotone revealed that land uses exerted relatively minor effects on soil bacterial communities in the FPEN. Across all samples, soil bacterial diversity, including Shannon and Faith's PD, followed a trend of grassland < ecotone < cropland. Interestingly, the entire ecotone acted as a semipermeable barrier, blocking 10.1 % of OTUs from moving from grassland to cropland and 9.3 % from cropland to grassland, while 3.6 % of OTUs were confined to the ecotone as endemic taxa. This led to the highest OTU richness in the FPEN. Compared to the adjacent ecosystems, the soil bacterial community composition in the ecotone showed significant differences and the lowest similarity. These results underscore the importance of identifying and preserving the intact FPEN to maintain soil bacterial diversity. Furthermore, the genera Ferruginibacter and Opitutus and the families Sporichthyaceae and P3OB-42 were identified as biomarkers of the ecotone, providing essential references for its accurate localization. In addition, the community similarity decreased with increasing environmental distance across all three ecosystems but was consistently lowest within the ecotone, indicating its susceptibility to environmental changes. Climate and vegetation conditions, particularly the normalized difference vegetation index (NDVI) and mean annual wind speed (MAWS), were the primary drivers of changes in soil bacterial diversity and community composition. In summary, soil bacterial community composition rather than diversity exhibited edge effects in the FPEN.