Specialist species of fungi and bacteria are more important than the intermediate and generalist species in near-urban agricultural soils

Near-urban farmland is an important component of agricultural soils in China. Understanding the distribution and functional patterns of soil microbial communities is essential for accurate fertilization and disease control in this area. By considering the generalists, intermediates, and specialists of soil microbiota, we studied the soil microbial community assembly process and functional transformations characteristics in near-urban farmlands. We found that the intermediate group has the highest relative abundance with 66.24 % and 50.33 % for fungi and bacteria, respectively. A steeper geographic distance decay relationship (DDR) was found for the specialist bacteria than the generalist and intermediate taxa in the near-urban farmlands. However, the generalist species of fungi showed a steeper DDR than the intermediate and specialist. Both bacterial and fungal generalists have the widest niche, followed by the intermediate and specialists, respectively. The null model indicated that the deterministic processes dominate the community assembly process for bacteria (56%) and fungi (54 %). Variation partitioning analysis (VPA) showed that both the specialists and intermediate groups were more susceptible to geospatial distance than the generalists. Soil water content, C and N nutrients were the main factors driving the changes in the community structure of dominant generalist, intermediate and specialist species. The main functional groups of the generalists and specialist bacteria were chemoheterotrophy. Moreover, specialist bacteria have much more functions related to nitrogen transformation and organic matter degradation than generalist species. For fungi, the specialist species mainly are responsible for saprophytic, pathotrophic_saprotrophic and symbiotrophic nutritional types. Interestingly, the specialist fungi also consisted of a higher number of saprophytic, pathotrophic and symbiotic trophic functions. Thus, fungal and bacterial specialist species, rather than the intermediate or generalist groups support the soil microbial ecosystem functions in the near-urban farmland.