The processes governing soil bacteria biogeography are still not fully understood. It remains unknown how the importance of environmental filtering and dispersal differs between bacterial taxonomic and functional biogeography, and whether their importance is scale-dependent. We sampled soils at 195 plots across the Tibet plateau, with distances among plots ranging from 20 m to 1,550 km. Taxonomic composition of bacterial community was characterized by 16S amplicon sequencing, and functional community composition by qPCR targeting 9 functional groups involved in N dynamics. Twelve climatic and soil characteristics were also measured. Both taxonomic and functional dissimilarities were more related to environmental dissimilarity than geographic distance. Taxonomic dissimilarity was mostly explained by soil pH and organic matter, while functional dissimilarity was mostly linked to moisture, temperature and N, P and C availabilities. The roles of environmental filtering and dispersal were, however, scale-dependent and varied between taxonomic and functional dissimilarities, with distance affecting taxonomic dissimilarity over short distances (<~300 km) and functional dissimilarity over long distances (>~600 km). The importance of different environmental predictors varied across scales more for functional than taxonomic dissimilarity. Our results demonstrate how biodiversity dimension (taxonomic versus functional) and spatial scale strongly influence the conclusions derived of bacterial biogeography.

The processes governing soil bacteria biogeography are still not fully understood. It remains unknown how the importance of environmental filtering and dispersal differs between bacterial taxonomic and functional biogeography, and whether their importance is scale-dependent. We sampled soils at 195 plots across the Tibet plateau, with distances among plots ranging from 20 m to 1 550 km. Taxonomic composition of bacterial community was characterized by 16S amplicon sequencing, and functional community composition by qPCR targeting 9 functional groups involved in N dynamics. Twelve climatic and soil characteristics were also measured. Both taxonomic and functional dissimilarities were more related to environmental dissimilarity than geographic distance. Taxonomic dissimilarity was mostly explained by soil pH and organic matter, while functional dissimilarity was mostly linked to moisture, temperature and N, P and C availabilities. The roles of environmental filtering and dispersal were, however, scale-dependent and varied between taxonomic and functional dissimilarities, with distance affecting taxonomic dissimilarity over short distances (<~300 km) and functional dissimilarity over long distances (>~600 km). The importance of different environmental predictors varied across scales more for functional than taxonomic dissimilarity. Our results demonstrate how biodiversity dimension (taxonomic versus functional) and spatial scale strongly influence the conclusions derived from bacterial biogeography studies.

Ammonia oxidizers are the primary agents for nitrification. The most significant change in an ecosystem’s nitrogen (N) cycle is the stimulation of ammonia oxidizer activity caused by N enrichment and the subsequent development of a highly nitrifying soil environment. However, few studies have examined the duration of N-induced impacts on nitrification after the cessation of N inputs. We conducted a 15 years nutrient addition experiment in a Tibetan alpine meadow, and then nutrient addition was ceased for 7 years, and the ammonia-oxidizer communities were examined. Ammonia oxidizers, especially ammonia-oxidizing bacteria (AOB), exhibited a continuous hysteretic response to N enrichment and subsequent cessation. Soil ammonia availability has been proposed as a primary driver controlling AOB abundances under nutrient addition. Although plant N and soil inorganic N concentrations showed rapid recovery in plots where nutrient additions were discontinued, soil microbial N mainly controlled changes in N mineralization and availability of soil N to ammonia oxidizers. Pyrosequencing data revealed that nutrient cessation significantly impacts AOB, and AOB composition established connections than AOA and comammox Nitrospira, indicating the greater ecological importance of AOB in the overall network. Our findings suggest that a high nitrification environment induced by N enrichment is not possible simply by reducing N input, and it requires a long recuperation time.