Changing communities of ammonia-oxidizing Betaproteobacteria and Thaumarchaea at discrete vegetation zones along a hill slope grassland gradient

To understand the effect of vegetation types on microbial ammonia-oxidizing communities, we analyzed quantitative and qualitative features of the communities of ammonia-oxidizing Betaproteobacteria and Thaumarchaea along a grassland gradient with distinct vegetation zones, i.e. , acidic grassland (AG), matgrass sward (MS), and calcareous grassland (CG). Based on measured abiotic soil characteristics, soil from CG was significantly different from soils collected from AG and MS, which was mainly due to pH. With respect to the communities of the ammonia-oxidizing microorganisms, soil from CG differed also significantly from soils collected from AG and MS. Numbers of Thaumarchaea determined by qPCR based on the amoA gene, were significantly larger in soil from CG compared to soils from AG and MS. Potential ammonia-oxidizing activities (PAA) increased significantly from MS soil via AG soil to CG soil. No effect of vegetation zone on the numbers of ammonia-oxidizing Betaproteobacteria was found. Overall, Thaumarchaea outnumbered ammonia-oxidizing Betaproteobacteria, and PAA correlated with the numbers of Thaumarchaea but not with the numbers of ammonia-oxidizing Betaproteobacteria. Significantly higher ammonium to nitrate ratio and significantly lower PAA in the soil from the MS vegetation compared to the AG and CG vegetation may imply the presence of biological nitrification inhibition. Since especially the soils from the MS and AG vegetation are abiotically rather similar, the vegetation itself might be responsible for the inhibition of ammonia oxidation as is discussed. • Thaumarchaea outnumbered ammonia-oxidizing Betaproteobacteria along the grassland gradient. • Potential ammonia-oxidizing activities (PAA) correlated only with Thaumarchaea. • Soil pH was the first hierarchical factor that distinguished between communities. • High NH 4 + to NO 3 − ratios, and low PAA implied nitrification inhibition in the matgrass sward.