Abstract Fluctuations in wetland hydrology create an interplay between aerobic and anaerobic conditions, control領(lǐng) vegetation composition and microbial community structure and activity in wetland soils. In this study, we investigated the vegetation composition and microbial community structural and functional changes along a wetland 5 hydrological gradient. Two different vegetation communities were distinguished along the hydrological gradient; Caricetum gracilis at the wet depression and Arrhenatherum elatioris at the drier upper site. Microbial community structural changes were studied by a combined in situ 13CO2 pulse labe領(lǐng) and phospholipid fatty acid (PLFA) based stable 10 isotope probing approach, which identifies the microbial groups actively involved in assimilation of newly photosynthesized, root-derived C in the rhizosphere soils. Gram negative bacterial communities were relatively more abundant in the surface soils of the drier upper site than in the surface soils of the wetter lower site, while the lower site and the deeper soil layers were relatively more inhabited by gram positive bacte15 rial communities. Despite their large abundance, the metabolically active proportion of gram positive bacterial and actinomycetes communities was much smaller at both sites, 土壤快速粉碎儀