An understanding of the distribution of soil microorganisms and enzyme activities at different soil aggregate level could help to understand the mechanisms operating behind different tillage management on soil structure and function. Our objective was to determine if the microbial community structure and soil enzymes activity (EA) associated with different aggregate fractions changed within the transition at switching between no-till and conventional tillage at 30 months after the switch of

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... gement on a base line field of 27 years long were no-till (NT) and conventional tillage (CT) were side by side compared. Part of NT plot was turned into new CT (n-CT) while part of CT plot was turned into new NT (n-NT). Aggregate fractions of 2000-250, 250-63, 63-20, 20-2 and 2-0.1 micrometers were obtained from soil samples taken at 30 months after the switch. Specific microbial abundances, measured by qPCR, and EAs were greatest on 2-0.1 micrometers following by 20-2and 2000-250 micrometers. The EAs showed the highest activities in the CT (Glucosidase and Cellobiosidase) and in the nCT (Phosphatase and Glucosaminidase) in 2000-250 micrometers. In contrast, in the intermediate fractions (250-63 micrometers and 63-20 micrometers), the highest activities were observed in NT soil. Microbial communities were significantly different among different aggregates. In the 20-2 micrometers fraction, fungi were able to differentiate between current treatments, and bacteria and archaea showed similar trends. In 2000-250 micrometers, the treatments were associated by their historical management, and the abundances in CT samples were superior to those of the NT. In contrast, in the fractions 250-63 and 63-20 micrometers, the NT samples showed greater abundances to those of the CT and the new treatment samples have suffered differences from historical treatments. In conclusion, tillage systems influenced the spatial distribution of soil enzymes as well as the abundances of microbial communities in the different soil aggregate size fractions.