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RESUMO Uma das metas do melhoramento genético de arroz no Sul do Brasil ao longo dos últimos 40 anos foi modificar a duração do ciclo de desenvolvimento das cultivares. Em geral, o ciclo de desenvolvimento foi encurtado, com o objetivo de aumentar a competição com o arroz vermelho (2003/2004 and 2004/2005

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Soil management systems cause many changes in the microenvironment that directly affect activity and diversity of microorganisms. In lowlands, there is a gap in relation to the adoption of no-tillage (zero-tillage) and the impact it has on soil under cultivation of irrigated rice. This study, an in-field experiment, evaluated the microbial enzymatic activity and diversity in an Entisol cultivated with rice under different managements for 22 years. The experiment started in the 1994/95 growing season, and the treatments were no-tillage, conventional, and pregerminated management systems. After 22 years, the data obtained on most of the evaluation dates indicated that no-tillage increased microbial biomass carbon (+45%), microbial biomass nitrogen (+54%), and basal respiration (+54%). No-tillage compared to management under soil tillage (pregerminated and conventional tillage) increased the activity of β-glucosidase (+43%), acid phosphatase (+68%), diacetate fluorescein (+34%), and urease (+96%).The enzyme activity was correlated with the soil organic carbon content and particulate fraction. Despite the relatively high enzyme activity with no-tillage, bacterial richness was maintained in this soil management system. The Proteobacteria phylum has a greater abundance in the NT (43.2%) in relation to the CT (32.3%). Bacteroidetes phylum has a lower abundance in the NT (10.0%) in relation to the CT (15.2%). The Verrucomicrobia phylum has a greater abundance in NT (8.9%) in relation to CT (4.9%). The results suggest that no-tillage is an important management tool in the recovery of irrigated rice areas whose soil has undergone microbiological degradation.