The numbers, types, and activities of soil microorganisms are important to the productivity of soil through their regulatory effect on soil carbon and nitrogen levels. Conservation and conventional tillage systems in the presence of barley stubble on forage corn biomass were established to assess the impact of management practices on microbial populations. Surface soils from different management systems were characterized for aerobic and anaerobic microbial populations and denitrification. Numbers of aerobic and anaerobic microorganisms in surface (0-10 cm) minimum-till soils averaged 1.6 and 1.4 times greater, respectively, than in surface plowed soils. Deeper in the soil (10-30 cm), however, aerobic microbial populations were significantly greater in conventionally tilled soils. In contrast, below 20 cm, the numbers of anaerobic microorganisms differed little between tillage treatments. In reduced tillage management and no-till soil, however, these organisms were found to comprise a greater proportion of the total bacterial population than in conventionally tilled soils. Measurements of the denitrification potential from soils generally followed the observed differences in anaerobic microbial populations. In surface no-till treatment soils, increased numbers of anaerobic microorganisms and a substantially greater denitrification potential, following irrigation, indicate the presence of less-aerobic conditions in comparison to conventionally tilled soils. This condition appears to result from greater soil bulk densities and water contents of no-till soils, which act to increase water-filled porosity and the potential for water to act as a barrier to the diffusion of oxygen through the soil profile. The numbers of total anaerobes in no-till soils, however, decreased much less with depth than did the populations of aerobic bacteria and fungi. Furthermore, at the 10 to 20 cm and 20 to 30 cm sampling depths, total anaerobes represented a greater percentage of the total bacterial community in no-till compared to plowed soils.