Soil microbial population and nitrogen-transforming bacteria from 11-year sorghum tillage plots

Soil tillage alters crop residue placement, soil moisture and soil physical properties, which in turn may affect soil chemical and microbial properties. The impact of tillage on microbial populations was investigated by studying soil microbiological, physical and chemical properties after 11 years of a tillage management experiment with continuous sorghum (Sorghum bicolor L. Moench) on an Orelia sandy clay loam (fine-loamy, mixed, hyperthermic Typic Ochraqualf). Soil from 0–7.5 and 7.5–15 cm depths was sampled from four replications of moldboard plow (MB), conventional till (CT), minimum till (MT) and no-till (NT) treatments in March (before planting) and May (during the growing season), 1989. Heterotrophic bacteria (HB) and actinomycete populations differed among tillage treatments before planting. Soil microbial populations differed with soil depth among all tillage treatments before the growing season (when no plants were present), but were similar among tillage treatments when counts by soil depth were composited, indicating that, although microbial distribution differed, total populations were similar. During the growing season, actinomycetes were lower in NT treatments, while HB counts were not significantly different between treatments. The similarity between treatments was attributed to drier soil conditions and the effect of plant roots in all treatments, which may have stimulated microbes similarly in all treatments. Ammonium oxidizers were lowest and dentrifiers highest in the NT soil a the 0–7.5 cm depth before planting. The sample obtained during the growing season indicated that soil nitrifier populations were usually greater in MT and NT treatments. The MB and CT treatments had significantly larger amounts of soil moisture, clay and nitrate than the NT and MT treatments, indicating that soil physical properties and nitrate levels were altered by tillage treatment. Soil nitrate was significantly correlated with nitrogen-transforming bacteria in several instances, although there was no predictable effect from soil depth or sampling date. In some instances, nitrifier counts fell and denitrifier counts increased as soil nitrate increased. HB and actinomycete population levels were not significantly correlated with the soil physical or chemical properties measured in this study. Although there were tillage treatment differences in soil physical (moisture, bulk density and clay content), chemical (nitrate-nitrogen, NO₃---N) and microbiological properties (HB, actinomycetes, and denitrifier, nitrifier and ammonium oxidizer activity), generalizations about tillage system effects on soil microbiological properties were difficult to confirm because of the temporal nature of these differences.