Soil microbial biomass and diversity respond to tillage and sulphur fertilizers

There is little information on the effects of S management strategies on so il microorganisms under zero tillage systems on the North American Prairies . Experiments were conducted to examine the effects of tillage and source a nd placement of S on soil microbial biomass (substrate induced respiration) and functional diversity (substrate utilization patterns) in a canola-whea t rotation under conventional and zero tillage systems at three sites in Gr ay Luvisolic and Black Chernozemic soils. Conventional tillage significantl y reduced microbial biomass and diversity on an acidic and C-poor Luvisolic soil, but it had mostly no significant effects on the near-neutral, C-rich Luvisolic and Chernozemic soils, which underlines the importance of soil C in maintaining a healthy soil. Sulphur had no significant effects on soil microbial biomass, and its effects on microbial diversity were more frequen t on the near-neutral Luvisol, which was more S-deficient, than on the acid ic Luvisol or the Chernozem. Significant S effects on microbial diversity w ere observed both in the bulk soil (negative effects, compared with the con trol) and rhizosphere (positive effects) of the acidic Luvisol, but all sig nificant effects (positive) were observed in root rhizospheres in the other soils. Sulphur by tillage interactions on acidic Luvisolic soil indicated that the negative effects of S in bulk soil occurred mostly under zero till age, presumably because the fertilizer is concentrated in a smaller volume of soil than under conventional tillage. Sulphate S effects, either negativ e or positive, on microbial diversity were usually greater than elemental S effects. Therefore, S application can have direct, deleterious effects on soil microorganisms or indirect, beneficial effects through crop growth, th e latter presumably due to increased root exudation in the rhizosphere of h ealthy crops.