Occasional tillage has no effect on soil microbial biomass, activity and composition in Vertisols under long-term no-till

Heavy rains in recent years have triggered an increase in herbicide-resistant weeds and crop diseases in long-term no-till (NT) farming systems in Queensland, Australia. As a possible solution, occasional or strategic tillage (ST) has been applied during summer fallow in two farms located near Jimbour and Biloela, Queensland, Australia. We investigated the impact of different frequencies (one to three passes) and timings (December, January and March) of tillage imposition on microbial indicators of soil health. Tillage implements included chisel plow sweeps at the Biloela site and narrow chisel point and offset disc at the Jimbour site. Seven soil samples were collected from each plot in April 2013 at 17, 10 and 2 weeks post-ST from 0 to 0.1 and 0.1 to 0.2 m depths and composited separately for each soil depth. Samples were analysed for microbial biomass C, enzyme activity, community-level physiological profiling (CLPP) via microrespirometry method and bacterial genetic fingerprinting. Overall, there were no significant differences for any of these parameters between NT and ST at both sites. However, irrespective of tillage treatments, significant differences between soil depths were found for enzyme activity (Biloela), substrate utilisation (Jimbour and Biloela) and bacterial genetic fingerprinting (Jimbour). There were no major effects of ST on the microbial indicators used either under different timings, frequencies or type of tillage implement. Therefore, ST with chisel plow sweeps, narrow chisel point and offset disc may be undertaken with minimal impact on soil microbial communities to combat problems associated with long-term NT Vertisols, such as weed and soil-borne disease outbreak in Queensland, Australia.