In the last decades, tendency amongst growers in Australia has been towards practising minimum disturbance for soil preparation and complete stubble retention.Consequently, farmers have reduced labour, energy and machinery costs, and improved soil quality and crop profitability. However, there is scepticism around such practices given the build-up of herbicide resistant weeds, stubble-borne diseases and nutrient stratification. To address these issues, occasional tillage (or strategic tillage, ST) has been applied to fields under long-term no-tillage (NT). ST aims to overcome constraints arisen by no-tillage and takes into account soil water content, tools for deployment and timing of application. Soil microbial communities play an important role in soil function and are often used as early indicators of soil disturbance due to their prompt response to environmental changes. I compared biological indicators of soil health including microbial biomass, microbial enzymatic activity, metabolic diversity, genetic structure; diversity of bacterial, archaeal and fungal communities, and abundances of nitrogen cycle genes to assess the impact of ST on long-term NT. In Hermitage, ST was applied to soils under NT and conventional tillage (CT), in which stubble was retained. Soil samples were collected 3.5 and 13 months after ST operations. While it is clear that there would be immediate effects on soil biology, I chose these time points to allow a reasonable amount of time to evaluate whether microbial communities can restore and provide the ecosystem function needed for crop performance. I observed differences between NT and CT in enzymatic activity after 3.5 months, possibly attributed to physico-chemical properties and land management. However, one year after tillage deployment there were no changes in the biological indicators, microbial structure and diversity that were measured after ST.High throughput 16S rRNA gene amplicon sequencing targeting bacteria and archaea revealed 69 operational taxonomic units (OTU's) at relative abundances higher than 1%. The most abundant phyla were Crenarchaeota (25%) and Acidobacteria (28.1%).Fungal communities profiled by internal transcribed region (ITS) amplicon high throughput sequencing showed no significant differences between treatments. The 3 most abundant phylum was Ascomycota (60%) with a relative abundance higher than Nevertheless, assessment of abundance of nitrogen cycle genes for the three sites showed a significant impact of ST management. In Hermitage and Jimbour, nitrogen fixers tended to be enriched after ST application probably due to organic residue incorporation, previously shown to influence the abundance of these organisms. In contrast, the abundance of nitrogen fixing communities tended to decrease on surface soils in Biloela. The decrease may be related to inorganic forms of N affecting the nitrogenase activity in diazotrophic communities. Ammonia oxidizing bacteria (AOB) had an increase in abundance after ST application in Hermitage for soils under NT...

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