Bacterial community response to tillage and nutrient additions in a long-term wheat cropping experiment

The effects of agricultural management practices on bacterial community structure and function are not well defined, yet are of concern for long-term soil resilience. In this study, soil microbial biomass, bacterial community structure (determined by TRFLP), and function, (determined by enzyme assays and Biolog assays), and soil physicochemical properties were investigated in a wheat cropping system subjected to long-term tillage (20 years) and short-term nutrient addition treatments. Samples were taken over a full cropping cycle. Tillage, nutrient addition, and time all significantly affected bacterial community structure (rDNA and rRNA), which showed considerable shifts across the sampling period. Microbial biomass and Biolog profiles changed significantly with time, but were not affected by treatment. With regard to specific enzyme assays, there were significant main effects of treatment and time on glucosidase, phosphatase and phenol oxidase enzyme activity, while for cellobioside and peroxidase, treatment and time had significant interactive effects. For the hydrolases significant effects were observed between nutrient treatments, while for the oxidases they were observed between tillage treatments. Overall, however, we found little evidence of major detrimental long-term effects of tillage on the soil bacterial communities or their important functions in the dryland wheat system studied. The bacterial communities showed both long and short-term trajectories that could be disentangled with appropriate experimental design. Concerns over significant long-term detrimental impacts of tillage on the soil bacterial communities appear unfounded, at least under systems similar to those studied here.