Reduced tillage increases soil biological properties but not suppressiveness against Rhizoctonia solani and Streptomyces scabies

Intensive agricultural crop production can lead to a decline in biological soil characteristics and functions, such as soil microbial biomass and activity, carbon and nutrient cycling and soil suppressiveness, important for the sustainable production of food and feed. There is a need to understand how those soil functions can be improved by agricultural practices. In a long-term field study, we assessed whether reduced tillage could enhance soil biological parameters and soil suppressiveness against the plant-pathogenic fungus Rhizoctonia solani AG2-2IIIB and bacterium Streptomyces scabies. Soil suppressiveness was assessed in bioassays with a susceptible crop while adding pathogen inoculum. Undisturbed soil cores (0–12 cm) were used in these bioassays to include the soil structural aspects, which were likely affected by the tillage treatments. Reduced i.e., non-inversion, tillage was compared with conventional ploughing treatment over 6 years. We found that reduced tillage led to an increase in bacterial and fungal biomass, labile carbon and nitrogen and an increase in the abundance of potential bacterial antagonists, compared to conventional tillage in the upper 12 cm. However, the increase of these microbial parameters did not lead to consistent changes in soil suppressiveness against both R. solani and S. scabies in response to the tillage treatment. Rather, disease suppressiveness varied significantly between field and year of sampling but was not correlated to any of the assessed soil parameters. Thus while reduced tillage can be beneficial for soil biology, other measures will have to be investigated for inducing R. solani and S. scabies disease suppressiveness.