Impacts of long-term plant residue management on soil organic matter quality, Pseudomonas community structure and disease suppressiveness

Diverse microbial communities harboured by grassland soils provide ecosystem services essential to soil health and plant productivity, including the suppression of soil-borne plant diseases. Understanding the soil properties impacting upon these microbial communities will provide opportunities by which indigenous soil microbes and their functions may be managed to enhance soil suppressiveness. Here, a long-term grassland field trial was used to investigate the impact of 20 years of plant residue management, as a factor influencing soil chemical and biological properties, on the suppression of Rhizoctonia solani AG 2-1 induced damping-off of kale. Plant residue management led to significant variation in Pseudomonas bacterial community structure between treatments. Soil organic matter quality (carbon recalcitrance) was responsible for 80% of the observed variation in Pseudomonas community structure. Pseudomonas species diversity and richness were identified as the primary parameters explaining the greatest proportion (>30%) of variation in the suppressive capacity of soils across treatments. Further, increased Pseudomonas species diversity, microbial activity, soil organic matter content, and carbon availability distinguished suppressive (low disease) soils from high disease soils. Collectively, our results suggest that management-induced shifts in Pseudomonas community composition, notably species diversity and richness, provide a better indicator of soil susceptibility to disease caused by a broad host range pathogen than soil chemical parameters. Furthermore, we demonstrate that management practices that result in the frequent addition of organic residues to agricultural grassland soils enhance the diversity and activity of plant-beneficial bacterial taxa.