Soil properties associated with organic matter-mediated suppression of bean root rot in field soil amended with fresh and composted paper mill residuals

The ability of an organic amendment to suppress soil-borne disease is mediated by the complex interactions between biotic and abiotic soil factors. Various microbiological and physicochemical soil properties were measured in field soils with histories of receiving 4 or 5 years of spring additions of paper mill residuals (PMR), PMR composted alone (PMRC), PMR composted with bark (PMRB), or no amendment under a conventionally managed vegetable crop rotation. The objectives of this study were to (i) determine the residual and re-amendment effects of the organic materials on root rot disease severity; (ii) determine the influence of amendment type on the structure of bacterial communities associated with snap bean roots grown in these soils; and (iii) quantify the relative contributions of microbiological and physicochemical properties to root rot suppression in the field and greenhouse. While all amendment types significantly suppressed root rot disease compared to non-amended soils in both environments, only soils amended with PMR or PMRB sustained suppressive conditions 1 year after the most recent amendment event. Disease severity was inversely related to microbial activity (fluorescein diacetate assay) in recently amended soils only. Terminal restriction fragment length polymorphism (T-RFLP) analysis of the 16s rRNA gene was performed to obtain bacterial profiles. Principal component analysis (PCA) of terminal restriction fragments (TRFs) revealed general differences in bacterial community composition (PC1) among amendment types, and specific TRFs contributed to these differences. Correlation and multiple regression analyses of the measured soil variables revealed that the composition of root-associated bacterial communities and the amount of particulate organic matter—carbon in bulk soils imparted independent and relatively equal contributions to the variation in disease severity documented in the field and greenhouse. Together, our findings provide evidence that disease suppression induced by annual PMR inputs was mediated by their differential effects on bacterial communities and the amount and quality of organic matter in these soils.