Soil suppressiveness to Pythium ultimum in ten European long-term field experiments and its relation with soil parameters

Soil suppressiveness to pathogens is defined as the capacity of soil to regulate soil-borne pathogens. It can be managed by agricultural practices, but the effects reported so far remain inconsistent. Soil suppressiveness is difficult to predict and for this reason different soil properties have been linked to it with the aim to find informative indicators, but these relationships are not conclusive. The objectives of this study were i) to test if soil suppressiveness is affected by long-term agricultural management such as tillage and organic matter (OM) addition; ii) to understand the direct and indirect relationships between soil suppressiveness and labile organic carbon fractions; and iii) to understand the relationship between soil suppressiveness and other chemical, physical and biological soil quality indicators. We measured soil suppressiveness with a bioassay using Pythium ultimum - Lepidium sativum (cress) as a model system. The bioassay was performed in soils from 10 European long-term field experiments (LTEs) which had as main soil management practices tillage and/or organic matter addition. We found that the site had a stronger influence on soil suppressiveness than agricultural practices. Reduced tillage had a positive effect on the suppressive capacity of the soil across sites using an overall model. Organic farming and mineral fertilization increased soil suppressiveness in some LTEs, but no overall effect of OM was found when aggregating the LTEs. Soil suppressiveness across LTEs was linked mainly to microbial biomass and labile carbon in the soil, but not to total soil organic matter content. From structural equation modelling (SEM) we conclude that labile carbon is important for the maintenance of an abundant and active soil microbial community, which is essential for the expression of soil suppressiveness. However, soil suppressiveness could only partly (25%) be explained by the soil parameters measured, suggesting that other mechanisms contribute to soil suppressiveness such as the presence and the activity of specific bacterial and fungal taxa with high biocontrol activity.