Soil health indicators and Fusarium wilt suppression in organically and conventionally managed greenhouse soils

• Fusarium wilt was suppressed more in organic than conventional greenhouse soil.
• Suppression was related more to biological than chemical soil characteristics.
• Microbial resistance and resilience after disturbance were greater in organic soil.
• Microbial resistance and resilience can be used as soil health indicators.

Soil health has been associated with internal cycling of nutrients, microbial activity and diversity as well as root disease suppression, which are frequently greater in organically than in conventionally managed soils. Resistance and resilience, measured as amplitude and frequency of oscillations in bacterial communities after a disturbance, were suggested as integral indicators of soil health, but until now there is little proof for this hypothesis. In this study, resistance and resilience of microbial communities and 24 soil chemical and biological parameters were analyzed and correlated to suppression of flax wilt (caused by Fusarium oxysporum f.sp. lini) in three experiments. Soil samples were collected on three different dates from a recently converted organic greenhouse and a similar, neighboring greenhouse under conventional management. The dynamics of copiotrophic and oligotrophic bacteria after a disturbance were monitored, and the resistance and resilience were calculated. The organic soil showed significantly higher water-holding capacity, organic matter content, total C and N contents, C:N ratio of the small particulate organic matter fraction, microbial biomass carbon, oxygen uptake rate, copiotrophic and oligotrophic bacterial communities and suppression of flax wilt incidence. After incorporation of a grass–clover mixture in both soils, the densities of copiotrophic and oligotrophic bacteria oscillated over time. The relative amplitudes of the oscillations (in grass–clover amended over non-amended soil) and the frequencies of the oscillations of both trophic groups were lower for the organic soil, indicating that the resistance and resilience of the microbial community were greater in this soil. These results support the hypothesis that the bacterial response to a disturbance can serve as an integral indicator for soil health, including disease suppressiveness.