Organic management of tilled agricultural soils results in a rapid increase in colonisation potential and spore populations of arbuscular mycorrhizal fungi

Arbuscular mycorrhizal fungi (AM fungi) are important members of the soil microbial community and can potentially offer benefits to low input and organic agricultural systems. While many agricultural practices can adversely affect AM fungi, organic agriculture prohibits some of the most detrimental practices and as a result AM fungal populations may be increased. We examined eleven sites to test the hypothesis that organic management increases AM fungal spore number and colonisation potential in tilled agricultural soils. Samples were taken from a conventional and an organically managed field at each site. AM spores were extracted and onions (Allium cepa L.) grown on field soils in the glasshouse, with AM colonisation measured to assess colonisation potential. Numbers of AM spores varied from 2g^-1 of soil to 27g^-1. Overall, spore numbers were significantly higher in the organically managed soils, though the number of AM spores was very low in some organically managed soils. Root colonisation varied from 4% to more than 80% and was significantly greater on soils from the organically managed fields. This result was consistent across arable (cereal based), arable/horticultural and horticultural systems. There was no overall difference in soil physicochemical characteristics, except total nitrogen, which was higher in organically managed soils. There was no relationship between the time organic management had been practiced and spore numbers or root colonisation, both responded rapidly to conversion, suggesting that changes in soil physicochemical characteristics are not an important factor. We suggest that the 2-year organic 'conversion' period is important for increasing spore number and colonisation potential.