The effectivity of arbuscular mycorrhizal fungi from high input conventional and organic grassland and grass-arable rotations

The effectivity of arbuscular mycorrhizal spores in promoting growth of All ium ameloprasum L. cv. Musselburgh and Trifolium repens L. cv. Menna was te sted for inocula from three soil series under long term organic or intensiv e, conventional grass and grass-arable rotations. For two soil series, Alli um responses to inocula from soils recently converted to organic fanning we re also assessed. Finally, Trifolium root fragments were used to inoculate Allium so as to evaluate responses to this inoculation procedure. Plants we re sown into previously sterilised, matched soils from organic farms with n o nutrient input. Mycorrhizal treatments generally increased growth for All ium. However, for Trifolium, infection decreased growth in the most fertile soil and gave an increase only in the least fertile. In the least fertile soil, inocula from organic farms were more effective than those from conven tional farms. For Trifolium (all soils) and for Allium (least fertile soil) , there was evidence of more efficient uptake of phosphorus in plants inocu lated with spores from organic farms. The pattern of Allium response to ino culation with spores from conventional, conversion and organic sources was not consistent between soil type, but there was evidence of lower root infe ction for conversion compared with organic inocula and of a trend towards h igher infectivity as the time period under organic management increased. In oculating Allium with AMF root fragments produced a plant response similar to that obtained when spores were used, confirming that spore viability was not the sole factor influencing AMF effectivity in earlier experiments. In tensive farming practices may reduce the effectiveness of indigenous arbusc ular mycorrhizal populations, particularly where fertiliser inputs are high and inherent fertility is low. This could have practical implications wher e high input farms are converted to organic management.