Inoculation with vesicular-arbuscular mycorrhizal fungi from cropped soil overcomes long-fallow disorder of linseed ( Linum usitatissimum L.) by improving P and Zn uptake

Abstract

Long-fallow disorder is a term describing poor crop growth that can occur after extended clean fallows of vertisol soils in the north-east grain belt of Australia. Although many crop species can suffer from this disorder, linseed ( Linum usitatissimum L.) is the most susceptible. A glasshouse experiment with linseed demonstrated that naturally-occurring propagules (spores and colonized roots) of vesicular-arbuscular mycorrhizal fungi (VAMF) extracted from a recently-cropped field soil and added into a long-fallow soil would overcome the disorder. Supplying additional VAMF propagules to the long-fallow soil had a similar effect on linseed as supplying Zn fertilizer, increasing growth in the presence of P fertilizer. Soil irradiation killed all VAMF inoculum in the long-fallow soil, resulting in uncolonized linseed plants that grew very poorly. Inoculation of irradiated soil with VAMF spores from the cropped soil resulted in early VAMF colonization of the linseed roots and good growth of the linseed tops. Inoculation of irradiated soil with VAMF-colonized roots sieved from the cropped soil resulted in delayed VAMF colonization and delayed growth of linseed tops, compared with inoculation with spores. Regression analysis including all treatments showed that dry weight of the linseed at 80 days was linearly dependent on the degree of VAMF colonization of the roots at 36 days. These results demonstrate the key role of natural VAMF inoculum in improving the nutrition and growth of field crops on farms subject to long-fallow disorder.