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Phosphorus (P) is essential for plant growth. Arbuscular mycorrhizal fungi (AMF) aid its uptake by acquiring P from sources distant from roots in return for carbon. Little is known about how AMF colonise soil pore-space, and models of AMF-enhanced P-uptake are poorly validated. We used synchrotron X-ray computed tomography to visualize mycorrhizas in soil and synchrotron X-ray fluorescence/X-ray absorption near edge structure (XRF/XANES) elemental mapping for P, sulphur (S) and aluminium (Al) in combination with modelling. We found that AMF inoculation had a suppressive effect on colonisation by other soil fungi and identified differences in structure and growth rate between hyphae of AMF and nonmycorrhizal fungi. Our results showed that AMF co-locate with areas of high P and low Al, and preferentially associate with organic-type P species over Al-rich inorganic P. We discovered that AMF avoid Al-rich areas as a source of P. Sulphur-rich regions were found to be correlated with higher hyphal density and an increased organic-associated P-pool, whilst oxidized S-species were found close to AMF hyphae. Increased S oxidation close to AMF suggested the observed changes were microbiome-related. Our experimentally-validated model led to an estimate of P-uptake by AMF hyphae that is an order of magnitude lower than rates previously estimated – a result with significant implications for the modelling of plant–soil–AMF interactions.

The concept of hormesis describes that the application of low concentrations of a toxic compound will stimulate growth and activity of an organism. Since it is unknown whether hormesis occurs in arbuscular mycorrhizal fungi (AMF) the present work was designed to reveal whether two fungicides would generate hormetic response curves for AMF performance. The effect of mancozeb and carbendazim on performance of three AMF in symbioses with pea was investigated. The fungicides were mixed uniformly into irradiated soil at three field dose equivalents, which were 1x, 5x and 25x for mancozeb and 0.01x, 0.1x and 1x for carbendazim. A nil fungicide treatment was included for each fungus and a mesh-enclosed, P-33-labelled soil patch enabled the measurement of AMF P uptake. Both fungicides generated biphasic response curves for AMF root colonization, which was largely enhanced by the two lower doses and suppressed by the highest. Besides, the lowest concentration of both fungicides increased the hyphal length-specific P-33 uptake by one of the fungi, while 0.1x carbendazim also increased the P-33 uptake by another. In contrast, the length of root-external hyphae was either decreased or unaffected by increasing fungicide doses. The biphasic fungicide responses of root colonization and hyphal P uptake were obtained in irradiated soil without AMF antagonists and therefore probably caused by direct effects on the AMF. Such hormetic response patterns may be common in ecosystems where AMF will usually be exposed to a range of abiotic and biotic stressors.

Arbuscular mycorrhizal fungi (AMF) are important in plant nutrient uptake, but their function is prone to environmental constraints including soil factors that may suppress AMF transfer of phosphorus (P) from the soil to the plant. The objective of this study was to disentangle the biotic and abiotic components of AMF-suppressive soils. Suppression was measured in terms of AMF-mediated plant uptake of 33P mixed into a patch of soil and treatments included soil sterilization, soil mixing, pH manipulation and inoculation with isolated soil fungi. The degree of suppression was compared to volatile organic compound (VOC) production by isolated fungi and to multi-element analysis of soils. For a selected suppressive soil, sterilization and soil mixing experiments confirmed a biotic component of suppression. A Fusarium isolate from that soil suppressed the AMF activity and produced greater amounts than other fungal isolates of the antimicrobial VOC trichodiene (a trichothecene toxin precursor), beta-chamigrene, alpha-cuprenene and p-xylene. These metabolites deserve further attention when unravelling the chemical background behind the suppression of AMF activity by soil microorganisms. For the abiotic component of suppression, soil liming and acidification experiments confirmed that suppression was strongest at low pH. The pH effect might be associated with changed availability of specific suppressive elements. Indeed 33P uptake from the soil patches correlated negatively to Al levels and Al toxicity seems to play a major role in the AMF suppressiveness at pH below 5.0–5.2. However, the documentation of a biotic component of suppression for both low and high pH soils leads to the conclusion that biotic and abiotic components of suppression may act in parallel in some soils. The current insight into the components of soil suppressiveness of the AMF activity aids to develop management practices that allow for optimization of AMF functionality.