Bacterial and arbuscular mycorrhizal fungal interactions for mobilisation of organically bound sulfur in soil

Sulfur (S) is an essential macronutrient that is increasingly limiting to crop yield as a result of reduced atmospheric deposition and high yielding crop varieties rapidly depleting soil S stocks. Plants obtain S from soil via their root networks, however, in soil up to 95% of S is organically bound as either sulfate esters or sulfonates which are not directly accessible. In this instance, plants rely on distinct functional soil microbial populations to mineralise the respective organo-S pool. Sulfonate mineralisation is catalysed solely by specific bacteria, while sulfate ester mineralisation is achieved by both bacteria and fungi. Arbuscular mycorrhizal (AM) fungi are ubiquitous soil microbes that have been shown to be very important for both nitrogen and phosphorus cycling, so intuitively they may play a role in the S cycle. AM fungi have many beneficial characteristics that would render them useful in organo-S mobilisation including improved nutrient exchange, extensive hyphal networks, modified rhizodeposition, and increased access to soil aggregates. The study set out to ascertain (i) the microbial diversity associated with AM hyphae in a native ecosystem and analyse their functionality, (ii) the effect of AM inoculation on plant growth and organo-S mobilising communities, and (iii) the interaction between AM fungi and bacteria to improve uptake of labelled organo-S. The results obtained demonstrate that (i) hyphospheric bacterial communities in native grassland swards are enriched in sulfonate mobilisers that can putatively attach to AM hyphae to explore the extended soil environment, (ii) AM inoculation promotes plant growth, significantly impacts bacterial community composition and abundance, and harbours distinct and abundant populations of sulfonate mobilising bacteria, and (iii) an undisturbed AM symbiosis increases uptake of 34S from organo-34S enriched soil at early stages of growth when S requirement is high. Mycorrhizal treatments harbour larger populations of cultivable heterotrophs and sulfonate mobilising bacteria. Sulfate ester (arylsulfatase enzyme assay, atsA gene) and sulfonate mobilising activity (asfA gene) is significantly enhanced/altered by an undisturbed AM symbiotic partnership. The results obtained demonstrate that AM fungi can promote plant growth and increase uptake of S from organo-S via functional interactions with AM stimulated organo-S mobilising bacterial communities. These results provide important evidence to suggest that AM inoculation and the associated functional interactions have great potential for use in agriculture to improve S supply to plants as available sulfate (SO42-) is increasingly a limiting factor to growth.