Sulfur is a macro-nutrient that is essential for growth and plants need 1-5mg sulfur per gram biomass. However, most of the sulfur in soils is bound to organic molecules (organo-S) as sulfate esters and sulfonates. While plants are almost entirely dependent on sulfate, many bacteria and fungi are capable to mobilize sulfur from sulfate esters, and specific bacterial guilds can utilize sulfonates as a sulfur source.Most plants receive nutrients not directly from the soil but largely through fungal networks. Mycorrhizal fungi are living in symbiosis with the plant host where the plant supplies the fungus with sugars and the fungus supplies the plant host with soil nutrients. Up to now, it is unclear where the soil sulfur comes from that mycorrhizal fungi supply to the plants. This is of special interest in agricultural systems where a sustainable sulfur supply through arbuscular mycorrhizas could replace inorganic fertilizer applications. The hypothesis of this fellowship is that sulfate esters and sulfonates are mobilised by certain fungi and bacteria in the mycorrhizosphere and arbuscular mycorrhiza transport the mobilised form to the plant host.The aim of this study is to i) determine the direct and indirect involvement of arbuscular mycorrhizal fungi, saprophytic fungi and bacteria in organo-S mobilization in the mycorrhizosphere and ii) to identify and quantify the activity of organo-S desulfurizing microorganisms.Fungi and bacteria will be isolated from mycorrhizal pastures. Monoxenic microcosms with Agrostis as host plant, Glomus as mycorrhizal symbiont and selected organo-S mobilizing microbes will be set up to quantify the flow of organically bound stable isotope S through this system. In addition, non-sterile mesocosm systems will be set up to follow the flow of S. Nucleic acids and proteins will be isolated from micro- and mesocosms to identify S cycle relevant expressed genes and to identify protein mass shifts in 36S containing key peptides.