Arbuscular mycorrhizal (AM) fungi establish symbiotic associations with many plant species, transferring significant amounts of soil nutrients such as phosphorus to plants and receiving photosynthetically fixed carbon in return. Functioning of AM symbiosis is thus based on interaction between two living partners. The importance of dead AM fungal biomass (necromass) in ecosystem processes remains unclear. Here, we applied either living biomass or necromass (0.0004 potting substrate weight percent) of monoxenically produced AM fungus (Rhizophagus irregularis) into previously sterilized potting substrate planted with Andropogon gerardii. Plant biomass production significantly improved in both treatments as compared to non-amended controls. Living AM fungus, in contrast to the necromass, specifically improved plant acquisition of nutrients normally supplied to the plants by AM fungal networks, such as phosphorus and zinc. There was, however, no difference between the two amendment treatments with respect to plant uptake of other nutrients such as nitrogen and/or magnesium, indicating that the effect on plants of the AM fungal necromass was not primarily nutritional. Plant growth stimulation by the necromass could thus be either due to AM fungal metabolites directly affecting the plants, indirectly due to changes in soil/root microbiomes or due to physicochemical modifications of the potting substrate. In the necromass, we identified several potentially bioactive molecules. We also provide experimental evidence for significant differences in underground microbiomes depending on the amendment with living or dead AM fungal biomass. This research thus provides the first glimpse into possible mechanisms responsible for observed plant growth stimulation by the AM fungal necromass.

Department of Ecosystem Biology Faculty of Science University of South Bohemia Branišovská 1760 370 05 České Budějovice Czech Republic

Institute of Geology Czech Academy of Sciences Rozvojová 269 165 00 Prague 6 Czech Republic

Laboratory of Biotransformation Institute of Microbiology Czech Academy of Sciences Vídeňská 1083 142 20 Prague 4 Czech Republic

Laboratory of Fungal Biology Institute of Microbiology Czech Academy of Sciences Vídeňská 1083 142 20 Prague 4 Czech Republic

Laboratory of Fungal Biology Institute of Microbiology Czech Academy of Sciences Vídeňská 1083 142 20 Prague 4 Czech Republic Faculty of Science Jan Evangelista Purkyně University in Ústí nad Labem České mládeže 8 400 96 Ústí nad Labem Czech Republic

Laboratory of Fungal Biology Institute of Microbiology Czech Academy of Sciences Vídeňská 1083 142 20 Prague 4 Czech Republic Isotope Laboratory Institute of Experimental Botany Czech Academy of Sciences Vídeňská 1083 142 20 Prague 4 Czech Republic

Laboratory of Fungal Genetics and Metabolism Institute of Microbiology Czech Academy of Sciences Vídeňská 1083 142 20 Prague 4 Czech Republic

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