Species-dependent partitioning of C and N stable isotopes between arbuscular mycorrhizal fungi and their C3 and C4 hosts

Natural C-13 and N-15 abundances of mycorrhizal fungi are increasingly used in ecology but reference data on arbuscular mycorrhizal fungi (AMF) are scarce. In experiments with nine phylogenetically dispersed AMF strains inoculated on leek (C3 plant) and sorghum (C4) in pot cultures, we measured the C-13/C-12 and N-15/N-14 ratios in shoots, roots, AMF spores as well as carbon isotope signature of the C16:1 omega 5 fatty acid (FA), which is diagnostic for AMF. Spore delta C-13 values varied among AMF strains on any given host. They were significantly lower than shoot delta C-13 for sorghum (-2.5 parts per thousand on average) while for leek, no clear C isotope partitioning between spores and host shoots was observed. The FA C16:1 omega 5 fatty acids were more C-13-depleted than spores, without correlation with spore delta C-13 values. For both, sorghum and leek, spore delta N-13 was higher (+1-2 parts per thousand on average) than for shoots. We found no evidence that isotopic partitioning between the partners drives C-13 and N-15 abundances in plant-AMF symbiosis. Mycorrhizal roots displayed relatively high delta C-13 typical for heterotrophic organs, and not a mix between AMF and plant signatures. Interestingly, inoculation slightly decreased shoot delta C-13 on leek but not on sorghum, as compared with non-mycorrhizal plants, suggesting that AMF improved the plant's water status, a parameter affecting the delta C-13 of C3 but not C4 plants. Phylogenetically closer AMF displayed more similar spore delta C-13 and induced similar C-13 and N-15 abundances on leek shoots, but this observation was not confirmed for sorghum. Plant and AMF isotopic abundances hardly correlated with other parameters related to plant development, mineral nutrition or root mycorrhizal colonisation, and these correlations were never consistent between sorghum and leek. Thus, isotopic abundances in plant-AMF symbiosis were rather constrained by each AMF/plant interaction. Nevertheless, our data provide a valuable reference for future investigations of AMF communities and AM symbiosis in situ. (c) 2014 Elsevier Ltd. All rights reserved.