Phosphorus uptake, not carbon transfer, explains arbuscular mycorrhizal enhancement of <i>Centaurea maculosa</i> in the presence of native grassland species

Summary Previous studies have shown that arbuscular mycorrhizas (AM) enhance the growth of the invasive forb Centaurea maculosa when growing with native grass species. Using 13 CO 2 , we tested the hypothesis that this enhancement is explained by carbon transfer from native species to C. maculosa via mycorrhizal hyphal linkages. A C. maculosa plant was paired with one of five native species – three grasses ( Festuca idahoensis , Koeleria cristata and Pseudoroegneria spicata ) and two forbs ( Achillea millefolium and Gaillardia aristata ) – in pots that separated the plants with either a mesh barrier (28 µm, excludes fine roots but not hyphae) or a membrane barrier (0·45 µm, excludes roots and hyphae). 13 CO 2 was added to the atmosphere of either Centaurea or the native species after 20 weeks’ growth. A 25 min pulse application was followed by 7 days’ growth and subsequent harvest. The biomass response of C. maculosa was consistent with previous experiments: C. maculosa was larger when growing in mesh barrier pots, when hyphae were able to access the opposite side of the pot; in mesh barrier pots only, biomass varied with neighbouring species. Native plant biomass did not vary between mesh‐ vs membrane‐barrier pots. There was no evidence of carbon transfer, either from the native plant to C. maculosa or in the reverse direction. Centaurea maculosa contained significantly more phosphorus in mesh‐divided pots, but this depended on the neighbouring plant. The P concentration in C. maculosa was significantly higher in mesh‐divided pots when growing with a grass and not a forb. Native species contained more P in mesh‐divided pots than membrane‐divided pots, and P concentration differed between species (higher in forbs than grasses), but did not vary between mesh‐ and membrane‐divided pots. Our study suggests that C. maculosa is able to exploit its mycorrhizal symbiosis more effectively than the native grassland species. The mechanism for this appears to be luxury consumption of P through efficient utilization of extra‐radical hyphae, but that effect is dependent on neighbouring species, and occurs when growing with a grass neighbour. Although no single study can disprove the carbon‐transfer hypothesis, our work suggests that AM‐mediated neighbour effects are the result of mycorrhizal networks that increase species’ access to P. Whether the synergistic effects of neighbours are due to complementarity of AM fungal symbionts utilized by different plant species, or have to do with the structure of AM networks that develop more extensively with multiple host plants, remains to be investigated.