Contrasting arbuscular mycorrhizal responses of vascular and non-vascular plants to a simulated Palaeozoic CO 2 decline

The arbuscular mycorrhizal (AM) fungal symbiosis is widely hypothesized to have promoted the evolution of land plants from rootless gametophytes to rooted sporophytes during the mid-Palaeozoic (480–360 Myr, ago), at a time coincident with a 90% fall in the atmospheric CO 2 concentration ([CO 2 ]a). Here we show using standardized dual isotopic tracers ( 14 C and 33 P) that AM symbiosis efficiency (defined as plant P gain per unit of C invested into fungi) of liverwort gametophytes declines, but increases in the sporophytes of vascular plants (ferns and angiosperms), at 440 p.p.m. compared with 1,500 p.p.m. [CO 2 ]a. These contrasting responses are associated with larger AM hyphal networks, and structural advances in vascular plant water-conducting systems, promoting P transport that enhances AM efficiency at 440 p.p.m. [CO 2 ]a. Our results suggest that non-vascular land plants not only faced intense competition for light, as vascular land floras grew taller in the Palaeozoic, but also markedly reduced efficiency and total capture of P as [CO 2 ] a fell.