Arbuscular mycorrhizae of Gutierrezia sarothrae and elevated carbon dioxide: evidence for shifts in C allocation to and within the mycobiont

In a complete 2×2 factorial greenhouse experiment we examined the responses of arbuscular mycorrhizal (AM) and non-mycorrhizal fungi to Gutierrezia sarothrae shrubs grown in elevated atmospheric carbon dioxide (750 μl l−1) and fertilized with N. AM percent infection did not change significantly with elevated CO2, but arbuscular infection increased 14-fold in the low-N treatment. Extraradical hyphal length increased on an absolute basis in elevated CO2, and also per infected root length. In the high-N treatments, increasing CO2 caused a decrease in hyphal length per infected root length, and an increase in vesicular infection. There was a significant positive response of AM infection intensity to increasing CO2 for the high N treatment, and a similar trend in the low N treatment. Infection intensity was positively correlated with arbuscular infection and with vesicular infection. Non-mycorrhizal fungi did not respond to any of the treatment combinations, as measured by percent root infection and external hyphal length. Our results indicate that C allocation to the AM fungi was increased in elevated CO2, and that the mycobiont in turn increased C allocation to external hyphae.