Elevated CO2 does not affect arbuscular mycorrhizal fungi in the Mojave Desert

Plants grown under elevated atmospheric CO₂ can experience increased nutrient deficiencies.  Arbuscular mycorrhizal fungi (AMF) reduce nutrient limitations of most terrestrial plants, and elevated CO₂ may allow plants to increase their use of these symbiotic organisms. This study was conducted at the Nevada Desert FACE Facility in 2006 after 9 years of CO₂ treatment. Soil cores were collected from ambient (380 ppm) and elevated (550 ppm) CO₂ plots in the spring and fall from three microsites: beneath Larrea tridentata and Ambrosia dumosa canopies and within shrub interspaces. We measured root length, root colonization, extraradical hyphal (ERH) length, and immunoreactive glomalin. Root length, percent root colonization, ERH lengths, and glomalin concentrations were not different between CO₂ treatments. Spring samples only differed from fall samples in root length and percent root colonization, and both variables were greater in the spring. Higher soil moisture values in the spring were likely responsible for seasonal differences in root length and colonization. Soils within the three microsites did not differ in any root or AMF response. Thus, AMF appear to explore all areas of the Mojave Desert equally although nutrient availability varies considerably among microsites. We conclude that unless changes in decomposition rates of hyphae and glomalin are comparable to any change in production, plant dependence on AMF is not altered by high CO₂ in the Mojave Desert. Stimulated nitrogen cycling by other soil microbes in the elevated CO₂ plots may eliminate the need for greater production of AMF by increasing nutrient availability in this system.