(Fungi) -- Summary
Nearly all of earth’s plants become involved in intimate relationships with different fungal species at one point or another in their life cycles. Among other things, the fungi commonly aid plants in the acquisition of water and important soil nutrients. In addition, fungal-plant interactions are often impacted by variations in both atmospheric CO2 and soil nitrogen concentrations. In this summary, we review how various aspects of fungal-plant interactions are influenced by elevated CO2 under varying soil nitrogen regimes.
In a one-year study conducted by Walker et al. (1998), ponderosa pine seedlings exposed to atmospheric CO2 concentrations of 525 and 700 ppm displayed total numbers of ectomycorrhizal fungi on their roots that were 170 and 85% greater, respectively, than those observed on roots of ambiently-grown seedlings.
In the study of Rillig et al. (1998), three grasses and two herbs fumigated with ambient air and air containing an extra 350 ppm CO2 for four months displayed various root infection responses by arbuscular mycorrhizal fungi, which varied with soil nitrogen supply. At low soil nitrogen contents, elevated CO2 increased the percent root infection by this type of fungi in all five annual grassland species. However, at high soil nitrogen contents, this trend was reversed in four of the five species.
Finally, in the study of Rillig and Allen (1998), several important observations were made with respect to the effects of elevated CO2 and soil nitrogen status on fungal-plant interactions. First of all, after growing three-year-old shrubs at an atmospheric CO2 concentration of 750 ppm for four months, they reported insignificant 19 and 9% increases in percent root infected by arbuscular mycorrhizal fungi at low and high soil nitrogen concentrations, respectively. However, elevated CO2 significantly increased the percent root infection by arbuscules, which are the main structures involved in the symbiotic exchange of carbon and nutrients between a host plant and its associated fungi, by more than 14-fold at low soil nitrogen concentrations. In addition, the length of fungal hyphae more than doubled with atmospheric CO2 enrichment in the low soil nitrogen regime. In the high soil nitrogen treatment, elevated CO2 increased the percent root infection by vesicles, which are organs used by arbuscular mycorrhizal fungi for carbon storage, by approximately 2.5-fold.
In conclusion, these observations suggest that elevated CO2 will indeed affect fungal-plant interactions in positive ways that often depend upon soil nitrogen status. Typically, it appears that CO2-induced stimulations of percent root infection by various fungal components is greater under lower, rather than higher, soil nitrogen concentrations. This tendency implies that elevated CO2 will enhance fungal-plant interactions to a greater extent when soil nutrition is less-than-optimal for plant growth, which is the common case for most of earth’s ecosystems that are not subjected to cultural fertilization practices typical of intensive agricultural production.
References
Rillig, M.C. and Allen, M.F. 1998. Arbuscular mycorrhizae of Gutierrezia sarothrae and elevated carbon dioxide: evidence for shifts in C allocation to and within the mycobiont. Soil Biology and Biochemistry 30: 2001-2008.
Rillig, M.C., Allen, M.F., Klironomous, J.N., Chiariello, N.R. and Field, C.B. 1998. Plant species-specific changes in root-inhabiting fungi in a California annual grassland: responses to elevated CO2 and nutrients. Oecologia 113: 252-259.
Walker, R.F., Johnson, D.W., Geisinger, D.R. and Ball, J.T. 1998. Growth and ectomycorrhizal colonization of ponderosa pine seedlings supplied different levels of atmospheric CO2 and soil N and P. Forest Ecology and Management 109: 9-20.