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Grasslands (Fungal Interactions with Roots) -- Summary
Nearly all grassland species form symbiotic relationships with mycorrhizal fungi.  Arbuscular mycorrhizal fungi commonly colonize roots of grasses and form symbiotic structures known as arbuscules, which are short-lived organs that facilitate carbon and nutrient exchange between the fungi and their host plants.  The presence of these symbiotic relationships often increases grassland vitality and productivity.  It is clearly important, therefore, to understand how rising atmospheric CO2 concentrations may affect these important relationships.  Hence, in this summary we review several recently published scientific papers that provide insight into the effects of atmospheric CO2 enrichment on these important fungal-plant interactions.

It is important to understand, first of all, that even under ambient atmospheric CO2 concentrations, symbiotic interactions between grasses and arbuscular mycorrhizal fungi often lead to significant increases in growth.  In the study of Wilson and Hartnett (1998), for example, the authors grew 36 grass and 59 forb species common to tallgrass prairie ecosystems with and without the presence (induced by inoculation) of arbuscular mycorrhizae.  Among the grasses, they reported that fungal inoculation increased the average dry mass of perennial C4 species by 85%.  However, fungal inoculation had no significant effects on dry mass production in perennial C3 species or in any annual grasses, regardless of their photosynthetic physiology.  With respect to the forbs studied, over 80% of the perennial species exhibited significant increases in dry mass with fungal inoculation, while only 15% of the annual species displayed enhanced growth with inoculation.  Thus, a large number of plant-fungal interactions exist at ambient CO2 concentrations that may be modified by exposure to elevated CO2.

Exploring this possibility, Rillig et al. (1998a) grew monocultures of three grasses and two herbs that co-occur in Mediterranean annual grasslands in pots placed within
open-top chambers receiving ambient and twice-ambient concentrations of atmospheric CO2 for a period of four months.  They reported that elevated CO2 significantly increased the percent root colonization by arbuscular mycorrhizal fungal hyphae in all five species, which could ultimately lead to greater biomass production in these annual grassland plants.  In another four-month study using the same experimental enclosures, Rillig et al. (1998b) grew the annual grass Bromus hordeaceus at ambient and elevated atmospheric CO2 concentrations.  In this study, however, elevated CO2 did not increase the percent root colonized by fungal hyphae; but it did significantly increase the percent root colonized by arbuscules, indicating that elevated CO2 can cause enhanced fungal-plant interactions by modifying fungal structures other than hyphae.

Lastly, in two related long-term studies, Rillig et al. (1999a,b) constructed open-top chambers on two adjacent serpentine and sandstone grassland communities in California, USA, fumigating them with air containing 350 and 700 ppm of CO2 for six years.  In corroboration of their earlier short-term results, they reported that elevated CO2 did not increase the percent root colonized by fungal hyphae (Rillig et al., 1999a) but that it enhanced the percent root colonized by arbuscules in serpentine and sandstone grasslands by three- and ten-fold, respectively (Rillig et al.,1999b).

These observations suggest that as the air’s CO2 concentration continues to climb, it will positively impact plant-fungal interactions on grasslands by increasing percent root colonization by either mycorrhizal fungal hyphae or arbuscules, both of which aid in carbon and nutrient exchanges between the two interacting symbionts.  Earth’s grasslands should thus exhibit increased productivity – even above and beyond that normally caused by atmospheric CO2 enrichment – due to these enhanced relationships that can make soil nutrients more available for plant uptake and usage.

Rillig, M.C., Field, C.B. and Allen, M.F.  1999a.  Fungal root colonization responses in natural grasslands after long-term exposure to elevated atmospheric CO2Global Change Biology 5: 577-585.

Rillig, M.C., Field, C.B. and Allen, M.F.  1999b.  Soil biota responses to long-term atmospheric CO2 enrichment in two California annual grasslands.  Oecologia 119: 572-577.

Rillig, M.C., Allen, M.F., Klironomous, J.N., Chiariello, N.R. and Field, C.B.  1998a.  Plant species-specific changes in root-inhabiting fungi in a California annual grassland: responses to elevated CO2 and nutrients.  Oecologia 113: 252-259.

Rillig, M.C., Allen, M.F., Klironomos, J.N. and Field, C.B.  1998b.  Arbuscular mycorrhizal percent root infection and infection intensity of Bromus hordeaceus grown in elevated atmospheric CO2Mycologia 90: 199-205.

Wilson, G.W.T. and Hartnett, D.C.  1998.  Interspecific variation in plant responses to mycorrhizal colonization in tallgrass prairie.  American Journal of Botany 85: 1732-1738.