But Tang et al. carried out an experiment - conducted under conditions of low soil phosphorus (P) content - and grew a group of three C4 grasses and three C3 grasses. They found the C4 species to respond better than their counterparts.
'C3' and 'C4' refer to different strategies with which plants fix carbon (binding the gaseous molecules to dissolved compounds inside the plant) for sugar production through photosynthesis. Evolutionary speaking, the C3 photosynthetic pathway is the oldest and covers approximately 95% of the world's plant biomass. Most grass species in temperate climates belong to this group.
The C4 strategy is younger and more efficient, resulting in higher biomass productivity (see image, click to enlarge). Many promising (tropical) energy crops follow this pathway. They include sugarcane, sorghum, and switchgrass (Panicum virgatum).
The three researchers from China's Zhejiang University grew the three C3 grasses (Poa annua L., Lolium perenne L., Avena fatua L.) and the three C4 grasses (Echinochloa crusgalli var. mitis (L.) Beauv., Eleusine indica (L.), Setaria glauca (L.) P. Beauv.) from seed to maturity under well watered conditions within controlled-environment chambers (maintained at a mean atmospheric CO2 concentration of either 350 or 700 ppm) in pots containing 2.5 kg of soil that was low in extractable P content. Under these conditions, total aboveground plus belowground plant biomass was enhanced by an average of 9.92% due to the doubling of the air's CO2 concentration in the group of C3 grasses, but by an average of 12.27% by the doubling of the air's CO2 concentration in the group of C4 grasses.
So how did it happen that the CO2-induced growth response of the C4 grasses was nearly 25% greater than that of the C3 grasses:
biomass :: bioenergy :: biofuels :: energy :: sustainability :: climate change :: CO2 :: plant biology :: photosynthesis :: energy crops :: sugarcane :: sorghum :: switchgrass ::
Tang et al. report that the C3 grasses they studied had low mycorrhizal colonization and that atmospheric CO2 enrichment did not significantly enhance this beneficial symbiosis, whereas injecting extra CO2 into the air did enhance mycorrhizal colonization in the C4 grasses. (Mycorrhizae are the result of the colonisation of the roots of the plants by a fungus, in a mutually beneficial relationship (either inside or outside of the root cells); the fungus survives by tapping energy (sugars) from the roots, but in exchange it allows the plant to make use of the fungus' tremendous surface area to absorb mineral nutrients from the soil.)
In addition, they say they observed "a positive correlation between mycorrhizal colonization rate and shoot P concentration, and between the increase in mycorrhizal colonization rate and the increase in total P uptake under elevated CO2," which findings they interpreted as suggesting that "mycorrhizae might enlarge P uptake for plants that have high mycorrhizal colonization and then promote host-plant growth response to elevated CO2." Or as they describe the situation in another place in their paper, "the C4 grasses ... used in our experiment were also significant mycorrhizal hosts and their mycorrhizal colonization was significantly stimulated by elevated CO2," which suggested to them that this situation may "promote the total P uptake of the C4 grass in low P soil and enhance the C4 grasses' response to CO2 enrichment."
As an added "bonus," so to speak, Tang et al. had also included three C3 forbs (Veronica didyma Ten., Plantago virginica L., Gnaphalium affine D.Don.) in their study, as well as three legumes (Vicia cracca L., Medicago lupulina L., Kummerowia striata (Thunb.) Schindl.), both of which plant groups responded better to the researchers' enriching of the air about them with CO2 than did the two groups of grasses. The C3 forbs, for example, exhibited a mean biomass increase of 35.61%, while the legumes exhibited a mean biomass increase of 41.48%. Of these latter champion responders, the researchers wrote that they too "had high levels of mycorrhizae, and their mycorrhizal symbionts were stimulated greatly by CO2." And they again noted that the "higher enhanced total P uptake of legumes under elevated CO2 concentrations implies that mycorrhizae may facilitate P uptake and enhance legume response to elevated CO2."
Tang, J., Chen, J. and Chen, X. Response of 12 weedy species to elevated CO2 in low-phosphorus-availability soil. Ecological Research 21: 664-670.