Rising CO2 Promotes the Growth of Important Soil Fungi
One of the most important secondary consequences of CO2-induced growth stimulation in resource-limited situations is the enhancement of the activity of a host of other root-zone or rhizosphere microorganisms. Supported by enhanced root exudation of organic substances, which may comprise as much as 40% of the photosynthates moving into a plant's roots, this CO2-augmented phenomenon typically stimulates a multiplicity of growth-promoting effects beneath the soil surface, one of the most important of which is the enhancing of the growth of a number of soil mycorrhizal fungi.
Possibly numbering in the hundreds of thousands of species, fungi are key players in most ecosystems. Indeed, there is evidence to suggest that they are absolutely essential for growth in stressful environments. And, the benefits plants receive from mycorrhizae are often proportional to mycorrhizal abundance, which has been demonstrated in a number of experiments to be enhanced by atmospheric CO2 enrichment.
So what are the benefits of enhanced soil mycorrhizal activity? For starters, as the "better-fed" hyphae of more numerous and robust symbiotic fungi extend outward from their CO2-enriched hosts, they lengthen the life of absorptive root hairs and increase the area of root surface available for water and nutrient uptake. They also secrete organic acids that hasten the chemical weathering of soil minerals; and they are especially adept at making phosphorus available to plants by this means. In addition, they are known to produce a variety of hormones that stimulate root growth, enhancing the production of lateral roots and root hairs. Finally, symbiotic soil fungi tend to protect their hosts from toxic materials in the soil, including heavy metals and salt.