). If its escape is prevented by high soil water, CO2 can accumulate up to 17.5% in the root zone (Jackson, 1979). Such increased levels of CO2 (and hence of carbonic acid which dissociates to form H+ at neutral to alkaline pH) can enhance the dissolution of soil CaCO3 to produce Ca2+ for plant uptake. Sim- ilarly, HCO3− excreted directly by plant roots into soil can aid the dissolution of calcite to yield soluble sup- plies of Ca2+ for plant nutrition (Rendig and Taylor, 1989). CO2 release by roots can also have other in- direct effects on N and P nutrition of plants that form symbiosis with Rhizobium bacteria and VA fungi. An exogenous supply of CO2 is required for better growth of rhizobia and VA fungi in the rhizosphere (Becard et al., 1992; Lowe and Evans, 1962); the increase in microsymbiont population should have the potential to provide adequate symbiotic N and P for the host plant.