. 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.