Carbon metabolism and costs of arbuscular mycorrhizal associations to host roots

Arbuscular mycorrhizal (AM) fungi form beneficial associations with host root systems, during which the soil nutrient acquisition by the fungal symbiont induces an alteration in AM root carbon metabolism and belowground carbon costs of the host plant. The reciprocal exchange of host-derived carbohydrates for the symbiont-acquired nutrients may be controlled by both partners in the symbiosis. The carbohydrates not only serve as the fuel for fungal growth and maintenance but also provide energy for nutrient uptake and assimilation of inorganic minerals such as nitrogen and phosphorus. The belowground carbon cost may be further influenced by biotic and abiotic factors that affect the basic carbon metabolism of the AM roots. In natural and agricultural soils, major factors affecting carbon economy include the concentration and source of nitrogen or phosphorus nutrition, the developmental stages of the host, and the presence of additional symbionts, such as nitrogen-fixing bacteria in legume nodules. Since each of these factors may complicate the measurement of the carbon economy, several methods are proposed to evaluate the carbon costs of nutrient uptake and assimilation by AM roots

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