Mycorrhiza assist plants in obtaining soil nutrients. Therefore, any resulting stimulations in plant growth provide additional plant residue, which in turn can lead to increased carbon storage in the soil (Lal et al., 1998; Smith et. al., 2008). However, mycorrhiza can also promote carbon sequestration through a second mechanism. Mycorrhizae release glomalin, which is a glycoprotein that serves as gluing agent that facilitates soil aggregate formation, improvement of soil physical properties, and sequestration of carbon in the soil (Rillig, 2004; Subramanian et al., 2009). The stability of soil aggregates is highly correlated with the length of mycorrhizal hypha in the soil (Jastrow et al., 1998).
One of the prime factors associated with enhancing soil carbon sequestration is the release of glomalin in mycorrhizal systems. Specific mycorrhizae: Glomus intraradices, Glomus mosseae, Glomus fascicullatum, Glomus margarita, and Glomus pellucida, have been reported to enhance soil carbon due to the release of glomalin. Glomalin is a glycoprotein that serves as gluing agent that facilitates soil aggregate formation and improves soil physical properties (Rillig, 2004). Glomalin secretion helps to conserve soil carbon besides increasing microbial biomass. Subramanian et al (2009) reported that glomalin is composed of 45% carbon, like most organic compounds, and it is considered to be a major compound that is a store of carbon in soil carbon sequestration. Since glomalin is a reservoir of carbon, examining it helps explain amounts of C sequestration in a maize-mycorrhizal system. Arbuscular mycorrhizal (AM) fungi release glomalin which stores about 30-40% carbon in the form of carbohydrates and proteins. It is a super glue that helps store carbon, nutrients, and beneficial microorganisms, as well as being involved in stabilising soil aggregates. It also offers protection against biotic and abiotic stress conditions that could decrease crop growth and therefore reduce carbon sequestration Subramanian et al., (2009).
Mycorrhizal inoculation resulted in colonisation of roots irrespective of fertility gradients and crop growth stages (Subramanian et al., 2009). The un-inoculated treatments registered less than 5% colonisation shortly after planting, but the percentage of colonisation tended to increase significantly with the advancement of plant growth. The glomalin content of the soil substantially increased with mycorrhizal association, suggesting that mycorrhiza plays a vital role in conserving the carbon in a long-lived pool, which prevents loss of carbon to the atmosphere while sustaining soil fertility. Although soil glomalin concentration was not affected by chemical fertiliser levels, combined application of fertiliser and rice straw significantly increased soil glomalin concentration, which result into the greater soil organic carbon conservation (Subramanian et al., 2009).
Mycorrhizal plants are generally photosynthetically more active and capable of converting more atmospheric CO2 into assimilates in the plants (Subramanian et al., 2009). Mycorrhizal symbiosis utilises at least 10% of the host plant’s photosynthetic carbon which helps the microbial activity in the rhizosphere and contributes to the enhancement of active carbon pool in the soil. Shoot and root biomass of Glomus intraradices mycorrhiza inoculated maize plants were significantly increased about 29% in comparison with uninoculated plants with there being more enhancement when soil zinc levels were low (Subramanian et al., 2009). Thus, arbuscular mycorrhizal fungi that form symbiotic relationship with more than 90% of terrestrial plant species are helpful in storing carbon in living soil pools. However, the degree of dependence on mycorrhizae varies with plant species, particularly root morphology, as well as soil and climate (Muchovej, 2001). Crops with thick roots, poorly branched, and with few root hairs are more dependent on mycorrhizae including onions, grapes, citrus, cassava, coffee, and tropical legumes.
In many parts of the world, phosphate fertilisers are relatively inexpensive, and therefore farmers do not have a great incentive to inoculate with mycorrhizae. Where phosphate fertilisers are relatively expensive or unavailable, the lack of commercial inoculums and the difficulty of culturing one’s own are significant barriers, although commercial sources are becoming available.
Inoculation with ectomycorrhizae is common in the forest industry, but the necessity for more difficult to produce arbuscular mycorrhizae has slowed penetration into agriculture. Nevertheless, practical applications include transplant media that have been treated to remove soil pathogens, re-vegetation of eroded or mined areas, and in arid and semi-arid regions (Muchovej, 2001).
- Mycorrhizal inoculated plants produce larger biomass as a direct consequence of improved photosynthetic activities, and they can translocate 20-30% of assimilated carbon to the rhizosphere (underground).
- Glomalin concentrations in the soil can be significantly enhanced by the mycorrhizal inoculation resulting in more durable soil carbon sequestration, as well as more stable soil aggregates with improved soil physical properties.
- Indigenous mycorrhizal fungal inoculation is not very effective and causes inhibitory effects when inorganic fertiliser is applied to the soil without any integration of organic manures.
- Cultures of arbuscular mycorrhizae for inoculation of agricultural crops require a host plant and therefore are difficult to grow. However, they are beginning to become commercially available, at least in the United States (Muchovej, 2001).
The potential for use is very high, especially to remove the need for phosphorous fertiliser in developing countries, and therefore the mitigation potential to reduce GHG emissions is also high.
Where phosphate fertilisers are relatively expensive or unavailable, the lack of commercial inoculums and the difficulty of culturing one’s own are significant barriers, although commercial sources are becoming available.
Jastrow, JD, Miller, RM, and Lussenhop, J. (1998): Contributions of interacting biological mechanisms to soil aggregate stabilization in restored prairie 30:905-916.
Lal, R., Kimble, JM, Follet, RF, and Cole, CV. (1998): The Potential of U.S. Cropland to Sequester Carbon and Mitigate the Greenhouse Effect, Ann Arbor Press, Chelsea, Michigan, USA.
Muchovej, R.M. (2001): Importance of mycorrhizae for agricultural crops. IFAS Extension Bulletin SS-AGR-170, University of Florida, Gainesville, Florida, USA. 5 pp.
Rillig, M.C., (2004): Arbuscular mycorrhizae, glomalin, and soil aggregation. Can. J. Soil Sci. 84: 355–363.
Smith P, Martino D, Cai Z, Gwary D, Janzen HH, Kumar P, Mccarl B, Ogle S, O’mara F, Rice C, Scholes RJ, Sirotenko O, Howden M, Mcallister T, Pan G, Romanenkov V, Schneider U, Towprayoon S, Wattenbach M and Smith JU (2008): Greenhouse gas mitigation in agriculture. Philosophical Transactions of the Royal Society B 363:789-813.
Subramanian, K.S., Tenshia, V., Jayalakshmi, K. and Ramachandran, V. (2009). Role of arbuscular mycorrhizal fungus (Glomus intraradices) – (fungus aided) in zinc nutrition of maize. Journal of Agricultural Biotechnology and Sustainable Development 1(1):029-038.