Networks of power and influence: the role of mycorrhizal mycelium in controlling plant communities and agroecosystem functioning, Leake 2004

Two major groups of mycorrhizal fungi are arbuscular mycorrhiza (AM) and ectomycorrrhiza (EM). Both form a symbiosis with plants by colonizing their roots and creating an interface where carbon from the plant can be exchanged for phosphorus, nitrogen and other nutrients from the soil and transferred to the plant by the fungi. The extraradical^[15] mycorrhizal mycelium (ERMM), which are the vast portion of the fungal network that branches out into the soil, is difficult to study and has therefore been considered the “hidden half” of the symbiosis.

Progress in understanding the nature, extent, functioning, and identity of mycorrhizal fungal networks has been seriously hampered by the difficulties inherent in observing and studying mycelial systems without disturbing and destroying them.… As a consequence, the external mycelium, which is the fungal structure of mycorrhiza that is most intimately associated with the soil and furthest from the roots, and by implication the most critical for nutrient uptake, is normally overlooked and has been rarely recorded. Only in the past decade have studies started to focus specifically on the extent and functioning of ERMM in the field [Leake 2004: 1017].

This article highlights the significant yet overlooked role of mycorrhizal fungi in ecosystem functioning and reviews some advances in the techniques used to study these hidden powerhouses.

ERMM is the hidden power behind plant community composition and ecosystem functioning through the major processes it carries out, such as nutrient uptake, weathering of minerals, soil aggregate stability, and the way in which it alters competition between plants [Leake 2004: 1039].

The symbiosis with plants is the source of power for these fungi, given that the carbon received from plant hosts is practically inexhaustible and costs the plants little.

Despite the substantial biomass and associated C drain on their hosts, the actual “cost” of mycorrhiza to plants may be negligible because mycorrhizal colonization can increase the rate of photosynthesis (Wright et al. 1998), alleviate shoot N and P limitation, and cause a substantial increase in leaf area arising from improved nutrition (Read and Perez-Moreno 2003) [Leake 2004: 1021].

Thus,

The empowerment of mycorrhizal networks with substantial amounts of host-derived C allows them to play central roles in major biogeochemical cycles [Leake 2004: 1030].

The article concludes by emphasizing the importance for sustainable agriculture of a broader public understanding of the role of mycorrhiza for improving soil health and crop yields.

AM [arbuscular mycorrhizal] hyphal lengths in soil show strong positive correlations with soil-aggregate stability (Rillig et al. 2002; Kabir and Koide 2002), P uptake efficiency (Schweiger and Jakobsen 2000), and crop-yield improvements (Kabir and Koide 2002). Interest in the development of less intensive management systems is presenting new opportunities for adapting agricultural production systems to enhance these benefits that can be gained from AM networks. Substantial improvements in “soil health” and AM functioning in field crops are gained by the doubling of lengths of AM hyphae in soil when tillage is reduced (Kabir et al. 1998a, 1998b). Similar gains are achieved by growth of AM-compatible cover crops in place of winter fallow (Kabir and Koide 2002) [Leake 2004: 1038].