In addition to their well-recognized roles in plant nutrition and communities, mycorrhizas can influence the key ecosystem process of soil aggregation. Here we review the contribution of mycorrhizas, mostly focused on arbuscular mycorrhizal fungi (AMF), to soil structure at various hierarchical levels: plant community; individual root; and the soil mycelium. There are a suite of mechanisms by which mycorrhizal fungi can influence soil aggregation at each of these various scales. By extension of these mechanisms to the question of fungal diversity, it is recognized that different species or communities of fungi can promote soil aggregation to different degrees. We argue that soil aggregation should be included in a more complete 'multifunctional' perspective of mycorrhizal ecology, and that in-depth understanding of mycorrhizas/soil process relationships will require analyses emphasizing feedbacks between soil structure and mycorrhizas, rather than a uni-directional approach simply addressing mycorrhizal effects on soils. We finish the discussion by highlighting new tools, developments and foci that will probably be crucial in further understanding mycorrhizal contributions to soil structure.

https://nph.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1469-8137.2006.01750.x

Mycorrhizas and soil structure

Despite the fact that arbuscular mycorrhizal (AM) associations are among the most ancient, abundant and important symbioses in terrestrial ecosystems, there are currently few unifying theories that can be used to help understand the factors that control their structure and function. This review explores how a stoichiometric perspective facilitates integration of three complementary ecological and evolutionary models of mycorrhizal structure and function. AM symbiotic function should be governed by the relative availability of carbon, nitrogen and phosphorus (trade balance model) and allocation to plant and fungal structures should depend on the availabilities of these resources (functional equilibrium model). Moreover, in an evolutionary framework, communities of plants and AM fungi are predicted to adapt to each other and their local soil environment (co-adaptation model). Anthropogenic enrichment of essential resources in the environment is known to impact AM symbioses. A more predictive theory of AM structure and function will help us to better understand how these impacts may influence plant communities and ecosystem properties.

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Resource stoichiometry elucidates the structure and function of arbuscular mycorrhizas across scales

Arbuscular mycorrhizal fungi and organic farming

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/nph.13045

Plant root and mycorrhizal fungal traits for understanding soil aggregation.

Improved soil structure and citrus growth after inoculation with three arbuscular mycorrhizal fungi under drought stress

Summary
• Arbuscular mycorrhizal fungi (AMF) and roots mediate soil stabilization, although the mechanisms and how their interactions affect soil stabilization are not known. We tested the effects of specific plant–fungus combinations on aggregate stabilization, and whether hyphal length and root biomass determine stabilization, predicting that fungi producing more hyphae, and plants with higher root biomasses, would better stabilize soils.
• The percentage of water-stable aggregates (%WSA1−2 mm), hyphal lengths, and root biomass were measured from a five AMF × nine plant factorial experiment. Arbuscular mycorrhizal fungi with greater extradical mycelium production were represented by the Gigasporaceae and plants of high root biomass by grasses. Other taxa represented lower hyphal lengths and root biomass.
• An interaction between symbionts with respect to %WSA1−2 mm was observed. Root biomass and total hyphal lengths were not positively correlated with %WSA. Combinations of grasses with Gigasporaceae fungi had the lowest %WSA.
• Mechanisms underlying aggregation were not elucidated by measuring root biomass and total hyphal lengths alone, suggesting other physiological or architectural mechanisms may be responsible.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.2004.01181.x

T. Denich

Papers

The effects of arbuscular mycorrhizas on soil aggregation depend on the interaction between plant and fungal species.