Mycorrhizae clearly improve soil quality by building soil structure. Different mechanisms are involved, whether they are biological, biochemical and biophysical mechanisms. Here they are explained:
From a biological perspective, deposition of mycelium and exudates (coming from mycorrhizae) serve as a substrate for bacteria growth and increase microbial biomass.
Glomalin, a biological glue secreted by mycorrhizal hyphae, is a major actor in soil particles' aggregation and in soil aggregate's water stability. Also, soil microbial biomass secretes compounds that reinforce soil particle attachment and lower water tension.
Mycorrhizal hyphae act as a tunnelling machine, exerting considerable pressure on soil particles and force organic material and clay particles together leading to micro-aggregate formation. The creation of these tunnels will contribute to enhancing air and water penetration and movement. Wet-dry cycles, promoted by mycorrhizal fungi, increase the binding of roots, fungal exudates and clay particles. Finally, mycorrhizal hyphae enmesh and entangle soil particles, organic matter and small aggregates together facilitating macro-aggregate formation.
Video showing how mycorrhizae work on plants' root system, leading to better soil structure.