What is the main function of mycorrhizal fungi in soil?

Mycorrhizal fungi play a crucial role in enhancing nutrient absorption for plants, which is why this choice is the correct answer. These fungi form symbiotic relationships with plant roots, effectively extending the root system's surface area. Through this association, mycorrhizal fungi help plants access essential nutrients such as phosphorus and nitrogen that may be less available in the soil. This mutualistic relationship not only boosts plant growth and health but also contributes to soil quality by encouraging more robust root systems that can stabilize soil structure.

In contrast, while soil aggregation is important for overall soil health, mycorrhizal fungi primarily impact nutrient dynamics rather than directly promoting soil structure. Furthermore, decreasing soil pH is typically associated with different soil processes and does not directly relate to the primary function of mycorrhizal fungi. Lastly, mycorrhizal fungi encourage water retention and absorption, whereas increasing water runoff contradicts the benefits these fungi bring to plant hydration.