Mycorrhizal fungi are ancient organisms forming mutually beneficial relationships with the roots of most plants. These fungi act as extensions of plant root systems, enhancing their ability to acquire resources from the soil. This partnership improves plant health and plays a role in the functioning and resilience of terrestrial ecosystems.
Understanding Mycorrhizal Fungi
Mycorrhizal fungi are a diverse group of fungi that establish symbiotic associations with plant roots, a partnership dating back over 400 million years. The term “mycorrhiza” originates from Greek words meaning “fungus” and “root.” These fungi colonize plant root tissues, acting as an interface between the plant and soil.
Two primary types of mycorrhizal fungi relevant to most plants are ectomycorrhizal (EcM) and arbuscular mycorrhizal (AM) fungi. Ectomycorrhizal fungi form a dense sheath around root tips and extend hyphae into the surrounding soil. They are commonly associated with woody plants like oaks, pines, and birches, and typically colonize about 10% of plant families.
Arbuscular mycorrhizal fungi penetrate root cells, forming branched structures called arbuscules for nutrient exchange. They are the most widespread type, associating with over 80% of plant families, including most herbaceous plants and important crops such as corn, wheat, and soybeans.
The Symbiotic Partnership
The relationship between mycorrhizal fungi and plants exemplifies mutualism, benefiting both organisms. Plants produce sugars through photosynthesis and transfer a portion to the fungi, providing their primary energy source. This carbon allocation can range from 5% to 20% of the plant’s total carbon uptake.
In return, the fungi act as an extensive extension of the plant’s root system. Their fine hyphal networks, much smaller in diameter than plant roots, can explore a far greater volume of soil, accessing water and nutrients otherwise unavailable. These networks are particularly effective at acquiring immobile nutrients like phosphorus, which often exist in forms difficult for plants to absorb directly.
They also enhance the uptake of other nutrients, including nitrogen, zinc, manganese, and copper. Fungal hyphae can also mobilize sparingly available nutrient sources by secreting enzymes. This increased access to resources improves the plant’s nutrient uptake efficiency, leading to enhanced growth, improved drought tolerance, and greater resistance to certain diseases. The fungal network essentially “mines” the soil, delivering essential elements to the plant in exchange for the carbon compounds needed for the fungus’s own growth and maintenance.
Broader Ecological Importance
Beyond individual plant health, mycorrhizal fungi are important for ecosystem stability and functioning. Their extensive hyphal networks form vast underground connections, often called the “wood wide web,” linking multiple plants, even different species. This network facilitates nutrient and water sharing, contributing to ecosystem resource distribution and resilience.
These fungi also play a role in soil structure and carbon sequestration. Fungal hyphae bind soil particles, forming stable aggregates that improve soil aeration, water infiltration, and reduce erosion. Mycorrhizal fungi contribute to soil organic matter when their biomass dies and decomposes, forming a stable carbon pool.
Plants transfer over 13 billion metric tons of carbon dioxide to mycorrhizal fungi annually, comparable to a portion of global fossil fuel emissions. Mycorrhizal associations enhance ecosystem resilience by improving plant tolerance to environmental stresses like drought, salinity, and heavy metal toxicity. Their ability to reduce nutrient leaching also helps maintain soil fertility and prevents groundwater contamination. These contributions show their role in supporting biodiversity and the long-term health of landscapes.
Fostering Mycorrhizal Growth
Encouraging healthy mycorrhizal populations involves practices that support their natural development. Minimizing soil disturbance, such as reducing tillage, is a step, as tilling can disrupt fungal networks. Allowing organic matter, like compost or mulch, on the soil surface provides a food source for fungi and improves soil structure.
Reducing reliance on synthetic fertilizers, particularly those high in phosphorus, is also beneficial. High levels of soluble phosphorus can decrease a plant’s reliance on fungi, inhibiting mycorrhizal colonization. Similarly, excessive use of fungicides can harm these organisms.
Selecting plants known to form strong mycorrhizal associations and promoting plant diversity also supports robust fungal communities. While commercial mycorrhizal inoculants are available, healthy soil practices often provide sufficient conditions for native mycorrhizal fungi to thrive.