In a changing climate, horticulturalists and landscape professionals are looking for ways to ensure the establishment and healthy growth of plants, whilst minimizing the use of both water and chemical fertilizers. The demands on potable water have never been greater than they are today and with rising populations demands are likely to increase further. Many soils have become nutrient deficient due to years of intensive farming, overuse of chemical fertilizers and from the effects of industrialization. Pressure on land is increasing rapidly and populations are developing in areas where natural soils have since been removed by human intervention or through desertification.
In the search for environmentally acceptable alternatives to chemical fertilizers, various materials have been used as soil improvers including peat, manure, compost and seaweed. Such materials have often been used successfully to enhance the nutrient content and water holding capability of soils but they are generally bulky materials that often come from finite sources and require expensive collection, treatment and transportation. Mycorrhizae are now being seriously considered as a means of improving nutrient deficient soils. It is considered that mycorrhizae can play an essential role in plant growth by enhancing plant vigour in poorly performing soils, and through their ability to store large amounts of carbon, they may ameliorate some of the effects of global warming.
Definition and function of mycorrhiza
Mycorrhiza refers to a mutually beneficial interaction, or symbiosis, between plants and fungi. These naturally occurring symbiotic associations are characterized by the exchange of nutrients during the growing season.
There are two main kinds of mycorrhiza:
- Arbuscular mycorrhizae (AM) penetrate the plant root tissues
- Ectomycorrhizae (EM) surround the roots without penetrating them.
AM are the most widespread across plant populations.
Typically, whichever the type of mycorrhizal association, the fungus acquires carbon from the plant, while the plant obtains otherwise unavailable mineral nutrients via the fungal hyphae (threads) which extend the effective absorptive area of the root system. The hyphae are narrower than root hairs, extend further and explore a greater volume of soil, thus having a much higher surface to volume ratio for absorption of nutrients and water. Additionally, the fungi secrete enzymes which break down organic matter and tightly bound micronutrients, such as phosphorus, zinc and copper, enabling the plant to absorb the required minerals from the surrounding soil.
Mycorrhizae are found in a variety of ecosystems and plant communities. The majority of land plants form symbiotic relationships with fungi in their native habitats, though in open habitats, where soil nutrients are not a limiting factor, non-mycorrhizal plants are more widespread. Fossil records from the Devonian period show that plants evolved alongside fungi, and the latter may have been critical to the evolution of plants and their land colonization hundreds of millions of years ago.
In their native habitats, mycorrhizal associations vary widely in structure, form and function, involving different combinations of plant and fungus, which allow plants to use a range of survival strategies, for example:
- Ericaceous plants on heather moorlands are colonized by AM fungi which grow extensively within (intracellular) root cells, but produce relatively insignificant growth in the surrounding soil. The fungi produce enzymes which release nitrogen from organic matter which can be a critical limiting factor for growth of these plants
- Forest trees associate with EM fungi that grow between (intercellular) root cells, producing large quantities of hyphae on the root and the surrounding soil. The hyphae release enzymes which extract nutrients from organic matter on the forest floor
- Grassland plants are colonized by fungi which form highly branched structures both within root cells and outside the root, and this significantly increases rates of phosphorus-uptake which is frequently a limiting factor for growth
- Some orchids differ from the above associations as they are parasitic on the fungus, and their seeds are extremely small with minimal nutrient reserves. Germination is triggered when the seeds are infected by a particular fungus which breaks down cellulose and other organic matter in the soil.
Most mycorrhizal fungi are not host-specific and several plants of differing species may be linked by a single hyphal network, just as individual plants may form AM and/or EM associations. In some cases, the combined associations act synergistically.
Benefits of mycorrhizal inoculation
In situations where naturally occurring mycorrhizal associations are infrequent or ineffective an appropriate inoculant can benefit new or established plants by:
- Reducing transplant shock
- Extending the growing season - plants grow larger, flower earlier and produce higher yields
- Providing protection from attack by soil-borne pathogens
- Buffering against toxic levels of trace elements on contaminated land
- Increasing the ability to tolerate environmental stresses.
Commercially available products often contain both ectomycorrhizal and endomycorrhizal species and allow the plant to select the appropriate species for its needs. Not all plants benefit equally from the symbiosis; plant responses to interaction with mycorrhizal fungi vary considerably and management practices such as tillage and crop rotation may adversely affect mycorrhizal structures. Mycorrhizae are especially beneficial for plants in degraded or infertile soils and generally coarse-rooted plants benefit more than fine-rooted plants.
Mycorrhiza also benefit the soil and the wider ecosystem in the following ways:
- Improving soil structure, particularly in manufactured or degraded soils
- Allowing survival of many kinds of seedling that would otherwise never compete, in effect increasing plant diversity
- Below-ground diversity of fungi is one of the major contributing factors to the above-ground plant biodiversity in ecosystems.