By Helen Lammers-Helps
Three decades ago researchers at the University of Guelph established the importance of mycorrhiza fungi in the soil for nutrient uptake. Today researchers continue to fine-tune this research to help farmers further exploit the benefits of these beneficial fungi.
While there are important implications for nutrient uptake, the benefits don’t stop there. These plant-fungi associations can also help combat the adverse effects of drought, nutrient deficiency, disease and climate change.
In the 1980s Murray Miller and his team at the University of Guelph wanted to understand how no-till corn got access to phosphorus in no-till systems. “They put together a picture that in the absence of tillage, arbuscular mycorrhiza fungi remain intact over winter and the new crop obtains phosphorus through the pre-existing network of fungal hyphae,” explains Mike Goss, professor emeritus with the University of Guelph.
These arbuscular mycorrhizae fungi form mutually beneficial associations with living plant roots. The mycorrhizae fungi have long, thread-like branches of cytoplasm called hyphae. One tip of the hyphae enters the plant root while the other explores the soil matrix, thereby substantially increasing the volume of soil accessible to the plant. The plant supplies sugars to the mycorrhizae and, in return, the mycorrhizae enhance the plant’s ability to take up important nutrients from the soil. The effect is significant, Goss says.
Goss’s research has focused on how to exploit the native soil mycorrhizae fungus for the benefit of agriculture. Working with soybeans, Goss’s research showed that when soybean plants were colonized with both mycorrhizae and rhizobia, the nodules on the soybean plant roots were bigger and the soybean plants were able to fix more nitrogen. His research showed the effect was greatest when the soil wasn’t disturbed. The presence of the existing fungal mycelium in the undisturbed soil resulted in earlier colonization, he says.
Goss says it is believed that a diverse mycorrhiza fungi community, made up of many different fungal species, will be more resilient to changes in environmental conditions such as drought or disease pressure. The mycorrhiza fungi have many benefits on soil health including improved soil structure, water holding capacity and resistance to erosion.
Most recently, Goss has been comparing mycorrhizae populations and diversity in wheat under no-till and conventional tillage in Portugal. Following harvest, soil samples were taken randomly from wheat fields, one of which had undergone conventional tillage and one of which had not been tilled for the past nine years. After isolating the DNA from these samples, a computer software program identified all the different mycorrhiza fungi species found in each field. Then Goss and his team created a family tree, which showed how the fungi were related to each other.
The study showed that the diversity of mycorrhizal fungi communities was greater in the no-till field compared to the conventionally tilled field. Tillage makes the formation of mycorrhizas dependent on spores present in the soil rather than on the extra-radical mycelium of pre-existing mycorrhiza.
There are three ways that mycorrhizal fungus colonization occurs: through spores, extra-radical hyphae and hyphae from colonized root fragments. Runner hyphae from a well-developed extra-radical mycelium are quicker to initiate colonization in a new host than the others, Goss explains. In his research, the pre-existing extra-radicle mycelia were more effective in supporting shoot growth in wheat.
At present, it’s not practical to inoculate fields with beneficial mycorrhizae, so it’s best for farmers to promote growth of mycorrhiza through management practices. The mycorrhiza diversity can be affected by several factors including tillage, soil nutrient levels, crop rotation, cover crops, weed populations, and the presence of bacteria or other fungi.
It is clear that tillage disrupts the extra-radicle mycelium of the fungi so less tillage is better, Goss says, adding it is not yet known how much tillage would be tolerable without causing negative consequences. “Zone tillage, or tillage that just scratches the soil surface would probably be okay, but further research is needed.”
Goss’s preliminary work shows that intact extra-radicle mycelium in undisturbed soil gives more effective protection from soil-borne diseases.
Crop selection will also impact the growth of mycorrhiza. In Goss’s research, when wheat followed a crop that doesn’t form associations with mycorrhiza fungi, the wheat crop grew poorly compared to wheat that followed a crop that had been colonized by the extra-radical mycelium. Goss’s research also showed differences in the diversity of mycorrhiza colonization with different cover crops. “Certain cropping sequences will be more beneficial so farmers should try to select crops carefully,” Goss says.