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Mycorrhizae in crop rotations

Many crops form arbuscular mycorrhizae, which are the combination of particular fungi with crop roots that enhance the uptake of phosphorus (P) from the soil. These fungi form tree-like structures called arbuscules in the roots and help with the growth of plants. Crops such as grasses, flax and legumes are host plants for mycorrhizae and are highly dependent on them, but crops such as canola and mustard are non-mycorrhizal. Crop rotation, tillage and other management factors can impact the level and benefits of mycorrhizae. “Tillage has an impact on mycorrhizal development,” explains Dr. Terence McGonigle, associate professor and chair, in the Department of Biology at Brandon University. “Going back to some earlier research I was involved in with corn in Ontario in the ‘90s, the results showed that no-till corn has more mycorrhizae than conventional till corn. The results clearly demonstrated that mycorrhizal development is promoted not only by avoiding fallow and non-mycorrhizal crops in rotation, but is also promoted by less soil disturbance.”

The mycorrhizae need to be interconnected to retain their ability to inoculate. Intact fungal filaments left in soil after harvest are able to colonize the roots of the following crop the next spring and access soil P quite early in the season. Tillage tends to break up the fungus in the soil and reduces the inoculum. Less inoculum delays the colonization of the roots and reduces the benefits mycorrhizae bring to early season P uptake, and causes impaired phosphate nutrition unless adequate fertilizer is provided.

McGonigle is interested in finding out more about the benefits of mycorrhizae to field crops and is conducting a four-year study with Viterra comparing mycorrhizal levels and other factors in flax grown on canola and wheat stubble. “In 2008 we seeded flax using uniform management on a field that had previously been seeded to a number of variety trials for wheat and canola,” explains McGonigle. “In early season scouting we were surprised to discover that we could determine which plots had previously been seeded to canola and the ones previously to wheat even without plot markers. The flax seeded on previous canola plots was much smaller, less advanced developmentally and flowered later. It appeared that flax following wheat was performing much better than flax following canola.”

Over the growing season, McGonigle collected root samples from various points in the field and found that levels of arbuscular mycorrhizae were greater on flax following wheat. Early season growth was also monitored and growth differences were more pronounced early in the year. Greater uptake was seen for flax shoots following wheat except for P, copper (Cu) and zinc (Zn), and this enhanced uptake was associated with more extensive mycorrhizal development of flax following wheat compared to that after canola. “What we were finding was that with crops like canola or with fallow that are non-mycorrhizal, there is a decrease in the natural inoculum of the mycorrhizal fungus in the soil over the growing season, and in the following year the mycorrhizae are slower to develop,” explains McGonigle. “In comparison with mycorrhizal crops like wheat, the inoculum levels are maintained and the mycorrhizae develop more quickly. Therefore, our interpretation was that flax was doing better following wheat than canola because it had more mycorrhizae.”

In 2009, McGonigle expanded the study to test this concept experimentally over the next four years. “Flax will be seeded on wheat stubble and canola stubble, but in this study, we have also added a phosphorus fertilizer variable,” he says. “We will also be recording differences in above-ground biomass early in the year and measuring final grain yields.”

Preliminary results from the 2010 flax harvest have confirmed some factors but also raised some questions. “As before, we found more mycorrhizae in flax following wheat as compared to canola. We also had a response to the P fertilizer applications,” explains McGonigle. “However we did not find any difference in final crop yields between the crop histories, which was surprising. We’re not sure how to interpret this, and are looking at factors such as soil and weather conditions, which were much wetter and cooler in 2010. We hope over the remainder of the study we will find a pattern and answers to some of these questions.”

Although inoculation may be cost effective in smaller scale production such as onions or strawberries, McGonigle does not recommend inoculation for larger-scale field crops. “There is natural inoculum in the field and probably the most effective approach for field crops is to manage the naturally occurring mycorrhizae,” says McGonigle. “Growers should avoid placement of flax or corn after canola in rotation, and where this does occur, additional use of P fertilizer should be considered.” 

November 30, 1999  By Donna Fleury


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