Researchers from the South Dakota State University have written a detailed review of the role of Arbuscular Mycorrhizal Fungi (“AM”), a fungus known to form a mutual association with a large percentage of land plants, in giving crops greater access to nitrogen in the soil.
AM are obligate biotrophs, requiring carbon from their host plants in order to complete their life cycles. Plants transfer carbon to AM, which form large networks between each other and numerous plant hosts and act as an extension of plant roots, in exchange for the fungi delivering a number of required nutrients to the plant host.
The role of AM in providing phosphate to host plants is well studied; being a relatively immobile nutrient, areas of phosphate depletion around the plant roots arise, and the AM provides an alternative method of access that results in the downgrade in activity of the plant uptake pathways.
But the role of AM in passing nitrogen to plants is unknown.
Roots of plants that form associations with AM have both plant uptake pathways and mycorrhizal uptake pathways for the intake of nutrients. Mycorrhizal uptake pathways are regions that form intimate connections between the fungi and plant roots, allowing the passing of nutrients between them.
Mycorrhizal uptake pathway and the Mycorrhizal interface. Sourced from article.
The review ultimately finds that the research into nitrogen uptake via AM are inconsistent in their findings and that further research is needed for a strong conclusion to be reached.
The researchers note that recent studies suggest that AM has something in the order of a five times greater affinity for ammonium than the plant uptake pathways, meaning that AM is more adept at sequestering nitrogen from the soil in low nitrogen conditions. Further, there is increasing evidence of the existence of pathways available for the transport of nitrogen through AM hyphae to their host plants and that some plants down-regulate their own N uptake pathways when AM-associated uptake is occurring.
Despite the findings of a positive relationship, the results of the studies reviewed show that no general rule is applicable. Whilst some plants are able to efficiently take up nitrogen via their associated AM network, other studies showed lower efficiency or an efficiency that varied as a result factors that aren’t completely understood.
Understanding with some clarity the plants and the environmental circumstances that result in significant nitrogen uptake via AM communities, and particularly the signals emitted by plants to direct this to commence, could provide a significant advance in the efficient use of nitrogen fertiliser. If certain crops are identified as having positive nitrogen uptake pathways via AM when a certain signal is emitted, and we are able to turn that signal on as required, nitrogen fertiliser utilisation could be increased significantly as can the problems of nitrification and eutrophication of nearby waterways be reduced.
Definitely a great area of research that could significantly improve food production if a breakthrough in our knowledge can be made.