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Fungi in Agriculture
by Eric Novey, NRCS
In a recent article by Dr. Wendy Taheri titled “The Role of AM Fungi in Agricultural Ecosystems”, she explored the ways arbuscular mycorrhizal (AM) fungi assist plants by supplying phosphorus along with additional benefits, while also considering the impact of chemical fertilizers on the connection between plants and fungi. Phosphorus often emerges as the most restrictive nutrient for plants, and the phosphorus cycle is interconnected with the nitrogen cycle. In agricultural systems where there is an overload of nutrients, carbon availability may become limited for soil microorganisms, which play a critical role in nutrient storage and preventing nutrient loss through leaching.
Accurately quantifying phosphorus poses challenges due to its existence in various forms. The Bray method assesses phosphorus that is bound to aluminum and iron, whereas the Olsen test is suitable for alkaline soils rich in calcium-bound phosphates. Both approaches concentrate on inorganic phosphorus (Pi), the form that plants can utilize directly via aqueous solution. Nonetheless, Pi’s presence in the soil is short-lived as it rapidly binds with other substances or is lost through leaching. Interestingly, natural ecosystems yield a higher biomass of plants with minimal available inorganic phosphorus thanks to efficient recycling through living entities.
In natural settings, microbes retain phosphorus by transforming organic phosphorus into forms accessible to plants. AM fungi are pivotal in the transfer of both organic and inorganic phosphorus to plants. Studies indicate that cropland soils contain significantly fewer viable AM fungal spores compared to natural environments, with prairie soils averaging over 1,300 AM spores per sample against fewer than 200 spores in croplands. In agricultural soils, non-AM fungi tend to predominate, competing with beneficial AM fungi for resources and offering minimal to no advantages for crops.
The reduction of beneficial fungi primarily stems from the excessive application of inorganic phosphate fertilizers. When phosphorus is abundantly available in the soil, plants cease to establish beneficial associations with AM fungi. Ordinarily, plants exchange sugars for phosphorus through these fungi.
However, in the presence of surplus phosphate, they decline colonization. Given that AM fungi rely on plants for their existence, their populations decline when hosts are absent. Some fungi may endure due to their high infectivity; however, the general trend is a reduction in beneficial microorganisms.
Additionally, the application of fungicides and pesticides to seeds adversely affects beneficial microorganisms significantly more than harmful counterparts, resulting in a shift from AM fungi to non-AM fungi. Pesticides can create vacant niches within ecological communities for other organisms to fill, leaving uncertainty about the replacements. This unpredictability underscores the necessity for prudent pesticide application.
To enhance soil management practices, adopt strategies that bolster beneficial organisms. Supporting these organisms facilitates their population growth, heightening the likelihood of successful coexistence post-chemical application. This arrangement restricts opportunities for detrimental ones and simplifies the establishment of beneficial new organisms. AM fungi stand out as highly advantageous for plants, offering numerous benefits without endangering other beneficial organisms.
AM fungi are vital symbiotic organisms for a myriad of plants, including most crops. It is believed that between 80-90 percent of flowering plants engage in symbiotic interactions with these fungi. This relationship has ancient origins, tracing back over 400 million years, yet remains intricate. The process begins when a seed germinates; the plant excretes hormones to draw in AM fungi, which primes its roots for colonization. The fungi then invade the roots, generating arbuscules for exchanging nutrients and extending hyphae into the soil to procure water and nutrients.
AM fungi primarily assist plants in phosphorus absorption. They rely on hospitable plants for survival, obtaining sugars through arbuscules. If they fail to locate a host, they perish. This relationship enables plants to manage the fungi by degrading arbuscules that do not supply phosphorus. Furthermore, if plants cease to provide sugars, the fungi can withhold phosphorus. However, excessive application of chemical phosphates can disrupt this relationship, rendering AM fungi redundant. Additionally, AM fungi contribute advantages such as greater drought and salt tolerance, enhanced nutrition, pest protection, improved soil health, and increased seed and fruit yield.
AM fungi can substitute for up to 25 percent of phosphorus consumed without compromising crop yields, although this doesn’t apply universally across all crops. Numerous studies concentrate on a select few easily cultivated species under low phosphorus conditions. Halting phosphate fertilizers may pose challenges for AM fungi that are acclimated to lower phosphorus levels to sustain yields. Natural ecosystems do not significantly rely on reduced phosphorus concentrations. To maximize phosphorus utilization, more research in high phosphorus soils is necessary to evaluate how AM fungal populations recover and their efficiency in phosphorus uptake for maintaining yields.
