As soil becomes more alive, plants become more talkative
March 30, 2017
This year CSANR sponsored registration for several WSU students to attend the Tilth Conference. We will be posting reflections written by the students over the next several weeks. Please feel free to comment and give these students your feedback.
My name is Aaron Appleby; I am a senior at Washington State University, studying Organic Agriculture. Living soil is a key aspect to sustainable farming, as it requires fewer inputs and promotes diversity amongst organisms, ensuring survival as different selection pressures are introduced. Carbon is essential for living organisms and although plants get their carbon from the air, the microorganisms that inhabit the soil need to ingest soil organic matter (SOM) to incorporate carbon into new cells and organic molecules for growth and reproduction. Thus, the main benefit of increasing SOM is to the ability of the soil to sustain life, which in return provides nutrients plants can utilize to grow.
The soil is not only alive, but plants have evolved ways of interacting with it to exchange chemicals needed by the different organisms. Dr. Mark Mazzola was the first speaker to present at the Soil Symposium on “Understanding Soil Microbiology to Build System Resilience and Enhance Plant Productivity.” His presentation described many key aspects of these interactions. Knowing that some mycorrhizal associations promoted root growth, I was always interested in the mechanism of this phenomenon. What I learned from Dr. Mazzola was that the fungus is exuding chemicals that alter the bacterial population dynamics in the rhizosphere to favor a type of bacteria that produces auxin. Auxin is a plant growth hormone that the plant then utilizes to increase root growth.
Not only is soil alive, but it “remembers” with the help of plants. This was another interesting phenomenon that Dr. Mazzola discussed in his lecture. Fields managed with continuous cropping (mono-cropping) were able to “remember” pathogens and respond accordingly during consecutive invasions; however, during a crop rotation the soil is unable to remember how it reacted to the pathogen. This results in the soil’s inability to quell the subsequent invasion as quickly as a mono-cropped system. By changing the plants sown into the soil during a rotation, the population dynamics of the soil microorganisms change and are less able to deal with previous problems, as easily as a population defined by and adapted to that specific selection pressure. The take home message from this discovery was not to use mono-crop systems, but that plants exude species-specific chemicals or chemical cocktails through their roots into the rhizosphere to “farm” certain populations of microorganisms.
This leaves the door open for more research on the different chemicals exuded by plants, and how they interact with the microbial populations in the soil. Perhaps these chemicals could be identified and synthesized to allow farmers to sway the microbial battles in the soil to benefit plant growth and development. Instead of adding anhydrous ammonia, which inhibits microbes in the soil, farmers could add a chemical that changes microbial populations, favoring a specific bacterium to the root zone. That, In turn, could attract increased populations of protozoa to feed on the bacteria and excrete plant-available nitrogen. Farmers may also be able to use this technology and research to manipulate the microbial populations into one that is beneficial to fight problematic pathogens or produce a particular growth hormone.
Dr. Mazzola’s presentation really got me excited about the role microbes play in agriculture. This summer I will be working with Dr. Lynne Carpenter-Boggs researching compost and have applied to Dr. Mazzola’s master’s program in plant pathology at WSU.