SOIL BIOLOGY 101: LEARNING OUR SOIL-PART 3
Three Part Harmony: Understanding Tripartite Association.
In the first two parts of this series, we looked at the symbiotic relationships agricultural plants establish with certain bacteria and fungi in the soil.
When we talk about symbiosis, we are usually describing a mutually beneficial relationship between two living organisms. However, in the case of a tripartite association, three parties are involved.
Such a relationship exists between pulses (and other select legumes) when colonized by two “good” microorganisms: rhizobium and arbuscular mycorrhizal fungi (AMF). Benefits to the plant include increased photosynthetic rate to supply more energy for nitrogen fixation activity, nodulation parameters, and a more balanced N:P:C supply ratio for better growth.
Let’s review these interconnected relationships and explore how it has led to the introduction of dual-inoculant products to provide an added advantage to pulse and select legume crops.
Relationship #1: In Part 2 of this series, we explained how pulse crops form a relationship with bacteria called rhizobium. To sum up, the plant forms new nodules, which are inhabited by the bacteria. From its new home in the nodules, the Rhizobium facilitates nitrogen fixation (converting N2 to plant-accessible ammonium).
In exchange, the pulse crop provides food (carbohydrates) and oxygen to the rhizobium. This process requires a great deal of energy on behalf of the plant. For this reason, pulse crops involved in nitrogen fixation require more Phosphorus than other plants.
Relationship #2: In Part 1, we discussed how arbuscular mycorrhizal fungi colonize the roots of the pulse plant. Then, the fungi’s long, web-like hyphae serve to extend absorption beyond the root system. This greatly increases the plant’s ability to uptake NPK, as well as micronutrients and water. The enhanced ability to find and absorb phosphorus is particularly important in pulse crops, due to the energy demands of nitrogen fixation.
Other benefits to the plant include increased tolerance to stress (biotic and abiotic), expanded root system growth and improved soil structure. In return, the AMF receive Carbon from the plant.
Relationship #3: Legumes have a lower phosphorus use efficiency (Vetonsek et al, 2002). By providing the plant with the Phosphorus required for nitrogen fixation, AMF are facilitating and improving the relationship between the pulse plant and the Rhizobium. And what is good for the plant is good for both microorganisms.
The plant gets more than it gives. When used in combination, the synergistic relationship of mycorrhizae and rhizobium increase the photosynthetic rate 2).
Encouraging tripartite association through dual-inoculant interaction.
While most plants can benefit from the addition of AMF as an inoculant, it has been shown that pulse and soybean crops gain the greatest productivity increase from a dual inoculant featuring AMF and Rhizobium.
According to a 2016 study “… It has been found that pea plants coinoculated with Rhizobium leguminosarum and AMF [Arbuscular Mycorrhizal Fungi] have shown best results regarding plant height, plant dry mass, nodule fresh weight, number of seeds, seed weight, seed yield, number of root nodules, number of pods per plant, average pod weight and pod length…“. (2)
Different crops. Different dual inoculants.
When purchasing a dual inoculant, it is important to specify which crop you will be using it on. That is because soybeans and pulses each interact with a different type of rhizobium, which is reflected in the product composition.
Pulses form a relationship with Rhizobium leguminosarum while soybeans form a relationship with Bradyrhizobium japonicum. So, make sure you purchase the right one!
It should be mentioned the biological actives that make up commercial inoculants (the rhizobium and mycorrhizae) are all manufactured in laboratory conditions to ensure consistency.
Tripartite cooperation – beyond the field.
Taurus is pleased to have shared this Learning Our Soil series with you. It reflects our role as an educator within a tripartite relationship involving Grower, Agronomic Advisor/Educator and Retailer. We hope you’ve found it helpful.
 Kaschuk et al. 2009. Soil Biol. Biochem. 41:1233-1244
 Shinde et al. 2016. Int. J. Bioassays. 5:4954-4957