Nitrogen-fixing bacteria are a well-known example of beneficial bacteria. They can make atmospheric N available to plants, lowering the need for N fertilizers. But some of the most important of the soil microorganisms are the plant growth-promoting rhizobacteria (PGPR) and arbuscular mycorrhizal fungi (AMF). Numerous studies have demonstrated the wide range of potential benefits that these microorganisms can provide to plants. While doing so, these beneficial microbes also stabilize the soil and add carbon as they decompose organic compounds produced by their host plants.

PGPR are a group of bacteria that form beneficial relationships with the roots of plants, enhancing their ability to uptake water and nutrients. PGPR produces phytohormones which stimulate plant growth, promoting root development and increasing plant biomass. They produce organic acids and other compounds that make nutrients like phosphate and potassium more soluble in water. These organic acids play many roles in soil and plant health. Other PGPR produce siderophores, which bind to iron and make it more bioavailable to the plant. PGPR can also induce pathogen defense mechanisms, or even produce compounds that make plants more drought- or salt-tolerant.

Similarly to PGPR, AMF have been shown to provide a wide range of benefits to plants, such as enhanced photosynthetic capacity, improved nutrient uptake, and increased antioxidant defense during periods of stress. AMF forms a network of fine mycelia through the soil and connect the roots of plants, increasing water and nutrient uptake by the plant. AMF can access microsites in the soil, improving mineral nutrition. They can also redistribute water during periods of drought, and can prevent toxic ions from entering plant roots in salty soils.

In many crops, inoculation with PGPR and AMF has been shown to enhance growth rates, macro- and micronutrient uptake, and biomass, even with reduced fertilizer applications.

Figure from Adesemoye, A. O., Torbert, H. A., and Kloepper, J. W. (2009). Plant growth-promoting rhizobacteria allow reduced application rates of chemical fertilizers. Plant Microbe Interactions.

• Crops grown in inoculated soils show an increase in the content of trace metals such as Zn, Fe and Cu.
• In one field study, the authors found that a 50% reduction in NPK fertilizer paired with PGPR inoculation did not decrease wheat yields.
• Tomatoes and wheat grown in PGPR- and AMF-inoculated soils treated with a 75% dose of fertilizer showed equivalent nutrient uptake compared to wheat treated with a 100% dose of fertilizer but non-inoculated.
• In a field study of tea (Camellia sinensis), the authors found that by inoculating plants with indigenous beneficial microbes, they could reduce N input by 33% and P inputs by 50% without a reduction in yield.
• In a four-year orchard study, AMF and PGPR inoculation significantly improved root structures and increased the trunk cross-sectional area 24% relative to non-inoculated trees.