Research Highlights

Soil Fungi Support Potassium Uptake

By Laura Temple

Soybeans need potassium (K) for biomass production, stress tolerance and reproductive growth. Those factors translate to yield. However, just a fraction of the potassium in soils is available to plants.

In response, plants have developed strategies to efficiently take up this nutrient.

Symbiotic, or mutually beneficial, relationships between land plants and soil microbes are the most important strategy for absorbing nutrients and water, notes Kevin Garcia, assistant professor at North Carolina State University. He leads research focused on understanding how beneficial fungi help soybeans acquire soil potassium, with funding from the North Carolina Soybean Producers Association checkoff. 

Garcia and Danielle Cooney, a Ph.D. graduate student working on this project, focus on how arbuscular mycorrhizal (AM) fungi transport K to soybean plants. The fungi name comes from Greek — arbuscular means tree-like structures and mycorrhizal combines the Greek words for “fungus” and “roots.”

“This fungus class creates tree-like structures in the roots of 80 percent of plants, including soybeans,” Cooney explains. “The fungi develop into cells in root walls. They get carbon from the plants in exchange for bringing in nutrients and water from a larger area in the soil, creating a mutually beneficial relationship.”

Most existing research on AM fungi studied acquisition of nitrogen and phosphorus. This multi-year project investigates the impact of AM fungi on K uptake in soybeans in the field and greenhouse. The laboratory portion of the study concentrates on understanding the mechanism and amount of K transported into soybeans.

“We think the breeding process, which happens in ideal conditions, may influence the strains of AM fungi attracted to specific soybean varieties,” Cooney says. “What we learn from this research could help breeders select for varieties that can efficiently interact with surrounding microbes and optimize the ability to efficiently absorb K at key stages.”

The long-term goal of this research is to learn how to reduce potash use without affecting soybean growth and yield.

Naturally Occurring and Added Fungi  

To better understand how AM fungi interact with soybeans, Garcia’s team investigated the impact of applying a commercial inoculant containing AM fungal spores to the natural soil microbe environment on K uptake and metabolism in soybeans. Lab work measured the ability of AM fungi to transport K and traced its flow in soybean plants.

Field trials compared treated and untreated soybeans, including varieties from maturity groups 4, 5 and 6. The team conducted field trials on research plots known to be low in K. Three different locations provided a variety of natural soil microbe communities. North Carolina soils vary between the sand hills, the upper Coastal Plain and the Piedmont regions. 

“AM fungi need about eight weeks to begin transporting K,” Cooney says. “We looked at several factors to understand what is happening above and below ground.”

They measured tissue chlorophyll content at eight weeks, at the R1 or R2 growth stage, and at 16 weeks, about R5 or R6. They also dug up roots samples to evaluate AM fungal colonization. At the end of the season, the team measured yield, along with protein and oil content of the soybeans.

“After three years, the data is still fairly inconclusive,” Cooney explains. “It’s complicated to determine which field-based factor may be having the most influence on our particular trial. Results have been mixed and inconsistent. The inoculant did not consistently benefit yield. However, a more noticeable, but still contradictory, effect was seen in protein content depending on location and year. In some plots, AM fungi increased protein content, while in others, treatment decreased protein content.”

These trials were repeated in field soils in the greenhouse to create a more controlled environment to strengthen correlations and shape future research, and that data is being analyzed.

“It appears that very nuanced factors influence soil–plant interactions,” Cooney says. “This work raised questions that will help us learn more. For example, we don’t know how introduced and native AM fungi strains interact with each other and other soil microbes.”

This research provides a baseline and tools to understand K uptake, leading to questions that can effectively evaluate inputs like the inoculant. Garcia’s team appreciates the willingness of the soy checkoff to support this type of research that leads into the next questions that will help scientists and farmers to better understand soybean and soil biology and could improve fertilizer efficiency.

Published: Sep 19, 2022