U of A-led research team will study how soil microbes could make or break hydrogen's clean energy promise

Molecular hydrogen (H2), a promising low-emissions alternative to fossil fuels, is gaining momentum as a potential solution for everything from heavy industry to electrical power generation. A University of Arizona-led research team aims to investigate the possible ecological impacts of large-scale hydrogen fuel adoption with a $1.7M grant from the National Science Foundation.

When consumed in a fuel cell, molecular hydrogen produces large amounts of energy through a process called electrochemical oxidation. It’s cleaner than burning fossil fuels, since it produces water vapor instead of harmful greenhouse gasses like carbon dioxide. But according to Laura Meredith, an associate professor in the School of Natural Resources and the Environment and the project’s lead principal investigator, large-scale use of hydrogen fuel would result in increased emissions of hydrogen gas into the atmosphere. Excess atmospheric hydrogen could adversely impact climate, air quality and the ozone layer. Understanding how hydrogen emissions could be mitigated is essential to preventing those adverse effects.

The three-year project will focus on hydrogen-consuming microbes found in soil, which account for approximately 80% of the total sink of atmospheric hydrogen.

“These soil microbes are the single most important factor in determining how much hydrogen stays in the atmosphere,” Meredith said. “There’s been some evidence that the microbes adapt to consume more hydrogen if more is available, but we don’t know if there’s a point at which they’ll no longer be able to consume that excess hydrogen. Since soil is such a significant sink for atmospheric hydrogen, understanding this is critical to understanding where those emissions are going to go.”

Studying emissions at the source – and modeling where they go

If hydrogen fuel technology is scaled up, it would result in increased hydrogen emissions near production and distribution sites. That, in turn, could increase ambient hydrogen in the atmosphere.

“As with natural gas, hydrogen fuel would likely be transported using pipelines that distribute it from its source to its local end user,” Meredith said. “We know there are a bunch of leaks in that natural gas infrastructure. Also, high volumes of liquified hydrogen could be transported across large distances, and sometimes it’s necessary to vent some of the gas during storage, transport and distribution.”

The new project will study how soil microbes respond to both high-concentration releases of hydrogen – the type of plume you’d expect near production and distribution facilities - and an increase in the overall level of atmospheric hydrogen.

“Now that hydrogen is being used at the scale that it is, we have the opportunity to go around and measure how much is actually coming out, and what its fate is,” Meredith said. “There’s a huge data gap that we’re poised to fill, based on our measurement expertise.”

The team will take continuous soil measurements over the course of multiple years using probes developed in partnership with Aerodyne Research. The partnership has also produced specialized gas analyzers that will be employed in a mobile lab to track hydrogen plumes produced during planned venting events. These will measure hydrogen concentrations in the atmosphere, how the emissions interact with nearby soils, and how those hydrogen molecules travel.

Ave F. Arellano, Jr., a professor in the Department of Hydrology and Atmospheric Sciences, will use data from the mobile labs and soil sensors to develop more accurate diffusion models than are currently available.

“Our models right now are very simplistic, and it’s high time to improve that,” he said. “We want to move towards a more nuanced – and more accurate – model, wherein we can really understand the limitations of temperature, moisture and microbial activity on the removal of hydrogen from the atmosphere.”

Moisture matters

Soil composition varies greatly across the globe, and one of the key elements of the project will be studying how different soil types react to elevated atmospheric hydrogen. Meredith said that soil moisture may be a key determining factor.

“Dry soils are more porous than wet soils, so gases like hydrogen can diffuse into arid soils more efficiently,” she explained. “But the microbes in arid soils tend to be more stressed, which might make them less efficient at consuming that hydrogen. Conversely, soils with more moisture tend to have more robust microbe activity, but it takes longer for gases to diffuse into soil that’s saturated with water.”

Researchers will measure the hydrogen uptake of arid soil at the Santa Rita Experimental Range station near Tucson, and semitropical soil at the Jones Ecological Research Center in southwestern Georgia.

“We wanted to have sites that were similar in terms of temperature so that we can better isolate the effects of soil moisture,” Meredith said. “Temperate forest soils tend to be overrepresented in our data sets, so having both arid and semi-tropical soils represented in our data will offer a more complete picture.”

Meredith also noted that arid soils cover almost a third of the planet’s soil surface cover, so understanding hydrogen uptake in arid soil will be essential to global hydrogen emission models.

Gathering enough data to improve global models of atmospheric hydrogen is one of the project’s ultimate goals, Arellano said. “The hope is that these detailed measurements will help us understand the underlying mechanisms and their limits, which will improve our models moving forward.”