Microbiomes essential to healthy soils may adapt poorly to changing climate

When we think of climate change’s disruption to terrestrial life, we tend to think of the big things — forests, polar bears, whole categories of birds — that are under pressure and not clearly capable of adapting to new conditions.

But it’s the microscopic soil organisms that form the foundation for almost all land-based life. Not only do they provide plants with nutrient-processing and water-transport services, they also regulate a large portion of the carbon cycle, including capture and re-release of atmospheric carbon dioxide.

Large gaps remain in our scientific understanding of these vast communities — there may be as many as a billion tiny critters in a spoonful of good soil — and a recent paper from the Pacific Northwest National Laboratory argues that these organisms may be far less adaptable to climate change than is often assumed.

The findings, which have received surprisingly light media coverage, are based on a long-term  study of what happens to soil microbes when their natural growing conditions become either hotter and drier, or colder and wetter.

The alarming answer is, not much. From the lab’s announcement:

The findings are based on a unique 17-year study of transplanted soils on a mountain in eastern Washington state. The team moved some samples of soil down the mountainside 500 meters to a warmer, drier climate, and other samples up 500 meters to a cooler, moister climate. After 17 years, they analyzed both sets of soil in the laboratory, as well as “control” samples from both sites that had never been moved.

The scientists analyzed the make-up of the microbial communities, their enzyme activity, and their rates of respiration — how quickly microbes convert carbon in the soil into carbon dioxide which is released to the atmosphere.

The scientists found less adaptability than they expected, even after 17 years. While the microbial make-up of the samples did not change much at all, the microbes in both sets of transplanted soils retained many of the traits they had in their “native” climate, including to a large degree their original rate of respiration.

The message, the authors say, is that scientists can’t simply assume that microbes will nimbly respond to climate change.

Longest-term study so far

While 17 years may not seem like a very long stretch in the timescales of evolution, it is apparently the longest frame so far in a study on the climate adaptability of soil biomes; more typical is a few years to perhaps a decade.  

The 1,640-foot elevation difference in this slope of Rattlesnake Mountain introduced some significant differences in climate: The lower site was about 9 degrees Fahrenheit warmer than the higher site, and got nearly two inches less rain in a year. This resulted in a microbial respiration rate in the cooler, wetter soils that was nearly double the rate at the lower elevation.

Because the sites lie within the Hanford Reach National Monument, they are considered pristine and free of confounding influences. So the scientists could be confident in their observation that transplanting had little effect on either the makeup of the soil samples’ microbial communities or on their functioning.

The microbes native to the higher site respired at a higher rate naturally, due to the moister climate and a more plentiful supply of carbon in their environment; when they were moved to the lower, warmer site, they continued to respire at a faster rate than the surrounding “native” soils and microbes. And the microbes transplanted from lower ground to higher ground had an unusually small response to the temperature change, though biological theory and climate models predict a larger change.

“With our changing climate, all microbes will be experiencing new conditions and more extremes,” said [Vanessa] Bailey, a soil microbiologist [and chief author of the paper]. “Climate change won’t translate simply to steady warming everywhere.

“There will be storm surges, longer droughts; some places may end up experiencing more mild climates. This study gives us a glimpse of how microbes could weather such changes under one set of conditions. They may be constrained in surprising ways.”

Scenarios of disruption

Constraints on adaptability to changing climate conditions remain the big question mark as science considers the impact of global warming.

The happy notion that ecosystems might evolve in more or less intact form — that Minnesota would just become more like Iowa — has gradually lost ground to the prospect that the pace of change in temperatures, precipitation and other key factors will simply be too rapid for systems to keep up.

And the alternative scenarios of disruption/disaggregation are grim: Predator species adapt just fine, but prey disappears. Plants exhibit resilience as the growing season changes shape, but become more vulnerable to pests.

The Bailey study doesn’t address the potential impacts of the changes discovered on Rattlesnake Mountain in detail. But it points out that one key function driven by the respiration rates of soil microbes is decomposition of soil organic matter — that is, its conversion into — nutrients usable by plants.

Another is the water transport. And still another, of critical importance to assessing the pace of climate change, is how soil buffers increases in atmospheric carbon — or doesn’t.

We think of trees as the main carbon sink in forested landscape, but in truth there’s more carbon stored below ground, in the soil, than in the tissues of the trees themselves.

Which is one reason there’s so much concern on accelerated rates of melting in the earth’s permafrost regions, which cover about one-fifth of the planet’s land area; another is that permafrost also tends to release a lot of methane when the ice goes out.

Out of the ‘black box’

As Jim Robbins wrote in Yale Environment 360, in one of the few pieces I’ve seen to address the Bailey paper and its findings:

“Soil was a black box,” said Janet Jansson, chief scientist for Biology Earth and Biological Sciences at the Pacific Northwest National Laboratory and president of the International Society for Microbial Ecology. “I have been working in microbial ecology for decades, and it has been difficult, if not impossible, to study them. Now we have these new molecular processes, and suddenly the whole field is exploding.”

Interest in microbiomes in the natural world is also exploding because many researchers realize that as the planet warms, essential diversity and function in the microbial world could be lost. Some areas may not be able to grow the same crops they are growing now — in the United States, for instance, no corn in Iowa or wheat in Kansas, because the microbes that currently fix nitrogen for the plants’ roots in the soil will no longer be able to do so. And, as we learn more about how microbes function, there may be ways to put them to work in the service of adaptation — enhancing plant growth, for example, in a warming climate.

Most urgent, though, is the fact that the earth has locked up a great deal of carbon and should it come unlocked as C02 it could dramatically speed up climate change. “The big question is whether soil will be a sink or source of greenhouse gases in the future,” said Jansson.

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The full paper, “Soil Respiration and Bacterial Structure and Function after 17 Years of a Reciprocal Soil Transplant Experiment,” was published in the journal PLOS One; it can be downloaded here without charge.