March 28, 2017, by Tim Radford
Soil tests on the Colorado Plateau in the US show that arid lands could reflect more light back into space. Image: Jennifer LaVista, USGS
Scientific studies cast new light on the complex role soil microbes play in affecting ecosystems, atmospheric carbon levels and the climate.
LONDON, 28 March, 2017 − Climate scientists puzzled by the traffic of carbon between soil and air may have to think more deeply about the role played by soil microbes − the planet’s smallest inhabitants.
One research team has just found that soil microbes could actually lighten the colour of arid land soils, to reflect more light and bounce more radiation energy back into space.
Another has identified an unexpected source of atmospheric carbon: 17% of the soil carbon that gets into the atmosphere from a floodplain has its origins among micro-organisms at depths of more than two metres.
And a third group has identified soil bacteria that could help plants survive drought, and enhance crop yields in drylands such as Arizona, Israel and the Nile Valley.
Species of soil microbes
Essentially, all three studies are reports from a new frontier. Almost the least known ecosystem on the planet is the one under our feet: a tiny pinch of soil is home to hundreds of species of microbes, and the numbers in each pinch could be counted in billions.
What these entities do and how they interact with each other is still a puzzle to be pieced together.
So, in all cases, the research is incomplete. But each study represents a new aspect of the complexity of the interactions between flowering plants, atmosphere and bedrock, And all these interactions are mediated by microbes.
Drylands cover 40% of the planet’s land surface. These arid zones may look lifeless, but the skin of the desert is alive with a mix of mosses, lichens and cyanobacteria that form a biological soil crust, or biocrust.
Scientists from the US Geological Survey write in Scientific Reports journal that they measured squares of these biocrusts on the Colorado Plateau and tested them with different levels of warmth and rainfall.
They monitored the responses of the biocrusts, and they also calculated how much solar energy they reflected back into the atmosphere.
Climate scientists call this the albedo effect, and it plays an important role in climate models.
The mix of more warmth and more water transformed dark surfaces to light soils at rates that might be enough to slow the rate of global warming.
“This information is an important step in understanding climate, and may be helpful
in developing future climate models”
But, equally, the switch from mosses and lichens to the cyanobacteria that lightened the soil might accelerate soil erosion, lower soil fertility and slow the removal of the greenhouse gas carbon dioxide from the atmosphere.
“The discovery that climate change impacts on biocrusts could feed back to future climate is a critical factor that hasn’t been considered in the past,” says Austin Rutherford, a biologist at the University of Arizona, who led the study.
“This information is an important step in understanding climate, and may be helpful in developing future climate models.”
A second group report in the Vadose Zone Journal that they looked a little deeper than the biocrust at the surface.
Researchers discovered three years ago that deep below America’s Great Plains lay a vast treasure trove of ancient carbon-rich soils. The assumption was that, for the moment, this carbon was safely buried.
But a team from the Lawrence Berkeley National Laboratory in California measured the flux of carbon dioxide from the Colorado River floodplain and found that almost one-fifth of the carbon that got into the atmosphere came from soil microbes busy at depths of between two metres and 3.5 metres − a contribution not so far included in models of the Earth system.
This is well below rooting depth, in the vadose zone far below the topsoil but above the water table. And even in this supposedly inert zone, soil microbes were still playing a role in the cycle of life.
According to a team from Northern Arizona University, some microbes could play an even more valuable one. Scientists report in Plant and Soil journal that they analysed 52 studies from around the world to identify a bit of microbial magic.
When crop plants were provided with growth-promoting rhizobacteria – microbes that colonise roots – the yields of vegetables and grain increased by 20% to 45% in a drought, compared with well-watered plants. So, with help from one suite of microscopic manipulators, the plants that were struggling for water did even better than those that were irrigated.
This really is unexpected. That plants depend on soil microbes for nutrients and protection against pests is well-known. But the latest research suggests that certain soil microbes confer a significant and fruitful protective effect in times of plant stress by drought.
Quite how and why these particular rhizobacteria confer this advantage to their plant hosts is an evolutionary puzzle that still has to be solved by experiment.
But according to UN estimates, the world loses 12 million hectares of arable land to drought and desert every year, so this discovery could ultimately be of value to farmers in drought-prone lands at a time when climates have begun to change, while human numbers keep on growing.
And the restoration of vulnerable drylands to green, photosynthesizing surfaces that consume atmospheric carbon would in turn feed back into the planetary climate system.
That, for the moment, remains only a hope, and a lot more research needs to be done. But, once again, it is a reminder that the most important creatures on the planet may be the most downtrodden: the invisible citizens of the ground beneath our feet. – Climate News Network
Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.