More trees and more vigorous vegetation growth may not soak up atmospheric carbon, according to new research.
Instead, more lusty tree roots could goad the soil microbe population into releasing as carbon dioxide so much more old carbon stored in the soil. And since the planet’s store of soil carbon is at least twice the quantity locked in the vegetation and the atmosphere, this could in turn accelerate global warming.
This is yet another example of what engineers call positive feedback, but the important word here is “could”. The question remains open.
Benjamin Sulman − a biologist at Indiana University, but then of the Princeton University Environmental Institute in the US − and colleagues report in Nature Climate Change that they have developed a new computer model to examine what really happens, on a global scale, when plants colonise the soil and start taking in moisture and carbon from the atmosphere.
The topsoil – the fertile mix of loam, rock dust, minerals, partly decomposed wood, straw and leaf litter, fungi, bacteria, invertebrates and moisture from which all of terrestrial life derives its nourishment – remains one of the great unexplored economies of the planet.
The puzzle is this: plants draw carbon dioxide from the atmosphere to build up their tissues, but some of this tissue decomposes and returns to the air, while some stays in the soil, locked away from the atmosphere.
Deforestation is well established as a major factor in the greenhouse gas budget, so more forests would be a good thing. More carbon dioxide should mean more vigorous growth, so more tree growth should start to reduce the atmospheric carbon levels.
But as the Princeton team have confirmed once again, it’s not so simple. “You should not count on getting more carbon storage in the soil just because tree growth is increasing,” said Dr Sulman.
“The goal was to take a very simple model and add some of the important missing processes. The main interactions between roots and soil are important and shouldn’t be ignored.
“Root growth and activity are such important drivers of what goes on in the soil, and knowing what the roots are doing could be an important part of understanding what the soil will be doing.”
Mechanics of life
Like all such research, the study offers a measure of how little we know of the mechanics of life, atmosphere, ocean and rock − and, in particular, the carbon cycle. Clearly, some of the most important things happen underfoot, literally buried from sight.
One study, published recently in Nature journal, tried to make an audit of the richness of life in the soil: there could be up to 9,000 different species of bacteria in a cubic centimetre, more than 200 different kinds of fungi in a gram of soil, and the total numbers of these microbes would add up to billions.
Add to this a dizzying variety of tiny invertebrates and other life forms, all playing a part in making growth happen and in disposing of the detritus, and the puzzle becomes even more perplexing.
Another study, also published in Nature, tried to work out how these rich and extraordinary microbial communities would respond to warmer temperatures.
The conclusion was that the soil would “breathe” faster, which means a greater traffic in carbon, especially in those high latitude places where there was a lot of stored carbon − in particular, the Arctic permafrost.
Since the soil microbes normally release at least 60 billion metric tonnes of carbon back to the atmosphere each year as carbon dioxide, any increase could have disconcerting consequences.
So researchers have repeatedly tried to make sense of the subterranean carbon cycle. They have established that fungi, in particular, play a role in the continuous traffic of energy and carbon that drives the plant world.
They have evidence that the soils may not store carbon as efficiently as they had once assumed, and that, to slow global warming, it may not be enough to just save the trees. Scientists must also consider the roles of such areas as grasslands, savannah and wetlands.
The message from all this research, and from the latest Princeton study, is that we may have mapped the planet Earth with exquisite precision, but we still don’t know much about the earth beneath our feet. There’s a whole new world down there.
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