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Look out, a soil model says more plants means massive carbon stores might be freed

Posted By Joanne Nova On December 27, 2014 @ 4:03 am In Global Warming | Comments Disabled

The Doom message version 48.2a (subclause i) has been released.

Forget methane clathrate pits, now extra plant growth (blame CO2) could cause  global soil to unleash massive amounts of carbon.

Carbon dioxide (aka “pollution”) feeds plants. This is now bad (didn’t you know?). An all new “first” computer model with plants, soil, and fungus, warns us that more plants could get soil microbes excited which might break down more soil carbon and release it into the air. Disaster! It’s a could-be-might-be-catastrophe. (At  least until paragraph 6 — see that caveat below.)

In the meantime this is is so big, it’s practically nuclear — the model reports that it could set off a “chain reaction”:

An increase in human-made carbon dioxide in the atmosphere could initiate a chain reaction between plants and microorganisms that would unsettle one of the largest carbon reservoirs on the planet — soil.

Did you know there is twice as much CO2, carbon in the soil as there is in Earth’s whole atmosphere?

Researchers based at Princeton University report in the journal Nature Climate Change that the carbon in soil — which contains twice the amount of carbon in all plants and Earth’s atmosphere combined — could become increasingly volatile as people add more carbon dioxide to the atmosphere, largely because of increased plant growth. The researchers developed the first computer model to show at a global scale the complex interaction between carbon, plants and soil, which includes numerous bacteria, fungi, minerals and carbon compounds that respond in complex ways to temperature, moisture and the carbon that plants contribute to soil.

(The “first”? Dr David Evans tells me that FullCAM — the Australian carbon accounting model he helped develop — did  this on an Australian scale years ago, and that they weren’t the first then.)

Note the politically-correct permitted phrasing next:

Although a greenhouse gas and pollutant, carbon dioxide also supports plant growth.

So after 500 million years of evolution of carbon based life-forms, carbon dioxide is first and foremost a greenhouse gas, secondly it’s a pollutant, but but… it does… “support” plant growth. (Could we make that weaker? Plants need CO2 so desperately that they suck out all the stuff they can get before morning tea, then they stop growing –  and the way to describe this is that CO2 “supports” them –   like a tomato stake, right?)

As trees and other vegetation flourish in a carbon dioxide-rich future, their roots could stimulate microbial activity in soil that in turn accelerates the decomposition of soil carbon and its release into the atmosphere as carbon dioxide, the researchers found.

Note the researchers found a “could”.

The next two paragraphs tell us how useful this work is — not for predicting the natural world necessarily — but for countering that annoying idea that plants might be a net sink of carbon:

This effect counters current key projections regarding Earth’s future carbon cycle, particularly that greater plant growth could offset carbon dioxide emissions as flora take up more of the gas, said first author Benjamin Sulman, who conducted the modeling work as a postdoctoral researcher at the Princeton Environmental Institute.

“You should not count on getting more carbon storage in the soil just because tree growth is increasing,” said Sulman, who is now a postdoctoral researcher at Indiana University.

Then there’s the killer caveat (maybe most readers won’t get this far). This is a doozy:

On the other hand, microbial activity initiated by root growth could lock carbon onto mineral particles and protect it from decomposition, which would increase long-term storage of carbon in soils, the researchers report.

So more plants with more roots will definitely cause bacteria to release more CO2 except  if they don’t.

But seriously, this is a really good model. It’s complex, it must be right:

Whether carbon emissions from soil rise or fall, the researchers’ model depicts an intricate soil-carbon system that contrasts starkly with existing models that portray soil as a simple carbon repository, Sulman said. An oversimplified perception of the soil carbon cycle has left scientists with a glaring uncertainty as to whether soil would help mitigate future carbon dioxide levels — or make them worse, Sulman said.

How lucky we are that the glaring uncertainty is resolved… I mean, might-be could-be settled.

The researchers’ soil-carbon cycle model has been integrated into the global land model used for climate simulations by the National Oceanic and Atmospheric Administration’s (NOAA) Geophysical Fluid Dynamics Laboratory (GFDL) located on Princeton’s Forrestal Campus.

Settled enough for NOAA – give us another grant.

Settled enough for Nature Climate Change.

REFERENCE

Sulman, et al (2014) Microbe-driven turnover offsets mineral-mediated storage of soil carbon under elevated CO2. Nature Climate Change; 4 (12): 1099 DOI: 10.1038/nclimate2436

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