Carbon and soils—plant roots may accelerate carbon loss from soils; increased atmospheric carbon dioxide limits soil storage

 

Climate change, plant roots may accelerate carbon loss from soils

Posted: 06 Apr 2015 01:55 PM PDT

Soil, long thought to be a semi-permanent storehouse for ancient carbon, may be releasing carbon dioxide to the atmosphere faster than anyone thought. In a new study, researchers showed that chemicals emitted by plant roots act on carbon that is bonded to minerals in the soil, breaking the bonds and exposing previously protected carbon to decomposition by microbes. In a study published in this week’s online edition of the journal Nature Climate Change, the researchers showed that chemicals emitted by plant roots act on carbon that is bonded to minerals in the soil, breaking the bonds and exposing previously protected carbon to decomposition by microbes. The carbon then passes into the atmosphere as carbon dioxide (CO2), said the study’s coauthor, Markus Kleber, a soil scientist in OSU’s College of Agricultural Sciences. He said the study challenges the prevailing view that carbon bonded to minerals stays in the soil for thousands of years. “As these root compounds separate the carbon from its protective mineral phase,” he said, “we may see a greater release of carbon from its storage sites in the soil.” It’s likely that a warming climate is speeding this process up, he said. As warmer weather and more carbon dioxide in the air stimulate plants to grow, they produce more root compounds. This will likely release more stored carbon, which will enter the atmosphere as CO2–which could in turn accelerate the rate of climate warming. “Our main concern is that this is an important mechanism, and we are not presently considering it in global models of carbon cycling,” Kleber said….

….”There is more carbon stored in the soil, on a global scale, than in vegetation or even in the atmosphere,” said Kleber. “Since this reservoir is so large, even small changes will have serious effects on carbon concentrations in the atmosphere, and by extension on climate.” One percent may not sound like much, he added. “But think of it this way: If you have money in the bank and you lose 1 percent per year, you would be down to two thirds of your starting capital after only 50 years.” Between 60 and 80 percent of organic matter entering the soil gets broken down within the first year in a chain of decomposition that ends with CO2, Kleber said. Most of the remaining carbon gets bound to the soil’s minerals through a variety of physical and chemical mechanisms. When this happens, the carbon is protected because the microbes can’t get at it to break it down. For the past couple of decades, scientists have assumed that these carbon-mineral bonds amounted to a long-lasting “sink” for soil carbon–keeping it out of the atmosphere by storing it in a stable form over many centuries.

“But from the beginning, there was a question that made a lot of folks uneasy,” said Kleber. “If carbon keeps going into the soil and staying there, then why aren’t we drowning in carbon? Isn’t there some process that takes it back into the cycle? That part was not very well researched, and it was what we were trying to find.” The researchers tested three model compounds for common “root exudates”–chemicals commonly excreted by plant roots–to see how strongly each one stimulated the microbes that drive organic-matter decomposition….

 

Marco Keiluweit, Jeremy J. Bougoure, Peter S. Nico, Jennifer Pett-Ridge, Peter K. Weber, Markus Kleber. Mineral protection of soil carbon counteracted by root exudates. Nature Climate Change, 2015; DOI: 10.1038/nclimate2580

 

 

Increased atmospheric carbon dioxide limits soil storage

Posted: 15 Apr 2015 06:00 AM PDT

Soil carbon may not be as stable as previously thought, scientists report, adding that soil microbes exert more direct control on carbon buildup than global climate models represent. This study, researchers say, provides insight into the mechanisms determining long-term soil carbon storage, knowledge that can be used to improve climate model representations of the global carbon cycle... Carbon dioxide, the major cause of global warming, is released to the atmosphere when oil, coal, and gasoline are burned. Soils contain the largest pool of terrestrial organic carbon, helping counteract rising carbon dioxide levels and thus potentially playing a key role in modulating climate change. Carbon accumulates in soil through many years of plant photosynthesis and is lost from soil as microscopic organisms, mostly bacteria and fungi, decompose soil carbon, converting it back to carbon dioxide and releasing it to the atmosphere. The balance of these two processes and the future of the soil carbon sink are uncertain. How much will soil organic carbon persist, and how much of this carbon will soil microorganisms convert back to carbon dioxide? By comparing data from experiments around the world with models of the soil carbon cycle, researchers have tested how soil carbon release by microbes responds to rising carbon dioxide. They found that higher levels of atmospheric carbon dioxide increase both carbon’s input and release from the soil. Thus, soil carbon may not be as stable as previously considered, and soil microbes have more direct control on carbon storage than is represented in today’s global climate models.

 

K. J. van Groenigen, X. Qi, C. W. Osenberg, Y. Luo, B. A. Hungate. Faster Decomposition Under Increased Atmospheric CO2 Limits Soil Carbon Storage. Science, 2014; 344 (6183): 508 DOI: 10.1126/science.1249534

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