Increased carbon dioxide in atmosphere linked to decreased soil organic matter

URBANA - A recent study at the University of Illinois created a bit of a mystery for soil scientist Michelle Wander – increased carbon dioxide in the atmosphere was expected to increase plant growth, increase plant biomass and ultimately beef up the organic matter in the soil -- but it didn’t. What researchers found instead was that organic matter decay increased along with residue inputs when carbon dioxide levels were increased and they think the accelerated decay was due to increased moisture in the soil.

“Going into the study, the assumption was that higher levels of carbon dioxide in the atmosphere will increase crop yield and soil organic matter,” said Wander. “We did see a 30 percent increase in above- and below- ground soybean biomass so we expected that to be mirrored in soil organic matter, but there wasn’t an increase. In fact, organic matter levels may have even been lower than in plots not exposed to elevated carbon dioxide levels.”

The study was conducted at U of I’s SoyFACE facility – an open air laboratory in which rings of pipes surround corn and soybean crops and can be exposed to various levels of carbon dioxide, ozone or both pumped through the pipes. The findings from the study are published in the February issue of Plant and Soil.

“My student Adriane Peralta and I were looking at younger soil organic matter that would be most influenced by today’s practices and we were expecting a big change -- a 30 percent increase in soil organic matter, reflecting the changes we saw above ground.

“The source of carbon is plant biomass, so we would expect increased yield, increased biomass, increased soil organic matter in the soil. This kind of positive feedback would be good because it could offset the increases in decay that will result from rising temperature,” said Wander. She explained that the increases in carbon dioxide levels in the atmosphere insulate the earth and contribute to global warming. Average annual air and soil temperatures are increasing while winters are getting shorter. By the end of the century, maximum daily temperatures could rise by 5 to 12 degrees Fahrenheit in winter and 5 to 20 degrees Fahrenheit in summer.

“We know that microbial activity is directly influenced by an increase in temperature if other factors, like moisture aren’t limiting their growth,” she said. “Increased decomposition of organic matter is undesirable from a soil quality and climate perspective; microbial degradation of organic stocks releases carbon and nitrogen and over the long term this reduces soil’s productivity and ability to resist erosion, plus it returns the carbon dioxide to the atmosphere.” All of this talk about using agricultural lands to mitigate climate change depends upon our ability to keep the carbon in soil reserves.