Increased carbon dioxide in atmosphere linked to decreased soil organic matter, decreased plant growth

Science Daily: 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."

…"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.

…"Most models or projections of the future assume the carbon dioxide fertilization effect would be a good thing for agriculture and the world's food supply and have a benefit to soil organic matter, but more and more we are finding things are a little more complicated. What our study shows is that in this system, rising carbon dioxide levels are not contributing to soil health after all.

…Wander and Peralta suspect soil moisture plays a role. Wander points out that changes in rainfall are another important aspect of climate change and notes that we are already seeing shifts in the distribution of rainfall with increases in winter and spring rains with drier summers. Dry conditions can constrain plant growth and microbial decay rates. So, what they saw in the SoyFACE plots, was evidence of an important feedback -- where crops exposed to elevated carbon dioxide became more water use efficient.

…"We have learned that we can't say 'yield equals organic matter.' We have to understand the nuances of the time and place. SoyFACE is giving us early clues about what could happen in the future and where to direct our research attentions." The frontier of science right now includes anticipation of these interactions --reality might be stranger than the fiction that we create in the laboratory- even in an open field study like SoyFACE….

Soil profile, US Department of Agriculture, Wikimedia Commons