Increased carbon dioxide in atmosphere linked to decreased soil organic matter

Wander said that carbon dioxide is rising every year in the atmosphere because of human use of fossil fuel and deforestation. “We attribute the higher soybean yields over the past several decades to the rising carbon dioxide levels in the Earth’s atmosphere – some attribute a 10 percent increase in soybean yields already due to this carbon dioxide fertilization effect.

“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.

“So, we had a bit of a mystery to solve. Where did the organic carbon that was added by increased plant growth go" We know for certain that soil organic matter stocks result from the balance of inputs and decay so we had to look at factors influence decomposition. Nutrient levels soil pH and available N were all high in this fertile field and so we ruled these factors out.”

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. “When plants take up moisture they open their stomata -- the pores through which they transport both carbon dioxide and water and when plants satisfy their need for carbon dioxide they can close those stomata and conserve water. This appears to have happened at SoyFACE in both corn and soybean crops. So, moisture feedbacks that increased microbial activity might solve the mystery”. Wander said it’s a little tricky to project the future with these findings, because they are manipulating carbon dioxide but not rainfall in the SoyFACE test plots.

“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.