Pesticides on Food Crops Might Backfire, Research Shows

Agricultural pesticides aimed at increasing crop yields might instead be disrupting natural nitrogen production in soil and leading to lower production or slower growth, according to a research team from the University of Oregon.

In a paper published online by the Proceedings of the National Academy of Sciences (PNAS), the Oregon team reported that pesticides intended to enhance crop production actually block interactions that let plants replace much-needed nitrogen in the soil. The pesticides interfere with bacteria living in the roots of rotation crops like alfalfa and soybeans, which typically help replenish nitrogen needed by primary crops such as corn, the researchers found.

Plants like alfalfa and soybeans, called legumes, normally send out chemical signals to recruit nitrogen-fixing bacteria. Under natural conditions, the bacteria subsequently help take nitrogen from the atmosphere and convert it into ammonia that other crops like corn can use as a natural fertilizer. However, synthetic pesticides interfere with this process by binding to and blocking connections to certain chemical receptors in the bacteria.

"In essence, the agrichemicals are cutting the lines of communication between the host plant and symbiotic bacteria," said
Jennifer E. Fox, a postdoctoral researcher at Oregon's Center for Ecology and Evolutionary Biology. "This is the mechanism by which these chemicals reduce symbiosis and nitrogen fixation."

Fox's team has studied the impact of more than 50 agricultural pesticides over the past six years. The researchers found that pentachlorophenol (PCP) was particularly strong in its impact on both nitrogen fixing and seed germination. Another 20-plus commonly used pesticides showed similar effects, although at varying levels, they reported.