Over the last forty years nitrogen fertilizer use has increased seven-fold and nearly every acre of intensively farmed, conventional cropland is treated with pesticides.  A team of scientists explored the impact of pesticides and other environmental toxicants on symbiotic nitrogen fixation (SNF) brought about by Rhizobium bacteria (Fox et al., 2007).  Their findings were published June 12, 2007 in the prestigious Proceedings of the National Academy of Sciences.

The team describes the critical role played by SNF in supporting crop yields and environmental quality.  SNF has great potential to reduce farm production costs – a factor of growing importance as rising natural gas prices push upward the cost of nitrogen fertilizers.  In Brazil, SNF from soybeans reduces production costs an estimated $1.3 billion per year.  The research by Fox et al. (2007) explored in depth the signaling processes between plants and bacteria colonizing plant roots – processes that govern the degree of SNF and the production of certain phytochemicals.  They focused on the ways that pesticides can disrupt signaling and impair the efficiency of SNF.  Some 30 pesticides are known to disrupt SNF; the most widely used pesticide in the United States, glyphosate (Roundup) is known to be toxic to nitrogen fixing bacteria.

The “Conclusions” section of the paper begins by stating:

“The results of this study demonstrate that one of the environmental impacts of pesticides and contaminants in the soil environment is disruption of chemical signaling between the host plants and N-fixing Rhiz(obia) necessary for efficient SNF and optimal plant yield.”

Drawing on their recent work and other published studies, the team projected that pesticides and other contaminants are reducing plant yield by one-third as a result of impaired SNF.  This remarkable conclusion suggests one mechanism, or explanation of the yield-enhancing benefits of well-managed, long-term organic farming systems.

Source: “Pesticides reduce symbiotic efficiency of nitrogen-fixing rhizobia and host plants”

Authors: Jennifer E. Fox, Jay Gulledge, Erika Engelhaupt, Matthew E. Burrow, and John A. McLachlan.

Proceedings of the National Academy of Sciences, Vol. 104, No. 24, June 12, 2007.