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 (read full study here).

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.