Pesticides adversely affect nitrogen fixation
The symbiotic relationship between nitrogen fixing bacteria and host leguminous plants is essential to fixing atmospheric nitrogen, and planting legumes in rotation thus reduces the need for synthetic nitrogen fertilizers. Such sustainable agriculture practices maintain soil fertility. This is essential in light of the fact that as synthetic nitrogen fertilizer application has increased exponentially, the yield potential of major staple food crops has become stagnant. Such diminishing returns have been explained by models showing that the first addition of nitrogen fertilizer induces the largest gain in crop yield with efficiency declining at higher levels of application.
Scientists have recently published a paper in the journal Proceedings in the National Academy of Sciences (June 12, 2007, vol, 104, no. 24, 10282-10287) demonstrating that pesticides can reduce the symbiotic efficiency of nitrogen fixing bacteria and the host leguminous plants. They showed that some agrichemicals, which are either applied to crops or found as contaminants in the soil, significantly disrupt symbiotic nitrogen fixation and subsequently lower plant yields.
The abstract of the paper is reproduced below. The full paper can be accessed online at www.pnas.org/cgi/doi/10.1073/pnas.0611710104
Jennifer E. Fox [1,2], Jay Gulledge , Erika Engelhaupt , Matthew E. Burow [2,5], and John A. McLachlan 
 Center for
Ecology and Evolutionary Biology,
80309; and  Department of Medicine and Surgery, Hematology and Medical
Unprecedented agricultural intensification and increased crop yield will be necessary to feed the burgeoning world population, whose global food demand is projected to double in the next 50 years. Although grain production has doubled in the past four decades, largely because of the widespread use of synthetic nitrogenous fertilizers, pesticides, and irrigation promoted by the "Green Revolution," this rate of increased agricultural output is unsustainable because of declining crop yields and environmental impacts of modern agricultural practices. The last 20 years have seen diminishing returns in crop yield in response to increased application of fertilizers, which cannot be completely explained by current ecological models. A common strategy to reduce dependence on nitrogenous fertilizers is the production of leguminous crops, which fix atmospheric nitrogen via symbiosis with nitrogen-fixing rhizobia bacteria, in rotation with nonleguminous crops. Here we show previously undescribed in vivo evidence that a subset of organochlorine pesticides, agrichemicals, and environmental contaminants induces a symbiotic phenotype of inhibited or delayed recruitment of rhizobia bacteria to host plant roots, fewer root nodules produced, lower rates of nitrogenase activity, and a reduction in overall plant yield at time of harvest. The environmental consequences of synthetic chemicals compromising symbiotic nitrogen fixation are increased dependence on synthetic nitrogenous fertilizer, reduced soil fertility, and unsustainable long-term crop yields.