In recent years it has been observed that there have been some declines in crop yield per unit of nitrogen fertilizer added, and a decline in symbiotic nitrogen fixation by rhizobia in legume crops. A novel explanation has been proposed.

The most common explanation for the observations is an overuse of agrichemicals applied to legume crops. That practice sets up "a vicious cycle," Fox said, because it reduces a legume crop's natural need for nitrogen fixation but leaves a shortage of natural nitrogen in the soil for the next year's crop to utilize. Thus, she said, there is the need for yet more fertilizer.

Other reasons, Fox said, have been poor soil quality due to overuse, which strips nutrients such as nitrogen and phosphorous from the soil, and to tillage, which interrupts root structures and disturbs the nitrogen-fixing bacteria when soil is turned.

"Our research provides another explanation for declining crop yields," Fox said. "We showed that by applying pesticides that interfere with symbiotic signaling, the overall amount of symbiotic nitrogen fixation is reduced.

Some pesticides, some times, seem to interfere with plant/rhizobia hook up.

In a paper appearing online this week ahead of the regular publication by the Proceedings of the National Academy of Sciences (PNAS), the five-member team reports that agrichemicals bind to and block connections to specific receptors (NodD) inside rhizobia bacteria living in root nodules in the soil. . .

"Agrichemicals are blocking the host plant's phytochemical recruitment signal," . . . "In essence, the agrichemicals are cutting the lines of communication between the host plant and symbiotic bacteria. This is the mechanism by which these chemicals reduce symbiosis and nitrogen fixation." . . .

None of the chemicals used in the research, including PCP, proved to be toxic to either the plants or bacteria, Fox said, "but PCP was unique in that it inhibited both seed germination and nitrogen fixation." More than 20 commonly used agricultural chemicals shared the same mechanism of action as PCP, but with varying amounts of signal disruption.

This isn't surprising. Though we lack specific information it has long been a concern that pesticides affected soil microbiology. I avoid them for that reason. Though I had no explicit information I worried that I would be nuking my good bugs or something. I've felt slightly foolish about this, especially when challenged to justify my aversions.

While we're on the NodD, there's some tangentially related work being done on NodDless symbiosis.

The team from the IRD’s ‘Laboratoire des Symbioses Tropicales et Méditerranéennes’ and its partners (1), taking as model a symbiosis between a tropical aquatic legume, Aeschynomene, and Bradyrhizobium, bacteria of the Rhizobia family, have just revealed a new mode of communication at molecular level between these two organisms. The bacteria of this original model have their own photosynthetic pathway, a unique property in the rhizobia (2). This special character confers on it the exceptional, rare ability to form nodules on the stems of its host-plant. The plant thus acquires the possibility of fixing much higher quantities of nitrogen than those usually measured in leguminous plants which have nodules only on their roots.

The researchers sequenced (3) the genes of two bacterial strains of Bradyrhizobium, ORS278 and BTAi1, in order to find out their genetic make-up and identify the genes involved in this rather special form of symbiosis. These bacteria were found to have no nod genes, usually essential for nodulation. Bradyrhizobium consequently appeared to use mechanisms that involved other genes. This surprising result calls into question the universally recognized model of molecular communication that initiates the rhizobia-legume symbiosis. This common model requires the presence of several nod genes which allow synthesis of the Nod factor, a compound elaborated by the bacterium which enables the plant to recognize it, by molecular recognition, thereby allowing the microorganism to penetrate inside the plant by the root hairs.

The finding raises the question as to what signalling pathway Bradyrhizodium might use to gain entry to the plant and set off nodulation.

The first observation was that the bacteria did not penetrate the roots of its host-plant by the hairs. It took advantage of “crack zones” comparable with wound areas. The set of results obtained from subsequent work, seeking to identify the genes involved in producing the unknown signal molecule that plays the role of Nod factor, prompted the team’s hypothesis that a molecule similar to a plant hormone (4), cytokinin, could act in the mechanisms by triggering nodulation. The discovery of the nature of the signal molecule itself, which remains to be fully determined, brings a glimpse of future agricultural applications.

Many plants live in symbiosis with bacteria, but the mechanisms are known for only a small number of these interactions. The demonstration of alternative pathways capable of triggering the nodulation signal in certain rhizobia is promising for future techniques for bringing these bacteria into association with different leguminous plants. It therefore becomes possible to increase agricultural production of a greater number of important plants, notably in tropical countries, while cutting down the use of fertilizers.

There's long been loose talk about finding ways to cause nitrogen fixing bacteria to hook up with non-legume plants. The more we know about signalling pathways and nodulation, the more likely we are to find a way to do this.