(Beyond Pesticides, June 8, 2007) New research shows that pesticides can reduce the efficiency of nitrogen-fixing plants, impacting soil fertility and ultimately reducing crop yields. The insecticides methyl parathion, DDT and pentachlorophenol were among the chemicals found to inhibit or delay the symbiotic relationship between the legume alfalfa and rhizobia bacteria, which is crucial to nitrogen fixation.
Legume crops are often cultivated to help replenish the vital soil nutrient nitrogen (N), but legumes cannot achieve this on their own — Rhizobium bacteria interacts with legumes to convert atmospheric N to nutrient compounds utilizable by plants. Legumes like alfalfa and soybeans are often included in crop rotations due to this beneficial relationship.
Nitrogen fixation resulting from the symbiotic relationship between leguminous plants and species of Rhizobium bacteria is an ecological service estimated to be equivalent to $10 billion worth of synthetic N fertilizer annually. Symbiotic nitrogen fixation (SNF) reduces the need for synthetic fertilizers, which cause environmental problems such as impaired water quality.
Acknowledging that SNF is both initiated and maintained by chemical signals between the host plant and the beneficial bacteria, the research team postulated that natural and synthetic chemicals could disrupt these signals. The study shows “previously undescribed in vivo evidence that a subset of organochlorine pesticides and pollutants inhibit symbiotic signaling between alfalfa and S. meliloti [rhizobia bacteria], resulting in delayed symbiotic recruitment, reduced SNF, and a decline in alfalfa plant yield.”
Alfalfa yields were affected by all chemical treatments applied, including methyl parathion, DDT, pentachlorophenol, and a commonly used plastic ingredient, bisphenol A. Methyl parathion is an organophosphate insecticide used to control a wide variety of insects on food and feed crops such as alfalfa, barley, onion and soybeans; DDT is a persistent organochlorine insecticide that is banned in several countries and identified as a chemical of concern by the Stockholm Convention on Persistent Organic Pollutants; and pentachlorophenol (PCP) is a chlorinated phenol wood preservative and multi-use pesticide. The health endpoints, as well as ecological effects, of all three of these pesticides are of concern.
Pointing to the fact that N fixation has declined over the past 25 years, the researchers estimate that the pesticide-induced SNF inhibition they have observed is ecologically relevant. Based on their model, they believe the findings translate to a one-third loss of plant yield per growing season. Additionally, they point to the need for more research to address real world scenarios as their data reflects a one-time treatment with a specific chemical. However, a typical field is treated multiple times throughout the growing season with a mixture of pesticides, suggesting additive and synergistic affects may further impact the capacity of N fixation.
The study, “Pesticides reduce symbiotic efficiency of nitrogen-fixing rhizobia and host plants,” is published in the Proceedings of the National Academy of Sciences.
To learn about other benefits of organic farming, visit http://www.beyondpesticides.org/organicfood/index.htm.