(Beyond Pesticides, November 21, 2013) Scientists in France and at the University of Calgary say that nitrogen fertilizer applied to crops lingers in the soil and for decades leaches toward groundwater —much longer than previously thought. The study was led by researcher Mathieu Sebilo, Ph.D. at the Université Pierre et Marie Currie in Paris, France, and by Bernhard Mayer, Ph.D. in the U of C’s Department of Geoscience, and included several research organizations in France. The study, Long-term fate of nitrate fertilizer in agricultural soils, was published in the Proceeding of the National Academy of Sciences in the U.S. in October.
The study shows that the loss of synthetic nitrogen (N) fertilizer in groundwater occurs at low rates over many decades, which means it could take years to reduce nitrate contamination in groundwater, including in aquifers that supply drinking water. The researchers recommend that as a result of the “legacies of past applications of synthetic fertilizers in agricultural systems,” mitigation or restoration measures must take into account this delay.
In fact, the scientists found that three decades after application of isotopically labeled fertilizer N to agricultural soils in 1982 12—15% of the fertilizer-derived N is still residing in the soil organic matter, while 8—12% of the fertilizer N had already leached toward the groundwater. The scientists estimate that it will continue to leach in low amounts for at least another 50 years, much longer than previously thought.
“There’s a lot of fertilizer nitrogen that has accumulated in agricultural soils over the last few decades which will continue to leak as nitrate towards groundwater,” Dr. Mayer says.
Nitrate is a common contaminant of drinking water, particularly in agricultural areas where nitrogen fertilizers are used. High rates of fertilizer application may also increase the natural nitrate levels found in certain vegetables, such as lettuce and root crops. Research has indicated that long-term exposure to nitrates through food and water may increase risk of thyroid disease. In the body, nitrate competes with uptake of iodide by the thyroid, thus potentially affecting thyroid function.
The researchers advocate that farmers apply the right fertilizer source at the right rate, the right time, and the right place in order to reduce nitrate contamination. Beyond Pesticides has long supported “feed-the-soil” approaches to lawn and landscape management. Understanding the role of healthy soils in creating healthy landscapes and plants, Beyond Pesticides promotes a systems approach that centers on management of soil health and proper fertilization that eliminates synthetic fertilizers and focuses on building the soil food web and nurturing soil microorganisms. Experience demonstrates that this approach will build a soil environment rich in microbiology that will produce a strong, healthy lawn that is able to withstand many of the stresses that affect lawns and landscape. For more information lawn and landscape management, visit Beyond Pesticides’ issues page.
Organic farming and land management uses natural, less soluble sources of nitrogen, phosphorous and magnesium, including cover crops, compost, manure and mineralized rock, in order to promote increases in soil organic matter and a healthy soil structure. For more information on the importance of an organic systems approach as a solution to pesticide use, see Beyond Pesticides’ article Increasing Biodiversity As If Life Depends on It. For more information on water quality and organic farming, see the fact sheet, “Organic Land Management and the Protection of Water Quality.”
All unattributed positions and opinions in this piece are those of Beyond Pesticides.