New Study Measures Crop Bactericide, Nitrapyrin, in Iowa Streams Completed

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Nitrification inhibitors are co-applied with inorganic nitrogen fertilizers (for example, anhydrous ammonia) to increase crop yields. Nitrapyrin, a widely used inhibitor, is a bactericide specifically designed to reduce activity of the natural soil-nitrifying bacteria Nitrosomonas, which limits nitrification and leaves more ammonium in the soil root zone for crop uptake (that is, to increase crop yields). Although nitrification inhibitors have been commonly used in the United States since the 1970's, there is very little information on their transport to aquatic systems or information on how they could affect non-target microbial community composition and function in streams.

U.S. Geological (USGS) scientists led the first reconnaissance study to assess potential off-field transport of nitrapyrin and its degradate, 6-chloropicolinic acid (6-CPA), to streams. To provide a comparison for nitrapyrin, this study also measured the off-field transport of three widely used and often detected corn herbicides: acetochlor, atrazine, and metolachlor. Water samples were collected from March to June 2016 at 11 sites ranging in basin size (12 to 220,000 square kilometers) across the State of Iowa to encompass the spring fertilizer application period and corresponding crop growth.

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This study was the first to document off-field transport of nitrapyrin. Nitrapyrin was detected in seven streams (39 percent of water samples) at concentrations ranging from 12 to 240 nanograms per liter even though its chemical properties indicate that it should adsorb to soils thus limiting its off-field transport. The degradate, 6-CPA, was never detected. The herbicides were detected in 100 percent of the samples at concentrations ranging from 28 to 16,000 nanograms per liter. Nitrapyrin detections were associated with rainfall events following the application of spring fertilizer, and nitrapyrin persisted in streams for up to 5 weeks. Nitrapyrin seems to follow the same spring flush pattern that has been well documented for herbicides.

Results from this study document nitrapyrin occurrence and transport from agricultural fields to adjacent streams. The concentrations observed in water samples were orders of magnitude below the median lethal concentration (LC₅₀— a statistically derived concentration of a substance that can be expected to cause death in 50 percent of test animals) values reported by the U.S. Environmental Protection Agency for freshwater vertebrates and invertebrates. Concentrations were also below the minimal toxicity level that can affect non-target soil organisms such as bacteria, fungi, and actinomycetes in soils, and were below the toxicity level for target soil-nitrifying bacteria. However, there are no studies about the effects of nitrapyrin on microbial function related to nitrogen processing in aquatic systems. The concentrations measured in this study provide a baseline for future studies to understand the effects of this nitrification inhibitor on instream processing of nitrogen.

This study was supported by the USGS Environmental Health Program (Toxic Substances Hydrology and Contaminant Biology) of the Ecosystems Mission Area.