A meta-evaluation of nitrapyrin agronomic and environmental effectiveness with emphasis on corn production in the Midwestern USA

The effectiveness of nitrification inhibitors for abatement of N loss from the agroecosystem is difficult to measure at typical agronomic scales, since performance varies at the research-field scale due to complex interactions among crop management, soil properties, length of the trial, and environmental factors. The environmental impact of the nitrification inhibitor nitrapyrin on N losses from agronomic ecosystems was considered with emphasis on the Midwestern USA. A meta-evaluation approach considered the integrated responses to nitrification inhibition found across research trials conducted in diverse environments over many years as measured in side-by-side comparisons of fertilizer N or manure applied with and without nitrapyrin. The resulting distributions of response indices were evaluated with respect to the magnitude and variance of the agronomic and environmental effects that may be achieved when nitrification inhibitors are used regionally over time. The indices considered (1) crop yield, (2) annual or season-long maintenance of inorganic N within the crop root zone, (3) NO(3)-N leached past the crop root zone, and (4) greenhouse gas emission from soil. Results showed that on average, the crop yield increased (relative to N fertilization without nitrapyrin) 7% and soil N retention increased by 28%, while N leaching decreased by 16% and greenhouse gas emissions decreased by 51%. In more than 75% of individual comparisons, use of a nitrification inhibitor increased soil N retention and crop yield, and decreased N leaching and volatilization. The potential of nitrification inhibitors for reducing N loss needs to be considered at the scale of a sensitive region, such as a watershed, over a prolonged period of use as well as within the context of overall goals for abatement of N losses from the agroecosystem to the environment.