Abstract
Nitrogen loss to the environment has been a constant problem for growers and timing of nitrogen (N)fertilizer application can be challenging due to seasonal precipitation in northern Idaho. Nitrogen fertilizer is especially vulnerable to loss through leaching, runoff and volatilization when applied in the fall. Prevention of nitrogen loss through the winter is crucial to supply winter wheat (Triticum aestivum L.) planted in the fall with sufficient nutrients. One solution to minimize nitrogen loss is the use of nitrogen stabilizers with the application of nitrogen fertilizers. The stabilizers slow down nitrate (NO3-) leaching by inhibiting the process of nitrification by nitrifying bacteria and archaea. However, improved crop performance after the application of nitrogen stabilizers has been variable, and little work has been done in the Pacific Northwest. Additionally, there is uncertainty in how nitrification inhibitors might influence nitrifying organisms. Therefore, experimental plots were established in Cottonwood and Cavendish, Idaho during the 2019-2020 and 2020-2021 growing seasons to test the efficacy of nitrogen stabilizers in the high rainfall zone of northern Idaho. Two wheat cultivars soft white winter wheat LCS Hulk and the hard red winter wheat LSC Jet were studied in separate trials with five urea ammonium nitrate (UAN, 32-0-0) fertilizer rates (0, 56, 112, 168, and 224 kg/ha) applied with and without the nitrification inhibitor Instinct® II (Corteva Agriscience, USA). Soil samples were taken at four dates during the growing season to monitor ammonium and nitrate soil concentrations: November after planting, March, May, and September after harvest. Agronomic measurements that were taken include yield and postharvest quality as well as monitoring populations of bacteria and archaea and soil nitrate and ammonium. Soil samples collected from both locations indicated that the nitrogen stabilizer helped to retain ammonium in the soil and decreased the concentration of nitrate in the top 15 cm and 30 cm of soil, with differences being most obvious in November and March. Most agronomic measurements were not influenced by nitrogen stabilizer application. There as not a significant impact on yield. For only the hard red winter wheat at Cavendish there was a small, but significant decrease in test weight and increase in grain protein in plots treated with Instinct® II compared to plots without Instinct® II. Nitrifying archaeal populations did not respond to nitrogen stabilizer treatment, nor were they impacted by nitrogen fertilizer rate. However, bacterial populations were significantly decreased in the Instinct® II treated soils in the spring. Populations of ammonia-oxidizing bacteria did significantly increase with increasing nitrogen fertilizer rates. The results from this research help to further understand nitrogen stabilizer efficacy on winter wheat in rainfed areas of Idaho as well as its effect on nitrifying microorganisms’ populations. This information will aid decision making for growers looking to improve their nitrogen use efficiency and crop quality while not harming their soil health.