Impact of stabilized urea fertilizers on gaseous nitrogen losses during forage seed production in Saskatchewan

Forage seed production requires significant fertility inputs, and differs from forage feed production in that fertilizer strategies focus on seed rather than biomass yield. Nitrogen fertilizer management is a particular challenge, because perennial grasses vary in their response to N due to differences in flower induction. Because forages grasses are typically grown for three or more years, N fertilizer typically is broadcast into standing vegetation. Unfortunately, the surface application of urea—the most commonly used form of N fertilizer in Western Canada—is subject to a variety of losses, such as volatilization of ammonia (NH3) and gaseous emissions of nitrous oxide (N2O), resulting in a decrease in N use efficiency (NUE) and causing a risk for the environment. Furthermore, if fertilizers are applied in the fall, subsequent spring snowmelt can promote N2O losses. One promising method to reduce these losses is to use stabilized fertilizers. Stabilized fertilizers contain either a urease or a nitrification inhibitor, or a combination of both, thereby blocking key pathways in the N cycle involved in NH3 volatilization and N2O emissions. The performance of stabilized fertilizers in soils of the Boreal Transition Zone, particularly under forage seed production management, is not well understood. The performance of stabilized N fertilizers in reducing gaseous N losses was investigated by quantifying and comparing gaseous NH3 and N2O losses in forage seed production systems. A novel and cost-effective closed, dynamic flux chamber (CDFC) system for measuring NH3 emissions in remote field sites was developed and validated. Utilizing the CDFC system, a field study was conducted to assess the efficacy of surface-applied stabilized urea fertilizers in reducing gaseous NH3 and N2O losses from forage seed production sites after application either in fall or spring. The study identified application timing (i.e., fall vs. spring) as a dominant factor governing the magnitude of gaseous N losses, with the majority of NH3 losses occurring after spring application, whereas N2O losses were greatest from fall-applied fertilizers during spring snowmelt. Soil properties influenced the potential for gaseous N losses, and stabilized fertilizers containing urease inhibitors reduced NH3 emissions significantly when the loss potential was high. The effect of stabilized fertilizers on N2O emissions, on the other hand, varied strongly between field sites. Soils were collected from the field sites and used in a series of bench-scale experiments to assess the efficacy of stabilized urea fertilizers in reducing NH3 losses under different soil environmental conditions (i.e., soil pH, moisture, and temperature). The study identified strong differences between the NH3 loss potential of the soils. Enhanced urea hydrolysis rates coupled with lower soil water content were the dominant factor governing the magnitude of NH3 losses. Stabilized fertilizers containing both urease and nitrification inhibitors were most effective in reducing NH3 losses.

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