• nitrate leaching,
• ammonia volatilization, or
• emissions of other N gases such as ammonia with some transformed into N₂O.

Although indirect losses are much higher than direct N₂O emissions, similar fertilizer management strategies can be used to minimize both types of loss and increase the availability of fertilizer N for crop growth.

Why does direct N₂O emission occur?

The major cause of direct N₂O emission from agricultural fields is soil microbial activity, primarily through the processes of nitrification (which occurs in well-aerated soils) and denitrification (which occurs in saturated soils). Because these loss pathways are biological, soil N₂O emissions are driven by temperature and soil water conditions. In a long-term field study in eastern Nebraska, soil N₂O emission from an irrigated continuous corn site was highest when the soil was warm (> 21 °C, or >70 °F) and relatively wet (water-filled pore space more than 60%) (Jin et al., 2017) (Figure 1).

Direct N₂O emissions will occur regardless of whether fertilizer is applied because the decomposition of soil organic matter also leads to the same soil microbial processes that release N₂O. The application of fertilizer, however, will significantly increase the amount of direct N₂O emission and indirect N losses because of greater N availability.

How can management decrease direct N₂O emissions?

The choice of fertilizer management strategy has a major impact on direct N₂O emissions. Outcomes of fertility management decisions will vary from field to field, and will depend on other management practices (i.e., tillage, rotation sequence, irrigation, cover crop use), soil type, and local climate (Figure 2). In general, using the 4R nutrient management principles (right source, right rate, right time, right place) will reduce both direct and indirect N₂O losses.

More specifically, fertility practices that maximize the opportunity for crops to take up N are expected to reduce N losses overall. Increased N uptake as a percent of total available N can be accomplished by minimizing the length of time between N application and crop N uptake (i.e., by using in-season and split fertilizer applications). The use of stabilized fertilizers, specifically slow- or controlled-release fertilizers, urease inhibitors, and nitrification inhibitors (NIs), can also sustain the availability of fertilizer N to crops for a longer time by delaying nitrification and protecting N from nitrate leaching, denitrification, and ammonia volatilization. Stabilized fertilizers also have been shown to boost yield, in some cases by 3% to 13% (Halvorson et al., 2014; Hatfield and Parkin, 2014; Shapiro et al., 2016).

The effectiveness of stabilized N fertilizers to protect against N losses and increase yield will vary with management, soils, and climate. In Merrick County the use of controlled-release urea significantly reduced indirect N₂O emission risks and increased crop yield in a coarse-textured soil, regardless of inter-annual differences in weather (Figure 3) (Maharjan et al., 2016). In extreme weather conditions, NIs failed to improve conventional fertilizer performance, but split application of conventional N fertilizers reduced N losses and improved yield (Maharjan et al., 2017). In other Nebraska studies (Clay, Dixon, Merrick, and Pierce counties), the use of stabilized fertilizers increased grain yield only when N rate approached the optimum fertilizer rate for a given field, or when weather (i.e., excessive rain) promoted N losses through leaching/denitrification (Bastos and Ferguson, unpublished data; Shapiro et al., 2016). Although the use of stabilized N fertilizers does not always lead to higher yield, it can reduce the uncertainty of N availability due to year-to-year variation in weather conditions.

Soil scientists at the University of Nebraska have developed the Nitrogen Loss Assessment Tool (N-LAT, Wortmann et al., 2014). This spreadsheet tool takes into account different user-defined parameters related to N fertilizer management (e.g., N source, rate, timing, placement, use of stabilized fertilizer) and county-level soils and weather data to estimate N losses. Producers can use N-LAT to simulate different scenarios of N management and their impact on N losses in their county. For more information about N-LAT, including a link to download the spreadsheet, see the Nebraska Extension publication, Nitrogen Loss Assessment Tool (N-LAT) for Nebraska: Background and Users Guide