The Impact Of Nitrogen Use Efficiency Improvement On Nitrous Oxide Emission From Cropland.

Nitrous oxide (N₂O) is the third most prominent greenhouse gas and the most abundant ozone-depleting gas in the atmosphere.  Its concentrations are projected to continue to increase in the atmosphere due to the expansion and intensification of crop production.  The agricultural sector is responsible for approximately 80% of anthropogenic N₂O emissions, thus making reducing N₂O emissions from cropland critical if we are to slow the increase of N₂O in the atmosphere.  However, such reductions are challenging given increasing global food demand due to an increasing population, greater meat consumption, and the expansion of biofuel production.

Because applying synthetic nitrogen fertilizer to cropland drives N₂O production, improving nitrogen use efficiency (NUE) in crop production could mitigate N₂O emission.   As agronomic research has shown, technologies like genetic improvement and slow-release fertilizers have improved NUE on a plot scale. Potentially, these technologies could reduce N₂O emission via two pathways: 1. reducing the proportion of added reactive nitrogen that is emitted as N₂O; 2. reducing nitrogen fertilizer use per unit of crop production, so that less reactive nitrogen is available for conversion to N₂O.  However, each of these N₂O reduction pathways is associated with large uncertainties, which need to be constrained by further research.  

Regarding the first pathway, due to limited scientific understanding and measurement availability, it is difficult to quantify how NUE improvement technologies affect direct N₂O emissions from cropland and indirect emissions from leached nitrogen.  To date, researchers have evaluated direct N₂O emissions from cropland, but only a few studies have tracked indirect N₂O emissions.  Analysis of both indirect and direct N₂O emissions is vital, making additional research necessary.   For example, nitrification inhibitors are usually effective for approximately 10-20 days, thus potentially only delaying the release of N₂O.  Assuming indirect emissions are negligible may result in an overestimate of the benefits of nitrification inhibitors on N₂O emissions.

Regarding the second pathway, NUE improvement technologies could be effective at reducing N₂O emissions because with lower fertilizer application they can achieve the same crop yields as conventionally fertilized plots (N_T1'<N_con, Figure 1).  In reality, however, a farmer usually determines the amount of fertilizer necessary to maximize profit and not to minimize expenditures on fertilizer. Consequently, as shown in Figure 1, the profit-maximizing fertilizer rate (N_T1) may frequently be higher than the fertilization rate (N_T1') used to achieve the same yield.  If a NUE improvement technology can lead to increasing yields when the fertilization rate is higher than the conventional plot, adopting the NUE improvement technology may lead to a higher fertilizer application rate for the farm (e.g., NUE Tech2 in Figure 1).

To explore these issues, we are developing a model to investigate the impact of NUE improvement technology on fertilizer application in the context of farmers profit maximizing objectives.  Our model uses a quadratic-plus-plateau yield response relationship, reported in the literature as the best for describing the fertilizer-yield response. The quadratic-plus-plateau yield response relationship means yields increase quadratically with fertilizer application until a yield ceiling is reached at which point increasing fertilizer application no longer increases yield.  It is designed to examine how profit-maximizing fertilizer rates and NUE changes as NUE improvement technologies shift the yield curve.  It also can be used to examine how fertilizer price, crop prices, and the cost of the NUE improvement technology affect the changes.

Based on a preliminary investigation, it appears that improving NUE will not necessarily reduce fertilizer use and N₂O emissions at the farm level. However, NUE improvement technologies may still benefit the environment by improving crop yields and thus reducing the need to clear additional land to produce more crops.

 

Figure 1.  A demonstration of how NUE improvement technology shifts the yield response curve to nitrogen fertilization (different line types) and the resulting profit-maximizing fertilizer rates (circles). The yield curve for conventional farming was fit from a field test conducted at Monsanto Water Utilization Learning Center at Gothenburg, Nebraska (http://www.monsanto.com/). The dash-dot line, dotted line, and the dashed line show several potential ways that NUE improvement technologies (NUE Tech 1-3) can change the yield curve. N_con, N_T1, N_T2, and N_T3 denote the profit-maximizing fertilizer application rate for each yield response curve. N_T1^' is the fertilization rate needed by NUE Tech 1 to reach the same yield level as the conventionally fertilized plots.