Challenges and Opportunities for Mitigating Nitrous Oxide Emissions from Fertilized Cropping Systems

Nitrous oxides are a potent greenhouse gas emitted from a multitude of sources.  Agricultural processes constitute a significant portion of these emissions, and attempts have been made at reducing nitrous oxide.  This paper intends to be a thorough review of the potential strategies and future research needs specific to nitrous oxide emissions by focusing on the management of individual fertilized cropping systems.   Venterea et al. (2012) investigate methods in reducing nitrous oxide emissions, and goes into detail about what has and hasn’t worked, as well as why.  What follows is a summary of this investigation as well as recommendations that the authors have in reducing nitrous oxide emissions. — Anthony Li
Venterea, R. T., Halvorson, A. D., Kitchen, N., Liebig, M. A., Cavigelli, M. A., Del Grosso, S. J., Motavalli, P. P., Nelson, K. A., Spokas, K. A., Singh, B. P., Stewart, C. E., Ranaivoson, A., Strock, J., Collins, H. 2012. Challenges and opportunities for mitigating nitrous oxide emissions from fertilized cropping systems.  Frontiers in Ecology and the Environment 10. 10: 562-570

Nitrous oxide emissions are the product of several processes occurring in the soil, which include nitrification, nitrifier-denitrification, chemo-denitrification, and denitrification.  All these processes are exacerbated in crop systems, as the addition of nitrogen fertilizers provide the necessary ingredients.  Because these processes can occur under a range of soil conditions, optimizing soil conditions to reduce nitrous oxides production can be difficult and may not be feasible.  Nitrous oxide is of particular concern, as it is 300 times more potent as a greenhouse gas than carbon dioxide, and is expected to increase by 2% per year through 2015.  Experts agree that the main challenge in reducing agroecosystem nitrogen losses and nitrous oxide emissions is to maximize the amount of nitrogen fertilizer that is actually used by the crops, or the optimization of nitrogen use efficiency (NUE).  The authors investigate current methods of optimizing NUE, and how they can be improved upon.
The synchronization of the application of nitrogen fertilizers with the demand for nitrogen in plants holds a lot of potential in improving NUE.  Currently, large amounts of nitrogen fertilizers are applied before the growing season, resulting in lower than 40-50% recovery in crops while contributing to unnecessary nitrous oxide production.  The nitrogen that is not recovered by plants is often moved to downwind/stream ecosystems where it is still converted into nitrous oxides.  However, the timing of fertilizer use not only has to be in sync with crop demands, but also during appropriate climatic conditions.  In past cases, applying nitrogen fertilizers during the growing seasons in warmer or wetter conditions has led to an increased amount of nitrous oxides emitted.  With this in mind, synchronizing fertilizer application will require the need for accurate systems that can predict nutrient demand in crops while accounting for climatic conditions.  It’s also common for large amounts of nitrous oxides to be emitted in response to management practices and climatic events.  We can address these emissions by investigating these practices and events individually and seeing how they affect nitrous oxides production.  On a broader scale, we can reduce nitrous oxide emissions from soils overall by using smaller and more frequent nitrogen applications, which would lessen fertilizer-induced pulses of nitrous oxides emissions, and by using carbon rich residues for short-term nitrogen immobilization, which can reduce pulses of nitrous oxides emissions during the decomposition of nitrogen rich residues.
Another solution for the nitrous oxides is reducing the nitrogen rate, or the amount of nitrogen applied per area of field during a growing season.  This cuts straight to the source of the emissions, while also solving problems such as nutrient runoff and a dwindling nitrogen supply.  However, “because crop yields, and therefore farmers’ profits, are also highly sensitive to nitrogen rate, the feasibility of nitrogen rate reduction as a strategy for mitigating nitrous oxide must consider economic impacts and other policy ramifications.”  Solutions that result in economic damage are unattractive and will unlikely be implemented.  It should also be noted that reducing nitrogen rate in one area might increase the nitrogen rate, and its associated nitrous oxide emissions, in another area via leakage effect.
In dealing with this nitrous oxides issue, the authors have a number of suggestions they believe to be most effective.  They recommend frequent additions of nitrogen fertilizer that are applied to coincide with crop demand and avoid wet conditions.  This would maximize amount of nitrogen absorbed by crops, while avoiding conditions that can result in elevated nitrous oxides production.  The authors also recommend using nitrogen rates that are adjusted spatially to match in-field variations in crop nitrogen demand and soil nitrogen supply, as the demand and supply of nitrogen in a field varies spatially and would benefit from maximum nitrogen use efficiency.  The authors finally recommend developing a system that can deliver nitrogen close to the root system in a chemical form that is stabilized to minimize losses of all other reactive nitrogen species.  This solution would reduce nitrogen runoff, which would in turn reduce eutrophication and the likelihood of nitrous oxides being produced downstream.  It should be noted that all these solutions are in common in that they focus on improving the efficiency of nitrogen use.  Increasing nitrogen use to compensate for low nitrogen recovery rate in crops would only produce more nitrous oxides, while decreasing nitrogen will result in a leakage effect that would offset any amount of nitrous oxide saved.  The lesson we should take away from this is that improving efficiency should be our goal in addressing any environmental issue.  The consequences of increasing/decreasing the inputs for a greenhouse gas are reviewed here and should be considered when addressing other greenhouse gases.

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